Neurol Sci DOI 10.1007/s10072-014-2015-y
ORIGINAL ARTICLE
Relationship of high blood lactate levels with latency of visual-evoked potentials Marinella Coco • Giovanna Alagona • Giulia De Maria Giuseppe Rapisarda • Erminio Costanzo • Vincenzo Perciavalle • Valentina Perciavalle
•
Received: 14 October 2014 / Accepted: 18 November 2014 Ó Springer-Verlag Italia 2014
Abstract We studied, in healthy adult subjects, the association of high blood lactate levels, induced with an exhaustive exercise (12 subjects) or an intravenous infusion (four subjects) of a lactate solution (3 mg/kg in 1 min), with amplitude and latency of visual-evoked potentials. Amplitude of N75, P100, and N145 components did not show significant changes, whereas latency of P100 was reduced at exercise’s end and that of N145 increased 10 min after the conclusion. Therefore, an increase of blood lactate induced by an exhaustive exercise or an intravenous infusion appears to induce an improvement in the conduction time between eye and striate cortex, while it seems to evoke a worsening of intracortical communication between striate and extrastriate areas. Keywords Maximal exercise Blood lactate Visualevoked potentials Man
Introduction After an intense dynamic exercise there is a transient muscle’s inability to maintain an optimal performance, called fatigue [1], and an increase of pyruvate and lactate
M. Coco V. Perciavalle (&) Department of Bio-Medical Sciences, Section of Physiology, University of Catania, Catania, Italy e-mail: [email protected] G. Alagona G. De Maria G. Rapisarda E. Costanzo Neurological Operative Unit, Cannizzaro Hospital, Catania, Italy V. Perciavalle Department of Sciences of Formation, University of Catania, Catania, Italy
[2], with the latter released into the blood [3]. It has been observed that oxygen, glucose and lactate extractions by the CNS increases during maximal exercise, supporting the hypothesis that brain activation is enhanced during intense exercise [4, 5]. However, it has been observed that an increase of blood lactate is associated with a worsening of attentional processes [6], an improvement of excitability of primary motor cortex [7] and no significant changes of excitability of spinal cord [8] and brainstem structures [9]. There is no information, to our knowledge, about the possible effects of blood lactate increases, as those evoked by an exhaustive exercise, on excitability in other part of the brain. Visual-evoked potentials (VEPs) have been used for studying the cortical mechanisms of visuoperceptual processing [10]. The VEP [11] is characterized by an initial negative component at around 75 ms (N75) followed by a positive component at around 100 ms (P100) and a second negativity at around 145 ms (N145). Aerobic exercise decreased the mean latency of VEPs immediately after exercise in rats that ran for 5 min on a motor-driven ¨ zmerdivenli treadmill below the lactate threshold [12]. O et al. [13] noted significant differences in recordings of preexercise VEPs between athletes and non-athletes; specifically, the latency of N145 was shorter and its amplitude was lower in athletes. The P100 amplitude was also lower in athletes. More recently, Zwierko et al. [14] observed that P100 latency increased and P100 amplitude decreased after an aerobic test at 80 % of VO2max in non-athletes but not in athletes. Since it has been noticed that, when aerobic activity exceeds 80 % of the VO2max, the blood lactate levels go over 4 mmol/l [15], there is the possibility that the observed changes of VEP waves could be related to a significant increase of blood lactate levels.
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The aim of the present study was to examine the relationship of high blood lactate levels, induced with a maximal exercise or an intravenous lactate infusion, with the amplitude and/or latency of the N75, P100, and N145 components of VEP waveform.
Materials and methods Subjects The subjects who volunteered for this study were 12 healthy adults, 6 women and 6 men, aging between 22 and 42 years (mean age 31.9 ± 6.1). All the subjects signed informed consent documentation in accordance with the Ethical Committee of our University prior to their participation in the study.
elicited by monocularly presented checkerboard patternreversal stimuli. Visual stimuli were presented on a 21-inch CRT monitor with a refresh rate of 75 Hz. The stimulus consisted of a central fixation point and black and white checks that changed phase (i.e. black to white and white to black). An active-gold disk electrode was placed on the scalp over the visual cortex at Oz and the reference-gold disk electrode was placed at Fz. The ground-gold disk electrode was placed on the forehead at Fpz. The electrodes were placed relative to bony landmarks according to the International 10/20 System [19]. Parameters of the recording system were: digitation, 250 Hz; bandpass filter, 1–100 Hz; notch filters, switched off; sweep time, 300 ms; artifact rejection threshold, 50 lV; averaging, 100 responses. Participants’ fixation was monitored using a video camera. Two consecutive waveforms were recorded. Data were averaged and analyzed off-line. The mean amplitude (lV)
Protocol The study previewed two different experimental sessions carried out at interval of 1 week: (1) assessment of VEP after an exhaustive exercise on all the 12 subjects, (2) assessment of VEP after an intravenous infusion of lactate in 4 out of 12 subjects (2 women and 2 men). Venous blood lactate levels, venous blood glucose levels and VEP waveforms were measured before, at the end, as well as 10 and 20 min after the conclusion of the exercise. Blood lactate level was measured using a ‘‘Lactate Pro’’ portable lactate analyzer (FaCT Canada Consulting Ltd). In fact, it has been shown [16] that this automated lactate analyzer has a good reliability. Blood glucose was measured with an ‘‘Ascensia Elite’’ portable blood glucose monitoring system (Bayer AG, Germany), that assures an acceptable consistency [17]. Exercise The participants performed a maximal multistage discontinuous incremental test on a mechanically braked cycloergometer (Monark, Sweden), at a pedaling rate of 60 rpm, while an electrocardiogram was monitored. Each subject started with unloaded cycling duration of 3 min, and the load was increased by 30 Watt every 3 min until volitional exhaustion or the required pedaling frequency of 60 rpm could not be maintained [18]. Visual-evoked potentials VEPs were recorded using a Medelec Synergy equipment (Oxford Instruments, Oxford, UK), according to the protocol established by the International Society for Clinical Electrophysiology of Vision [11]. VEPs were
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Fig. 1 Blood lactate values and blood glucose levels of the 12 subjects performing the exhaustive exercise. Symbols from ANOVA with Dunn’s Multiple Comparison Test: *p \ 0.05, **p \ 0.01, ***p \ 0.001
Neurol Sci
and latency (ms) of waves N75, P100, and N145 were calculated based on two sets of 100 stimulations of the right eye.
mean ± standard deviation (SD). All analyses were performed by means of using GraphPad Prism version 6.03 for Windows (GraphPad Software, San Diego, CA, USA).
Lactate infusion Lactate infusion was carried out as previously described [6, 7]. Briefly, four out of our 12 subjects accepted to receive an intravenous infusion of a 2 mEq/ml lactate solution. The apyrogenic and sterile solution was prepared by mixing 180 mg/ml of L(?)-lactic acid and 80 mg/ml of NaOH, to a pH of 6.5–8.0 (Monico S.p.A., Venice, Italy). Systemic lactate infusion was carried out at dose of 3 mg/kg in 1 min [20]. This dose allowed us to exceed the onset blood lactate accumulation (OBLA; 4 mmol/l) [21]. Statistical analysis Data were collected and averaged, and then compared using the data analyzed with one-way repeated measures analysis of variance (ANOVA; Friedman test), followed by Dunn’s Multiple Comparison Test. The relationship between variables was analyzed with linear regression. Significance was set at p \ 0.05 and all data are reported as
Results Assessment of VEP after an exhaustive exercise As can be seen in Fig. 1, after an exhaustive exercise, blood lactate levels increased from 1.3 mmol/l (±0.30 SD) before the exercise, to 12.9 mmol/l (±2.54 SD) at its end and returned to pre-exercise values within 20 min (1.5 mmol/l ± 0.33 SD). On the contrary, blood glucose levels did not exhibit any significant change. Table 1 shows mean values (±SD) of latency (ms) and amplitude (lV) of N75, P100 and N145 components of VEP before the maximal exercise (pre), at its end as well as 10 and 20 min after the conclusion. The same table displays oneway ANOVA, followed by Dunn’s Multiple Comparison Test, carried out on data shown in Table 1. It can be seen that amplitude of the N75, P100, and N145 components of VEP did not show significant changes. On the contrary, latency of
Table 1 Latency and amplitude (mean values ± standard deviation) of N75, P100 and N145 components of VEP waveform recorded in participants (N = 12) before the exhaustive exercise (pre), at its conclusion (end), as well as 10 and 20 min after its end Exercise
Pre
End
10 min
20 min
One-way ANOVA (p value)
N75 Latency (ms) Amplitude (lV)
69.1 (±3.32) 9.5 (± 3.83)
69.1 (±4.55) 9.7 (± 3.26)
67.9 (±2.69) 9.7 (± 3.50)
70.0 (±2.90) 9.4 (± 3.58)
0.1091 0.9025
P100 Latency (ms) Amplitude (lV)
101.0 (±3.62) 11.8 (± 4.41)
97.7 (±4.82)
100.0 (±4.67)
11.7 (± 3.26)
11.6 (± 4.17)
100.9 (±3.49) 11.9 (± 4.58)
>0.001*** 0.4098
N145 Latency (ms) Amplitude (lV)
138.4 (±4.00)
140.1 (±4.69)
145.4 (±4.33)
141.5 (±3.67)
8.6 (±3.20)
8.4 (±3.06)
8.3 (±3.09)
8.6 (±3.15)
Dunn’s Multiple Comparison Test
Latency
Pre vs. end Pre vs. ? 10 min
N75
0.0441* 0.5690
Amplitude P100
N145
N75
P100
N145
P [ 0.05
P < 0.01
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P < 0.05
P [ 0.05
P [ 0.05
P [ 0.05
Pre vs. ? 20 min
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
End vs. ? 10 min
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
End vs. ? 20 min
P [ 0.05
P < 0.001
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
?10 min vs. ? 20 min
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
One-way ANOVA (Friedman test) followed by Dunn’s Multiple Comparison Test carried out on measured values are also shown Significant differences are in bold
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Fig. 2 In the upper part, relation between blood lactate levels and latency of P100, in the 12 participants, at the end of the exercise. The insert shows the results of linear regression analysis. In the lower part, examples of VEPs from three participants with low, middle, high blood lactate at the end of exercise
P100 was significantly reduced (P \ 0.001) at the end of exercise, while latency of N145 resulted significantly (P \ 0.05) increased 10 min after the exercise’s conclusion. Figure 2 shows, for each of the 12 subjects, the relation between blood lactate levels and latency values of P100 component of VEP at the end of the exercise. Note that, after exercise completion, subjects with higher lactate levels had greater reduction of the P100 latency. This is highlighted by the negative linear relationship between these two variables (P \ 0.05). Three examples of this relationship are illustrated in the lower part of the figure. It should be noted that we have not found significant relationships between the levels of blood lactate and the latency of the N75 and N145 components of VEP. Assessment of VEP after an intravenous infusion of lactate Figure 3 shows the blood lactate and glucose levels of the four subjects before the intravenous lactate infusion (pre),
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Fig. 3 Blood lactate and glucose levels of the four subjects which accepted to receive an intravenous infusion of a 2 mEq/ml lactate solution, before the injection (pre), at its conclusion (end), as well as 10 and 20 min after its end
at its conclusion (end), as well as 10 and 20 min after its end. As can be seen, blood lactate increased from a mean value of 1.18 mmol/l (±0.15) before the infusion to a mean value of 5.18 mmol/l (±0.33) at its conclusion; the lactate concentration decreased to a mean value of 1.95 mmol/l (±0.40) 10 min later and decreased afterwards to a mean value of 1.23 mmol/l (±0.05) 20 min after the exercise’s termination. Mean value of blood glucose concentration was 5.35 mmol/l (±0.10) before the lactate infusion, 5.35 mmol/l (±0.08) at its end, 5.38 mmol/l (±0.10) and 5.34 mmol/l (±0.08) after 10 and 20 min from its conclusion, respectively. Table 2 shows mean values (±SD) of latency (ms) and amplitude (lV) of N75, P100 and N145 components of VEP before the lactate infusion (pre), at its end as well as 10 and 20 min after the conclusion. The same table displays one-way ANOVA, followed by Dunn’s Multiple Comparison Test, carried out on data shown. It can be seen that amplitude of the N75, P100, and N145 components of VEP did not show significant changes. On the contrary,
Neurol Sci Table 2 Latency and amplitude (mean values ± standard deviation) of N75, P100 and N145 components of VEP waveform recorded in participants (N=) which accepted to receive an intravenous infusion of Infusion
Pre
a 2 mEq/ml lactate solution, before the injection (pre), at its conclusion (end), as well as 10 and 20 min after its end
End
10 min
20 min
One-way ANOVA (p value)
N75 Latency (ms)
70.21 (±0.96)
68.0 (±0.81)
70.3 (±0.50)
70.3 (±0.99)
0.0341
11.8 (±1.71)
11.3 (±1.26)
12.0 (±2.45)
11.3 (±1.26)
0.4560
Latency (ms)
99.3 (±0.96)
96.0 (±0.82)
98.5 (±0.58)
99.0 (±1.41)
0.0341*
Amplitude (lV)
11.8 (±1.71)
11.3 (±1.26)
12.0 (±2.45)
11.3 (±1.26)
0.4560
139.3 (±5.57)
140.8 (±4.11)
140.0 (±4.55)
138.8 (±5.38)
7.0 (±1.41)
6.8 (±1.26)
7.5 (±1.29)
7.3 (±1.71)
0.6597
Amplitude (lV) P100
N145 Latency (ms) Amplitude (lV) Dunn’s Multiple Comparison Test
Latency
Pre vs. end Pre vs. ? 10 min
N75
0.0255*
Amplitude P100
N145
N75
P100
N145
P [ 0.05
P < 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
Pre vs. ? 20 min
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
End vs. ? 10 min
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
End vs. ? 20 min ?10 min vs. ? 20 min
P [ 0.05 P [ 0.05
P [ 0.05 P [ 0.05
P [ 0.05 P [ 0.05
P [ 0.05 P [ 0.05
P [ 0.05 P [ 0.05
P [ 0.05 P [ 0.05
One-way ANOVA (Friedman test) followed by Dunn’s Multiple Comparison Test carried out on measured values are also shown Significant differences are in bold
latency of P100 was significantly reduced (P \ 0.05) at the end of exercise, while latency of N145 resulted significantly (P \ 0.05) increased 10 min after the exercise’s conclusion.
Discussion In the present study we observed that, after an acute exhaustive exercise, a strong increase of blood lactate is associated with significant changes of latency of P100 and N145 components of VEP. In particular, P100 shows a significant decrease of latency at the end of exercise whereas N145 displays an increase of latency 10 min after the effort’s conclusion. No significant change was detected with regard to the latency of N75 as well as regarding the amplitude of all the three components of VEP waveform. In addition, at the end of the exercise, a significant negative correlation was found between the blood lactate and the latency of the P100 component of VEP. Since during an exhaustive exercise several variables will change besides increase lactate, as systemic hormones, cytokines, increased levels of adrenergic hormones both peripherally and centrally [22], we analyzed the effects on
VEP of an intravenous infusion of lactate, in subjects not performing any exercises. The neural generators of the pattern-reversal VEP components have been much debated [10]. While there is widespread agreement (based on studies in both animals and humans) that the N75 component originates from the primary visual cortex, the source of the P100 component is controversial, although the majority of investigators has proposed that the P100 is generated (like the N75) in the striate cortex. Concerning the N145 component, some studies have identified a source in the extrastriate visual cortex, while others concluded that the N145 arises from the calcarine cortex or from both striate and extrastriate areas. Therefore, an increase of blood lactate appears to induce an improvement in the conduction time between the eye and striate cortex, while it seems to evoke a worsening of intracortical communication between striate and extrastriate areas. It has been observed that O2, glucose and lactate extractions by the CNS increase during maximal exercise, supporting the hypothesis that brain activation is enhanced during intense exercise [4]. This elevation, however, is not equally distributed, but it is localized in the regions of the
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CNS related to locomotion, the maintenance of equilibrium, cardiorespiratory control and vision [23]. We observed that a strong increase of blood lactate, induced both with a strenuous exercise and an intravenous infusion of sodium lactate, is associated with an improvement, in the frontal lobe, of the excitability of primary motor cortex [7]. However, an increase in blood lactate induced in the same two ways, produces worsening of attentional mechanisms, which involves primarily prefrontal cortical regions [12, 24]. It has been suggested that lactate acts at cortical level since improvements in excitability of motor cortex are not associated with changes of excitability of spinal [8] or brain stem [9] structures. Furthermore, it has been recently detected in vitro that lactate may stimulate astroglial brain-derived neurotrophic factor expression connected with an increase in heat shock protein 70 in adverse situations [25]. These results suggest that the improvement in the conduction time between the eye and striate cortex and the worsening of intracortical communication between striate and extrastriate areas, observed after an exhaustive exercise, could be dependent on a strong increase of blood lactate levels. In this way, blood lactate appears to have a protective effect against fatigue, at least at level of primary cortical areas (as striate cortex or primary motor cortex), although at the expense of efficiency of adjacent areas. Conflict of interest The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
8.
9.
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11. 12.
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18.
19.
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associated with increased motor cortex excitability. Somatosens Mot Res 27:1–8 Coco M, Alagona G, Va Perciavalle, Cicirata V, Perciavalle V (2011) Spinal cord excitability is not influenced by elevated blood lactate levels. Somatosens Mot Res 28:19–24 Coco M, Alagona G, Va Perciavalle, Rapisarda G, Costanzo E, Perciavalle V (2013) Brainstem excitability is not influenced by blood lactate levels. Somatosens Mot Res 30:90–95 Di Russo F, Pitzalis S, Spitoni G, Aprile T, Patria F, Spinelli D, Hillyard SA (2005) Identification of the neural sources of the pattern-reversal VEP. Neuroimage 24:874–886 Halliday AM (1993) Evoked Potentials in Clinical Testing. Churchill Livingstone, Edinburgh Ozkaya YG, Agar A, Haciuglu G, Yargicoglu P, Abidin I, Senturk UK (2003) Training induced alterations of visual evoked potentials are not related to body temperature. Int J Sports Med 24:359–362 Ozmerdivenli R, Bulut S, Bayar H, Karacabey K, Ciloglu F, Peker I, Tan U (2005) Effects of exercise on visual evoked potentials. Int J Neurosci 115:1043–1050 Zwierko T, Lubin´ski W, Lubkowska A, Niechwiej-Szwedo E, Czepita D (2011) The effect of progressively increased physical efforts on visual evoked potentials in volleyball players and nonathletes. J Sports Sci 29:1563–1572 Va Perciavalle, Alagona G, Maci T, Petralia MC, Perciavalle V, Coco M (2014) Attentional processes during submaximal exercises. Somatosens Mot Res 31:1–6 Buckley JD, Bourdon PC, Woolford SM (2003) Effect of measuring blood lactate concentrations using different automated lactate analysers on blood lactate transition thresholds. J Sci Med Sport 6:408–421 Kristensen GB, Christensen NG, Thue G, Sandberg S (2005) Between-lot variation in external quality assessment of glucose: clinical importance and effect on participant performance evaluation. Clin Chem 51:1626–1632 Green JM, McLester JR, Crews TR, Wickwire PJ, Pritchett RC, Redden A (2005) RPE-lactate dissociation during extended cycling. Eur J Appl Physiol 94:145–150 Jasper HH (1958) The ten–twenty electrode system of the International Federation. Electroencephalogr Clin Neurophysiol 10:371–375 Mu¨ller M, Schimrigk K (1994) A comparative assessment of cerebral haemodynamics in the basilar artery and carotid territory by transcranial Doppler sonography in normal subjects. Ultrasound Med Biol 20:677–687 Sjodin B, Jacobs I (1981) Onset of blood lactate accumulation and marathon running performance. Int J Sports Med 2:23–26 Gleeson M (2007) Immune function in sport and exercise. J Appl Physiol 103:693–699 Delp MD, Armstrong RB, Godfrey DA, Laughlin MH, Ross CD, Wilkerson MK (2001) Exercise increases blood flow to locomotor, vestibular, cardiorespiratory and visual regions of the brain in miniature swine. J Physiol 533:849–859 Posner MI, Petersen SE (1990) The attention system of the human brain. Ann Rev Neurosci 13:25–42 Coco M, Caggia S, Musumeci G, Perciavalle V, Graziano AC, Pannuzzo G, Cardile V (2013) Sodium L-lactate differently affects brain-derived neurothrophic factor, inducible nitric oxide synthase, and heat shock protein 70 kDa production in human astrocytes and SH-SY5Y cultures. J Neurosci Res 91:313–320
ORIGINAL ARTICLE
Relationship of high blood lactate levels with latency of visual-evoked potentials Marinella Coco • Giovanna Alagona • Giulia De Maria Giuseppe Rapisarda • Erminio Costanzo • Vincenzo Perciavalle • Valentina Perciavalle
•
Received: 14 October 2014 / Accepted: 18 November 2014 Ó Springer-Verlag Italia 2014
Abstract We studied, in healthy adult subjects, the association of high blood lactate levels, induced with an exhaustive exercise (12 subjects) or an intravenous infusion (four subjects) of a lactate solution (3 mg/kg in 1 min), with amplitude and latency of visual-evoked potentials. Amplitude of N75, P100, and N145 components did not show significant changes, whereas latency of P100 was reduced at exercise’s end and that of N145 increased 10 min after the conclusion. Therefore, an increase of blood lactate induced by an exhaustive exercise or an intravenous infusion appears to induce an improvement in the conduction time between eye and striate cortex, while it seems to evoke a worsening of intracortical communication between striate and extrastriate areas. Keywords Maximal exercise Blood lactate Visualevoked potentials Man
Introduction After an intense dynamic exercise there is a transient muscle’s inability to maintain an optimal performance, called fatigue [1], and an increase of pyruvate and lactate
M. Coco V. Perciavalle (&) Department of Bio-Medical Sciences, Section of Physiology, University of Catania, Catania, Italy e-mail: [email protected] G. Alagona G. De Maria G. Rapisarda E. Costanzo Neurological Operative Unit, Cannizzaro Hospital, Catania, Italy V. Perciavalle Department of Sciences of Formation, University of Catania, Catania, Italy
[2], with the latter released into the blood [3]. It has been observed that oxygen, glucose and lactate extractions by the CNS increases during maximal exercise, supporting the hypothesis that brain activation is enhanced during intense exercise [4, 5]. However, it has been observed that an increase of blood lactate is associated with a worsening of attentional processes [6], an improvement of excitability of primary motor cortex [7] and no significant changes of excitability of spinal cord [8] and brainstem structures [9]. There is no information, to our knowledge, about the possible effects of blood lactate increases, as those evoked by an exhaustive exercise, on excitability in other part of the brain. Visual-evoked potentials (VEPs) have been used for studying the cortical mechanisms of visuoperceptual processing [10]. The VEP [11] is characterized by an initial negative component at around 75 ms (N75) followed by a positive component at around 100 ms (P100) and a second negativity at around 145 ms (N145). Aerobic exercise decreased the mean latency of VEPs immediately after exercise in rats that ran for 5 min on a motor-driven ¨ zmerdivenli treadmill below the lactate threshold [12]. O et al. [13] noted significant differences in recordings of preexercise VEPs between athletes and non-athletes; specifically, the latency of N145 was shorter and its amplitude was lower in athletes. The P100 amplitude was also lower in athletes. More recently, Zwierko et al. [14] observed that P100 latency increased and P100 amplitude decreased after an aerobic test at 80 % of VO2max in non-athletes but not in athletes. Since it has been noticed that, when aerobic activity exceeds 80 % of the VO2max, the blood lactate levels go over 4 mmol/l [15], there is the possibility that the observed changes of VEP waves could be related to a significant increase of blood lactate levels.
123
Neurol Sci
The aim of the present study was to examine the relationship of high blood lactate levels, induced with a maximal exercise or an intravenous lactate infusion, with the amplitude and/or latency of the N75, P100, and N145 components of VEP waveform.
Materials and methods Subjects The subjects who volunteered for this study were 12 healthy adults, 6 women and 6 men, aging between 22 and 42 years (mean age 31.9 ± 6.1). All the subjects signed informed consent documentation in accordance with the Ethical Committee of our University prior to their participation in the study.
elicited by monocularly presented checkerboard patternreversal stimuli. Visual stimuli were presented on a 21-inch CRT monitor with a refresh rate of 75 Hz. The stimulus consisted of a central fixation point and black and white checks that changed phase (i.e. black to white and white to black). An active-gold disk electrode was placed on the scalp over the visual cortex at Oz and the reference-gold disk electrode was placed at Fz. The ground-gold disk electrode was placed on the forehead at Fpz. The electrodes were placed relative to bony landmarks according to the International 10/20 System [19]. Parameters of the recording system were: digitation, 250 Hz; bandpass filter, 1–100 Hz; notch filters, switched off; sweep time, 300 ms; artifact rejection threshold, 50 lV; averaging, 100 responses. Participants’ fixation was monitored using a video camera. Two consecutive waveforms were recorded. Data were averaged and analyzed off-line. The mean amplitude (lV)
Protocol The study previewed two different experimental sessions carried out at interval of 1 week: (1) assessment of VEP after an exhaustive exercise on all the 12 subjects, (2) assessment of VEP after an intravenous infusion of lactate in 4 out of 12 subjects (2 women and 2 men). Venous blood lactate levels, venous blood glucose levels and VEP waveforms were measured before, at the end, as well as 10 and 20 min after the conclusion of the exercise. Blood lactate level was measured using a ‘‘Lactate Pro’’ portable lactate analyzer (FaCT Canada Consulting Ltd). In fact, it has been shown [16] that this automated lactate analyzer has a good reliability. Blood glucose was measured with an ‘‘Ascensia Elite’’ portable blood glucose monitoring system (Bayer AG, Germany), that assures an acceptable consistency [17]. Exercise The participants performed a maximal multistage discontinuous incremental test on a mechanically braked cycloergometer (Monark, Sweden), at a pedaling rate of 60 rpm, while an electrocardiogram was monitored. Each subject started with unloaded cycling duration of 3 min, and the load was increased by 30 Watt every 3 min until volitional exhaustion or the required pedaling frequency of 60 rpm could not be maintained [18]. Visual-evoked potentials VEPs were recorded using a Medelec Synergy equipment (Oxford Instruments, Oxford, UK), according to the protocol established by the International Society for Clinical Electrophysiology of Vision [11]. VEPs were
123
Fig. 1 Blood lactate values and blood glucose levels of the 12 subjects performing the exhaustive exercise. Symbols from ANOVA with Dunn’s Multiple Comparison Test: *p \ 0.05, **p \ 0.01, ***p \ 0.001
Neurol Sci
and latency (ms) of waves N75, P100, and N145 were calculated based on two sets of 100 stimulations of the right eye.
mean ± standard deviation (SD). All analyses were performed by means of using GraphPad Prism version 6.03 for Windows (GraphPad Software, San Diego, CA, USA).
Lactate infusion Lactate infusion was carried out as previously described [6, 7]. Briefly, four out of our 12 subjects accepted to receive an intravenous infusion of a 2 mEq/ml lactate solution. The apyrogenic and sterile solution was prepared by mixing 180 mg/ml of L(?)-lactic acid and 80 mg/ml of NaOH, to a pH of 6.5–8.0 (Monico S.p.A., Venice, Italy). Systemic lactate infusion was carried out at dose of 3 mg/kg in 1 min [20]. This dose allowed us to exceed the onset blood lactate accumulation (OBLA; 4 mmol/l) [21]. Statistical analysis Data were collected and averaged, and then compared using the data analyzed with one-way repeated measures analysis of variance (ANOVA; Friedman test), followed by Dunn’s Multiple Comparison Test. The relationship between variables was analyzed with linear regression. Significance was set at p \ 0.05 and all data are reported as
Results Assessment of VEP after an exhaustive exercise As can be seen in Fig. 1, after an exhaustive exercise, blood lactate levels increased from 1.3 mmol/l (±0.30 SD) before the exercise, to 12.9 mmol/l (±2.54 SD) at its end and returned to pre-exercise values within 20 min (1.5 mmol/l ± 0.33 SD). On the contrary, blood glucose levels did not exhibit any significant change. Table 1 shows mean values (±SD) of latency (ms) and amplitude (lV) of N75, P100 and N145 components of VEP before the maximal exercise (pre), at its end as well as 10 and 20 min after the conclusion. The same table displays oneway ANOVA, followed by Dunn’s Multiple Comparison Test, carried out on data shown in Table 1. It can be seen that amplitude of the N75, P100, and N145 components of VEP did not show significant changes. On the contrary, latency of
Table 1 Latency and amplitude (mean values ± standard deviation) of N75, P100 and N145 components of VEP waveform recorded in participants (N = 12) before the exhaustive exercise (pre), at its conclusion (end), as well as 10 and 20 min after its end Exercise
Pre
End
10 min
20 min
One-way ANOVA (p value)
N75 Latency (ms) Amplitude (lV)
69.1 (±3.32) 9.5 (± 3.83)
69.1 (±4.55) 9.7 (± 3.26)
67.9 (±2.69) 9.7 (± 3.50)
70.0 (±2.90) 9.4 (± 3.58)
0.1091 0.9025
P100 Latency (ms) Amplitude (lV)
101.0 (±3.62) 11.8 (± 4.41)
97.7 (±4.82)
100.0 (±4.67)
11.7 (± 3.26)
11.6 (± 4.17)
100.9 (±3.49) 11.9 (± 4.58)
>0.001*** 0.4098
N145 Latency (ms) Amplitude (lV)
138.4 (±4.00)
140.1 (±4.69)
145.4 (±4.33)
141.5 (±3.67)
8.6 (±3.20)
8.4 (±3.06)
8.3 (±3.09)
8.6 (±3.15)
Dunn’s Multiple Comparison Test
Latency
Pre vs. end Pre vs. ? 10 min
N75
0.0441* 0.5690
Amplitude P100
N145
N75
P100
N145
P [ 0.05
P < 0.01
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P < 0.05
P [ 0.05
P [ 0.05
P [ 0.05
Pre vs. ? 20 min
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
End vs. ? 10 min
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
End vs. ? 20 min
P [ 0.05
P < 0.001
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
?10 min vs. ? 20 min
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
One-way ANOVA (Friedman test) followed by Dunn’s Multiple Comparison Test carried out on measured values are also shown Significant differences are in bold
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Fig. 2 In the upper part, relation between blood lactate levels and latency of P100, in the 12 participants, at the end of the exercise. The insert shows the results of linear regression analysis. In the lower part, examples of VEPs from three participants with low, middle, high blood lactate at the end of exercise
P100 was significantly reduced (P \ 0.001) at the end of exercise, while latency of N145 resulted significantly (P \ 0.05) increased 10 min after the exercise’s conclusion. Figure 2 shows, for each of the 12 subjects, the relation between blood lactate levels and latency values of P100 component of VEP at the end of the exercise. Note that, after exercise completion, subjects with higher lactate levels had greater reduction of the P100 latency. This is highlighted by the negative linear relationship between these two variables (P \ 0.05). Three examples of this relationship are illustrated in the lower part of the figure. It should be noted that we have not found significant relationships between the levels of blood lactate and the latency of the N75 and N145 components of VEP. Assessment of VEP after an intravenous infusion of lactate Figure 3 shows the blood lactate and glucose levels of the four subjects before the intravenous lactate infusion (pre),
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Fig. 3 Blood lactate and glucose levels of the four subjects which accepted to receive an intravenous infusion of a 2 mEq/ml lactate solution, before the injection (pre), at its conclusion (end), as well as 10 and 20 min after its end
at its conclusion (end), as well as 10 and 20 min after its end. As can be seen, blood lactate increased from a mean value of 1.18 mmol/l (±0.15) before the infusion to a mean value of 5.18 mmol/l (±0.33) at its conclusion; the lactate concentration decreased to a mean value of 1.95 mmol/l (±0.40) 10 min later and decreased afterwards to a mean value of 1.23 mmol/l (±0.05) 20 min after the exercise’s termination. Mean value of blood glucose concentration was 5.35 mmol/l (±0.10) before the lactate infusion, 5.35 mmol/l (±0.08) at its end, 5.38 mmol/l (±0.10) and 5.34 mmol/l (±0.08) after 10 and 20 min from its conclusion, respectively. Table 2 shows mean values (±SD) of latency (ms) and amplitude (lV) of N75, P100 and N145 components of VEP before the lactate infusion (pre), at its end as well as 10 and 20 min after the conclusion. The same table displays one-way ANOVA, followed by Dunn’s Multiple Comparison Test, carried out on data shown. It can be seen that amplitude of the N75, P100, and N145 components of VEP did not show significant changes. On the contrary,
Neurol Sci Table 2 Latency and amplitude (mean values ± standard deviation) of N75, P100 and N145 components of VEP waveform recorded in participants (N=) which accepted to receive an intravenous infusion of Infusion
Pre
a 2 mEq/ml lactate solution, before the injection (pre), at its conclusion (end), as well as 10 and 20 min after its end
End
10 min
20 min
One-way ANOVA (p value)
N75 Latency (ms)
70.21 (±0.96)
68.0 (±0.81)
70.3 (±0.50)
70.3 (±0.99)
0.0341
11.8 (±1.71)
11.3 (±1.26)
12.0 (±2.45)
11.3 (±1.26)
0.4560
Latency (ms)
99.3 (±0.96)
96.0 (±0.82)
98.5 (±0.58)
99.0 (±1.41)
0.0341*
Amplitude (lV)
11.8 (±1.71)
11.3 (±1.26)
12.0 (±2.45)
11.3 (±1.26)
0.4560
139.3 (±5.57)
140.8 (±4.11)
140.0 (±4.55)
138.8 (±5.38)
7.0 (±1.41)
6.8 (±1.26)
7.5 (±1.29)
7.3 (±1.71)
0.6597
Amplitude (lV) P100
N145 Latency (ms) Amplitude (lV) Dunn’s Multiple Comparison Test
Latency
Pre vs. end Pre vs. ? 10 min
N75
0.0255*
Amplitude P100
N145
N75
P100
N145
P [ 0.05
P < 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
Pre vs. ? 20 min
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
End vs. ? 10 min
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
P [ 0.05
End vs. ? 20 min ?10 min vs. ? 20 min
P [ 0.05 P [ 0.05
P [ 0.05 P [ 0.05
P [ 0.05 P [ 0.05
P [ 0.05 P [ 0.05
P [ 0.05 P [ 0.05
P [ 0.05 P [ 0.05
One-way ANOVA (Friedman test) followed by Dunn’s Multiple Comparison Test carried out on measured values are also shown Significant differences are in bold
latency of P100 was significantly reduced (P \ 0.05) at the end of exercise, while latency of N145 resulted significantly (P \ 0.05) increased 10 min after the exercise’s conclusion.
Discussion In the present study we observed that, after an acute exhaustive exercise, a strong increase of blood lactate is associated with significant changes of latency of P100 and N145 components of VEP. In particular, P100 shows a significant decrease of latency at the end of exercise whereas N145 displays an increase of latency 10 min after the effort’s conclusion. No significant change was detected with regard to the latency of N75 as well as regarding the amplitude of all the three components of VEP waveform. In addition, at the end of the exercise, a significant negative correlation was found between the blood lactate and the latency of the P100 component of VEP. Since during an exhaustive exercise several variables will change besides increase lactate, as systemic hormones, cytokines, increased levels of adrenergic hormones both peripherally and centrally [22], we analyzed the effects on
VEP of an intravenous infusion of lactate, in subjects not performing any exercises. The neural generators of the pattern-reversal VEP components have been much debated [10]. While there is widespread agreement (based on studies in both animals and humans) that the N75 component originates from the primary visual cortex, the source of the P100 component is controversial, although the majority of investigators has proposed that the P100 is generated (like the N75) in the striate cortex. Concerning the N145 component, some studies have identified a source in the extrastriate visual cortex, while others concluded that the N145 arises from the calcarine cortex or from both striate and extrastriate areas. Therefore, an increase of blood lactate appears to induce an improvement in the conduction time between the eye and striate cortex, while it seems to evoke a worsening of intracortical communication between striate and extrastriate areas. It has been observed that O2, glucose and lactate extractions by the CNS increase during maximal exercise, supporting the hypothesis that brain activation is enhanced during intense exercise [4]. This elevation, however, is not equally distributed, but it is localized in the regions of the
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CNS related to locomotion, the maintenance of equilibrium, cardiorespiratory control and vision [23]. We observed that a strong increase of blood lactate, induced both with a strenuous exercise and an intravenous infusion of sodium lactate, is associated with an improvement, in the frontal lobe, of the excitability of primary motor cortex [7]. However, an increase in blood lactate induced in the same two ways, produces worsening of attentional mechanisms, which involves primarily prefrontal cortical regions [12, 24]. It has been suggested that lactate acts at cortical level since improvements in excitability of motor cortex are not associated with changes of excitability of spinal [8] or brain stem [9] structures. Furthermore, it has been recently detected in vitro that lactate may stimulate astroglial brain-derived neurotrophic factor expression connected with an increase in heat shock protein 70 in adverse situations [25]. These results suggest that the improvement in the conduction time between the eye and striate cortex and the worsening of intracortical communication between striate and extrastriate areas, observed after an exhaustive exercise, could be dependent on a strong increase of blood lactate levels. In this way, blood lactate appears to have a protective effect against fatigue, at least at level of primary cortical areas (as striate cortex or primary motor cortex), although at the expense of efficiency of adjacent areas. Conflict of interest The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
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