Biol. Neonate 31: 10-14 (1977)
Reactivity of Neonatal Canine Aortic Strips Sarah D. Gray1 Department of Human Physiology, School of Medicine, University of California, Davis, Calif.
Key Words. Adrenergic agents • Norepinephrine • Phenylephrine • Tyramine ■Neonatal ■Aortic strips • Vascular reactivity
Introduction
Arterial strips have been used extensively to assess the response characteristics of mam malian vascular smooth muscle stimulated by pharmacological agents in isolation from neural, humoral, and mechanical factors which might influence in vivo experiments. Reports in the literature indicate that vascular responsiveness is not a static quantity. There are species and tissue variations in reactivity (Mating et at., 1971), as well as differences associated with age. When compared to vessels from mature ani mals, response characteristics of vessels from 1 I gratefully acknowledge support for this project from the US Public Health Service, grant No. PHS HL 14780-03.
animals at both ends of the age spectrum are significantly different. In very old animals se nescent changes resulting in a thickening of smooth muscle and elastic elements account for the differences in responsiveness of arterial strips (Tuttle, 1966), and in very young ani mals, both fetal and neonatal, differences are due to developmental changes in the enzymatic and structural protein makeup of the cells (Mirkin, 1972). In in vivo experiments, Gero and Gerova (1971) measured the external diameters of femoral arteries in puppies and adult dogs, be fore and during sympathetic stimulation. They found that the greatest diameter changes (per cent decrease), at all rates of stimulation, oc curred in puppies up to the age of 12 weeks. The maximum decrease obtainable was 33 % in
Downloaded by: Vanderbilt University Library 129.59.95.115 - 1/13/2019 8:27:33 PM
Abstract. To determine whether vascular reactivity in the young animal is significantly different from that of the mature animal, helical strips of thoracic aorta were suspended in an organ bath of Krebs bicarbonate solution, and isometric tension was measured. The effects of norepinephrine, phenylphrine, tyramine, and KC1 were studied. The neonatal vascular strips were less sensitive than adult strips to the adrenergic agents, but responsiveness to KC1 was comparable in both groups.
11
Gray
Methods Thoracic aortae of mature dogs and neonatal pup pies, 4 -1 0 days of age, were excised and placed in ice-cold Krebs bicarbonate solution. The vessels were stripped of adventitia and cut helically into strips 2 mm wide by 2 cm long. Each strip was suspended in a 10-ml organ bath of Krebs solution maintained at 37 °C and bubbled with a 95 % 0 :/5 % CO: gas mix ture. After 90 min of equilibration at a resting (pas sive) tension of 1.0-1.6g, the strips were stimulated by one of the vasoactive agents under study: norepi nephrine, 1.2 X 1 0 '6 to 5.0 X 1 0 '5 mg/ml; phenyl ephrine, 1 X 1 0 '5 to 5 X 1 0 '2 mg/ml; tyramine, 1.0 X 10-“ to 2.0 X 10-* mg/ml; KC1, 0.93 to 9.3 X 10”3 g/ml. Starting with the lowest concentration, the agent was added to the bath in cumulative 0.05 ml volumes until the maximum effect was attained, after which the bath chamber was rinsed three times with Krebs solution and the strip was allowed to relax and come back to baseline tension. Log dose-response curves were obtained from twelve neonatal and eight adult strips.
Results
The vascular strips of both adult and neona tal dogs responded to the pharmacological agents, norepinephrine, phenylephrine, tyra mine, and KC1, by monophasic isometric con tractions. Log dose-response curves for each agent were plotted from data obtained from cumulative additions of each drug from the minimum to the maximum effective dose; ED50 and maximum tension were determined and recorded in table I and figure 1. The change in responsiveness to adrenergic agents during the early maturational stage can be seen in the data for norepinephrine, phenyl ephrine, and tyramine. The concentration cor responding to the half-maximal contractile re sponse, the ED50, is one indication of the re sponsiveness of the tissue to the vasoactive agent. The neonatal vascular strips required higher concentrations of each agent to produce the half-maximal response than did the strips from adult animals. During the maturation pe riod after the 10th day of life, the ED50 for norepinephrine decreased by 41 %, with phen ylephrine it decreased by 80 %, and with tyra mine it decreased by 54 %, to reach the adult values. There was very little change in the ED50 for KC1, a slight decrease of 6.7 %. Each vasoactive agent was different in its ef fectiveness in eliciting a strong contractile re sponse, both in the adult and the neonate. As can be seen in figure 1, the greatest tension was developed in response to increased potassium ion concentration, in both the adult (3.49 ± 0.58 g) and neonate (1.92 ± 0.18 g). Of the adrenergic agents the order of effectiveness is phenylephrine (2.59 ± 0.31 g), norepinephrine (2.35 ± 0.20 g), and tyramine (1.62 ± 0.17) in the adult, and norepinephrine (1.52 ± 0.17 g), phenylephrine (0.94 ± 0.01 g), and tyramine (0.45 ± 0.10 g) in the neonate.
Downloaded by: Vanderbilt University Library 129.59.95.115 - 1/13/2019 8:27:33 PM
puppies and 13 % in adult dogs, which might suggest a greater reactivity in the young ani mals. Contrarily, Boatman et al. (1965) found decreased responsiveness of arterial resistance vessels in young puppies to both exogenous cat echolamines and neural stimulation, as com pared with the responses in the adult dog. They also found that with sympathetic chain stimula tion there was a greater preponderance of cho linergic over adrenergic influence on vascular resistance in hindlimb vessels of the neonates. After the age of 4 weeks this reverted to the characteristic predominant constrictor response seen in the adult. The current study is an at tempt to determine under in vitro conditions the responses of aortic strips from mature dogs and 4- to 10-day-old puppies to adrenergic agents (norepinephrine, phenylephrine, and tyramine), and to KC1, a nonspecific membrane depolarizing agent.
Gray
12
Table I. Effect of vasoactive agents on adult and neonatal aortic strips (mean ± SEM) Agent
Adult EDS0
Neonatal EDS0
Norepineph- 7.13 ± 2.2 X IO’ 4 1.21 ± 0.43 X IO"3 rinc mg/ml mg/ml 6.10 ± 1.1 X IO’4 3.11 ± 0.40 X IO '3 mg/ml mg/ml
Tyramine
8.46 ± 2.1 X IO"2 1.84 ± 0.40 X IO’ 1 mg/ml mg/ml
KC1
3.34 ± 0.24 X IO '3 3.58 ± 0.29 X 10-3 g/ml g/ml
Discussion
Under the in vitro conditions of the experi ment, dog aortic strips showed an increase in sensitivity to adrenergic agents with matura tion, similar to that which has been shown in other mammalian large arteries {Gray, 1976). The sensitivity changes were also directionally similar to those described by Boatman et al. (1965) in canine arteriolar vessels of the hindlimb, namely that constrictor responses in small arterial vessels of the youngest animals were less easily evoked with sympathetic stimulation and norepinephrine and epinephrine infusion than in adults. Not until 4 - 8 weeks of age were con strictor responses elicited by neural stimulation. Although epinephrine and norepinephrine in creased vascular resistance at all ages, responses appeared to stabilize after the age of 4 weeks. With maturation less of the catecholamine was needed to produce the same response, i.e., the older animals were more sensitive. They suggest that the subsensitivity may be due either to a limited number of adrenergic receptors in the young animals, or to an inability of the con tractile protein within the muscle cells to de velop tension to the same degree as in the adult.
Fig. 1. A comparison of the maximal developed tension by adult and neonatal aortic strips stimulated by norepinephrine (NE), phenylephrine (PHE), tyramine (TYR), and KC1.
In the present in vitro experiments, respon siveness to adrenergic agents increases during maturation. Norepinephrine and phenylephrine, an almost pure a-adrenergic agonist, stimulated the vascular a-receptors directly, and tyramine which stimulated the receptors indirectly {Davey and Farmer, 1963) by releasing endo genous norepinephrine from the neural endings located in the vascular wall. The smaller tension development with tyramine would tend to in dicate that the amount of norepinephrine re leased in this preparation (the adventitia which may contain part of the neural apparatus has been stripped) does not produce maximum effect equivalent to that induced by a high con centration of exogenous norepinephrine. The ED50 for KC1 is essentially unchanged during maturation (3.58 X 10-3 in the neonate and 3.34 X 10~3 g/ml in the adult) and since in creased. [K+]0 is thought to induce contraction
Downloaded by: Vanderbilt University Library 129.59.95.115 - 1/13/2019 8:27:33 PM
Phenylephrine
Neonatal Vascular Reactivity
has been measured by Petersen et al. (1960). From their data it appears that R in canine aorta and femoral artery declines progressively after the age of 12 weeks. From the study of Gerrity and Cliff (1975), it is apparent that in rat aorta, R increases during the first 2 - 4 weeks of life, after which it declines. Sivertsson (1970) suggests that, at least in resis tance vessels, a decrease in R causes a decrease in the contractile response initiated by a stimu lation of the receptors. Aars (1971) also sug gests that the magnitude of the in vivo re sponses of large arteries to a norepinephrine infusion is strongly influenced by the prevailing diastolic blood pressure. All of these studies support the view that in the in vivo situation, reactivity and mechanical factors are interre lated. Under in vitro conditions in which a helical strip is allowed to contract isometrically, reactivity can be assessed without the inter ference of geometric factors. The present study indicates that as the animal matures, the arterial muscle develops greater tension, perhaps due to the increase in wall thickness (addition of con tractile protein), and that the smooth muscle cells become progressively more responsive to adrenergic agents.
References Aars, H.: Diameter and elasticity of the ascending aorta during infusion of noradrenaline. Acta physiol, scand. 83: 133-138 (1971). Boatman, D.L.; Shaffer, R.A.; Dixon, R.L., and Brody, M.J.: Function on vascular smooth muscle and its sympathetic innervation in the newborn dog. J . clin. Invest. 44: 241-246 (1965). Davey, M.J. and Farmer, J.B.: The mode of action of tyramine. J. Pharm. Pharmac. 15: 178-182 (1963). Gero, J. and Gerova, M.: Postnatal development of sympathetic constriction of the femoral artery in the dog. Physiologia bohemoslov. 20: 372 (1971).
Downloaded by: Vanderbilt University Library 129.59.95.115 - 1/13/2019 8:27:33 PM
by its ability to depolarize cell membranes generally, rather than by acting on a specific mechanism, it is likely that the contractile mechanism is well developed even in the young animal. The difference in maximal tension induced by [K+] in neonates (1.92+ 0.18 g) and adults (3.49 ± 0.58 g) is due to the increase in tissue thickness (increase in the amount of contractile protein) during development. Both in the newborn and the adult, increased [K+]0 induces greater tension development than does a maximal concentration of norepinephrine or phenylephrine. The data of Gero and Gerova (1971) which suggests that the constrictor response (percent decrease in diameter) of canine femoral artery to neural stimulation is 2.5 times greater in puppies than in the adult, appear to contradict the results of Boatman et al. (1965) and the findings in the present study. However, the measured parameters are not the same and they are not necessarily an indication of vascular re activity alone. The Geros used an inductive transformer to measure changes in the external diameter of the femoral artery during sympa thetic chain stimulation. In an in vivo prepara tion, however, the induced changes in external diameter are determined by many factors, among them the number and sensitivity of receptors and contractility of smooth muscle, as well as the distensibility, the prevailing blood pressure, and the geometric factor of the wall thickness/luminal radius ratio (R). They suggest that in their experiments the greater change in diameter in the young animals (up to 12 weeks of age) is due to the fact that a dense neural investment of the vascular wall is located at the medioadvential border in the young animal, but only scattered terminals are present in the adventitia of the adult, and so more neurotransmitter reaches the muscle cells in young animals. They did not measure R, but it
13
Gerrity, R.G. and Cliff, W.J.: The aortic tunica media of the developing rat. Lab. Invest. 32: 585-600 (1975). Gray, S.D.: Neonatal development of arterial respon siveness to adrenergic agents (in press, 1976). Maling, H.M.; Fleisch, J.H., and Saul, W.F.: Species differences in aortic responses to vasoactive amines. The effects of compound 48/80, cocaine, reserpine, and 6-hydroxytryptamine. J. Pharmac. exp. Ther. 176: 672 -683 (1971). Mirkin, B.L.: Ontogenesis of the adrenergic nervous system: functional and pharmacological implica tions. Fed. Proc. Fed. Am. Socs exp. Biol. 31: 65-73 (1972). Petersen, L.H.; Jensen, R.E., and Parnell, J.: Mechani cal properties of arteries in vivo. Circulation Res. 8: 622-639 (1960).
Gray
Sivertsson, R.: The hemodynamic importance of struc tural vascular changes in essential hypertension. Acta physiol, scand., suppl. 343, pp. 1-56 (1970). Tuttle, R.S.: Age-related changes in sensitivity of rat aortic strips to norepinephrine and associated chemical and structural alterations. J. Geront. 21: 510-516 (1966).
Dr. Sarah D. Gray, Department of Human Physiol ogy, School of Medicine, University of California, Davis, CA 95616 (USA)
Downloaded by: Vanderbilt University Library 129.59.95.115 - 1/13/2019 8:27:33 PM
14
Reactivity of Neonatal Canine Aortic Strips Sarah D. Gray1 Department of Human Physiology, School of Medicine, University of California, Davis, Calif.
Key Words. Adrenergic agents • Norepinephrine • Phenylephrine • Tyramine ■Neonatal ■Aortic strips • Vascular reactivity
Introduction
Arterial strips have been used extensively to assess the response characteristics of mam malian vascular smooth muscle stimulated by pharmacological agents in isolation from neural, humoral, and mechanical factors which might influence in vivo experiments. Reports in the literature indicate that vascular responsiveness is not a static quantity. There are species and tissue variations in reactivity (Mating et at., 1971), as well as differences associated with age. When compared to vessels from mature ani mals, response characteristics of vessels from 1 I gratefully acknowledge support for this project from the US Public Health Service, grant No. PHS HL 14780-03.
animals at both ends of the age spectrum are significantly different. In very old animals se nescent changes resulting in a thickening of smooth muscle and elastic elements account for the differences in responsiveness of arterial strips (Tuttle, 1966), and in very young ani mals, both fetal and neonatal, differences are due to developmental changes in the enzymatic and structural protein makeup of the cells (Mirkin, 1972). In in vivo experiments, Gero and Gerova (1971) measured the external diameters of femoral arteries in puppies and adult dogs, be fore and during sympathetic stimulation. They found that the greatest diameter changes (per cent decrease), at all rates of stimulation, oc curred in puppies up to the age of 12 weeks. The maximum decrease obtainable was 33 % in
Downloaded by: Vanderbilt University Library 129.59.95.115 - 1/13/2019 8:27:33 PM
Abstract. To determine whether vascular reactivity in the young animal is significantly different from that of the mature animal, helical strips of thoracic aorta were suspended in an organ bath of Krebs bicarbonate solution, and isometric tension was measured. The effects of norepinephrine, phenylphrine, tyramine, and KC1 were studied. The neonatal vascular strips were less sensitive than adult strips to the adrenergic agents, but responsiveness to KC1 was comparable in both groups.
11
Gray
Methods Thoracic aortae of mature dogs and neonatal pup pies, 4 -1 0 days of age, were excised and placed in ice-cold Krebs bicarbonate solution. The vessels were stripped of adventitia and cut helically into strips 2 mm wide by 2 cm long. Each strip was suspended in a 10-ml organ bath of Krebs solution maintained at 37 °C and bubbled with a 95 % 0 :/5 % CO: gas mix ture. After 90 min of equilibration at a resting (pas sive) tension of 1.0-1.6g, the strips were stimulated by one of the vasoactive agents under study: norepi nephrine, 1.2 X 1 0 '6 to 5.0 X 1 0 '5 mg/ml; phenyl ephrine, 1 X 1 0 '5 to 5 X 1 0 '2 mg/ml; tyramine, 1.0 X 10-“ to 2.0 X 10-* mg/ml; KC1, 0.93 to 9.3 X 10”3 g/ml. Starting with the lowest concentration, the agent was added to the bath in cumulative 0.05 ml volumes until the maximum effect was attained, after which the bath chamber was rinsed three times with Krebs solution and the strip was allowed to relax and come back to baseline tension. Log dose-response curves were obtained from twelve neonatal and eight adult strips.
Results
The vascular strips of both adult and neona tal dogs responded to the pharmacological agents, norepinephrine, phenylephrine, tyra mine, and KC1, by monophasic isometric con tractions. Log dose-response curves for each agent were plotted from data obtained from cumulative additions of each drug from the minimum to the maximum effective dose; ED50 and maximum tension were determined and recorded in table I and figure 1. The change in responsiveness to adrenergic agents during the early maturational stage can be seen in the data for norepinephrine, phenyl ephrine, and tyramine. The concentration cor responding to the half-maximal contractile re sponse, the ED50, is one indication of the re sponsiveness of the tissue to the vasoactive agent. The neonatal vascular strips required higher concentrations of each agent to produce the half-maximal response than did the strips from adult animals. During the maturation pe riod after the 10th day of life, the ED50 for norepinephrine decreased by 41 %, with phen ylephrine it decreased by 80 %, and with tyra mine it decreased by 54 %, to reach the adult values. There was very little change in the ED50 for KC1, a slight decrease of 6.7 %. Each vasoactive agent was different in its ef fectiveness in eliciting a strong contractile re sponse, both in the adult and the neonate. As can be seen in figure 1, the greatest tension was developed in response to increased potassium ion concentration, in both the adult (3.49 ± 0.58 g) and neonate (1.92 ± 0.18 g). Of the adrenergic agents the order of effectiveness is phenylephrine (2.59 ± 0.31 g), norepinephrine (2.35 ± 0.20 g), and tyramine (1.62 ± 0.17) in the adult, and norepinephrine (1.52 ± 0.17 g), phenylephrine (0.94 ± 0.01 g), and tyramine (0.45 ± 0.10 g) in the neonate.
Downloaded by: Vanderbilt University Library 129.59.95.115 - 1/13/2019 8:27:33 PM
puppies and 13 % in adult dogs, which might suggest a greater reactivity in the young ani mals. Contrarily, Boatman et al. (1965) found decreased responsiveness of arterial resistance vessels in young puppies to both exogenous cat echolamines and neural stimulation, as com pared with the responses in the adult dog. They also found that with sympathetic chain stimula tion there was a greater preponderance of cho linergic over adrenergic influence on vascular resistance in hindlimb vessels of the neonates. After the age of 4 weeks this reverted to the characteristic predominant constrictor response seen in the adult. The current study is an at tempt to determine under in vitro conditions the responses of aortic strips from mature dogs and 4- to 10-day-old puppies to adrenergic agents (norepinephrine, phenylephrine, and tyramine), and to KC1, a nonspecific membrane depolarizing agent.
Gray
12
Table I. Effect of vasoactive agents on adult and neonatal aortic strips (mean ± SEM) Agent
Adult EDS0
Neonatal EDS0
Norepineph- 7.13 ± 2.2 X IO’ 4 1.21 ± 0.43 X IO"3 rinc mg/ml mg/ml 6.10 ± 1.1 X IO’4 3.11 ± 0.40 X IO '3 mg/ml mg/ml
Tyramine
8.46 ± 2.1 X IO"2 1.84 ± 0.40 X IO’ 1 mg/ml mg/ml
KC1
3.34 ± 0.24 X IO '3 3.58 ± 0.29 X 10-3 g/ml g/ml
Discussion
Under the in vitro conditions of the experi ment, dog aortic strips showed an increase in sensitivity to adrenergic agents with matura tion, similar to that which has been shown in other mammalian large arteries {Gray, 1976). The sensitivity changes were also directionally similar to those described by Boatman et al. (1965) in canine arteriolar vessels of the hindlimb, namely that constrictor responses in small arterial vessels of the youngest animals were less easily evoked with sympathetic stimulation and norepinephrine and epinephrine infusion than in adults. Not until 4 - 8 weeks of age were con strictor responses elicited by neural stimulation. Although epinephrine and norepinephrine in creased vascular resistance at all ages, responses appeared to stabilize after the age of 4 weeks. With maturation less of the catecholamine was needed to produce the same response, i.e., the older animals were more sensitive. They suggest that the subsensitivity may be due either to a limited number of adrenergic receptors in the young animals, or to an inability of the con tractile protein within the muscle cells to de velop tension to the same degree as in the adult.
Fig. 1. A comparison of the maximal developed tension by adult and neonatal aortic strips stimulated by norepinephrine (NE), phenylephrine (PHE), tyramine (TYR), and KC1.
In the present in vitro experiments, respon siveness to adrenergic agents increases during maturation. Norepinephrine and phenylephrine, an almost pure a-adrenergic agonist, stimulated the vascular a-receptors directly, and tyramine which stimulated the receptors indirectly {Davey and Farmer, 1963) by releasing endo genous norepinephrine from the neural endings located in the vascular wall. The smaller tension development with tyramine would tend to in dicate that the amount of norepinephrine re leased in this preparation (the adventitia which may contain part of the neural apparatus has been stripped) does not produce maximum effect equivalent to that induced by a high con centration of exogenous norepinephrine. The ED50 for KC1 is essentially unchanged during maturation (3.58 X 10-3 in the neonate and 3.34 X 10~3 g/ml in the adult) and since in creased. [K+]0 is thought to induce contraction
Downloaded by: Vanderbilt University Library 129.59.95.115 - 1/13/2019 8:27:33 PM
Phenylephrine
Neonatal Vascular Reactivity
has been measured by Petersen et al. (1960). From their data it appears that R in canine aorta and femoral artery declines progressively after the age of 12 weeks. From the study of Gerrity and Cliff (1975), it is apparent that in rat aorta, R increases during the first 2 - 4 weeks of life, after which it declines. Sivertsson (1970) suggests that, at least in resis tance vessels, a decrease in R causes a decrease in the contractile response initiated by a stimu lation of the receptors. Aars (1971) also sug gests that the magnitude of the in vivo re sponses of large arteries to a norepinephrine infusion is strongly influenced by the prevailing diastolic blood pressure. All of these studies support the view that in the in vivo situation, reactivity and mechanical factors are interre lated. Under in vitro conditions in which a helical strip is allowed to contract isometrically, reactivity can be assessed without the inter ference of geometric factors. The present study indicates that as the animal matures, the arterial muscle develops greater tension, perhaps due to the increase in wall thickness (addition of con tractile protein), and that the smooth muscle cells become progressively more responsive to adrenergic agents.
References Aars, H.: Diameter and elasticity of the ascending aorta during infusion of noradrenaline. Acta physiol, scand. 83: 133-138 (1971). Boatman, D.L.; Shaffer, R.A.; Dixon, R.L., and Brody, M.J.: Function on vascular smooth muscle and its sympathetic innervation in the newborn dog. J . clin. Invest. 44: 241-246 (1965). Davey, M.J. and Farmer, J.B.: The mode of action of tyramine. J. Pharm. Pharmac. 15: 178-182 (1963). Gero, J. and Gerova, M.: Postnatal development of sympathetic constriction of the femoral artery in the dog. Physiologia bohemoslov. 20: 372 (1971).
Downloaded by: Vanderbilt University Library 129.59.95.115 - 1/13/2019 8:27:33 PM
by its ability to depolarize cell membranes generally, rather than by acting on a specific mechanism, it is likely that the contractile mechanism is well developed even in the young animal. The difference in maximal tension induced by [K+] in neonates (1.92+ 0.18 g) and adults (3.49 ± 0.58 g) is due to the increase in tissue thickness (increase in the amount of contractile protein) during development. Both in the newborn and the adult, increased [K+]0 induces greater tension development than does a maximal concentration of norepinephrine or phenylephrine. The data of Gero and Gerova (1971) which suggests that the constrictor response (percent decrease in diameter) of canine femoral artery to neural stimulation is 2.5 times greater in puppies than in the adult, appear to contradict the results of Boatman et al. (1965) and the findings in the present study. However, the measured parameters are not the same and they are not necessarily an indication of vascular re activity alone. The Geros used an inductive transformer to measure changes in the external diameter of the femoral artery during sympa thetic chain stimulation. In an in vivo prepara tion, however, the induced changes in external diameter are determined by many factors, among them the number and sensitivity of receptors and contractility of smooth muscle, as well as the distensibility, the prevailing blood pressure, and the geometric factor of the wall thickness/luminal radius ratio (R). They suggest that in their experiments the greater change in diameter in the young animals (up to 12 weeks of age) is due to the fact that a dense neural investment of the vascular wall is located at the medioadvential border in the young animal, but only scattered terminals are present in the adventitia of the adult, and so more neurotransmitter reaches the muscle cells in young animals. They did not measure R, but it
13
Gerrity, R.G. and Cliff, W.J.: The aortic tunica media of the developing rat. Lab. Invest. 32: 585-600 (1975). Gray, S.D.: Neonatal development of arterial respon siveness to adrenergic agents (in press, 1976). Maling, H.M.; Fleisch, J.H., and Saul, W.F.: Species differences in aortic responses to vasoactive amines. The effects of compound 48/80, cocaine, reserpine, and 6-hydroxytryptamine. J. Pharmac. exp. Ther. 176: 672 -683 (1971). Mirkin, B.L.: Ontogenesis of the adrenergic nervous system: functional and pharmacological implica tions. Fed. Proc. Fed. Am. Socs exp. Biol. 31: 65-73 (1972). Petersen, L.H.; Jensen, R.E., and Parnell, J.: Mechani cal properties of arteries in vivo. Circulation Res. 8: 622-639 (1960).
Gray
Sivertsson, R.: The hemodynamic importance of struc tural vascular changes in essential hypertension. Acta physiol, scand., suppl. 343, pp. 1-56 (1970). Tuttle, R.S.: Age-related changes in sensitivity of rat aortic strips to norepinephrine and associated chemical and structural alterations. J. Geront. 21: 510-516 (1966).
Dr. Sarah D. Gray, Department of Human Physiol ogy, School of Medicine, University of California, Davis, CA 95616 (USA)
Downloaded by: Vanderbilt University Library 129.59.95.115 - 1/13/2019 8:27:33 PM
14
