Journal of Dairy Research (1992) 59 469-478
Printed in Great Britain
469
Transepithelial potential difference in the goat mammary gland and its change during hand milking, and administration of oxytocin and catecholamines BY NIKOLAJ P. ALEKSEEV, ALEXANDER G. MARKOV AND YURI A. TOLKUNOV Laboratory of Physiology of Secretory Processes, Institute of Physiology, St Petersburg University, University Embankment 7/9, St Petersburg 199034, Russian Federation {Received 21 October 1991 and accepted for"publication21 May 1992) SUMMARY. The reaction of secretory epithelium and myoepithelial cells in the alveoli to hand milking and i.v. injection of oxytocin and catecholamines was studied in lactating goats. The reaction of secretory cells was assessed by changes in the transepithelial (blood—milk) potential difference (PD), and the contractile reaction of myoepithelial cells by the growth of intramammary pressure (IP). The initial value of PD was 24-6 + 06 raV, that of IP 3-32 + 0-08 kPa (24-9 + 0-6 mmHg). Milking and oxytocin administration caused a rise in PD and an increase in IP. After noradrenaline and adrenaline injections two-phase PD changes and a short-term rise in IP were recorded. Isoproterenol, a /?-agonist, caused a rise in PD but did not affect IP. Phenylephrine, an a-agonist, caused two-phase and one-phase changes in PD. Simultaneous!}7, a rise in IP was recorded. The results show that the reaction of the mammary gland to the substances administered is complex. Myoepithelial and secretory cells respond differently to short-term rises in the level of mediators and hormones in the blood.
To investigate the mechanisms of secretion and milk ejection it is necessary to know the changes taking place in the secretory epithelium of the mammary gland in response to a physiological stimulus (e.g. milking and suckling) and also to the biologically active substances involved in the regulation of milk production and ejection. In microelectrode experiments performed on the mammary glands of mice it was found that the membrane potential of the secretory cells responded differently in amplitude and sign to the administration of substances stimulating or inhibiting milk ejection. The transepithelial potential difference (PD), the potential difference between the intra-alveolar fluid (milk) and the intercellular fluid or blood, appeared to be closely connected with the membrane potential of secretory cells and to change similarly under the action of chemical and physical factors acting upon the alveoli (Tolkunov, 1989a, b, 1990). Unfortunately, problems with methods make it impossible to perform microelectrode studies in the alveoli of the goat's or cow's mammary gland. However, PD recording in goats or cows presents no difficulty (Linzell & Peaker, 1971a, b). It should be noted that the presence of PD in the goat's mammary gland is due to ionic processes in the alveolar secretory cells (Peaker, 1977; Blatchford & Peaker, 1988). The aim of the present study was to investigate changes in transepithelial PD during hand milking, and also after administration of oxytocin and catecholamines. 11)
DARS9
470
N. P. ALEKSEEV AND OTHERS MATERIALS AND METHODS
The experiments were conducted on three goats of 40-50 kg in established lactation. The three goats were kept under similar conditions and milked twice daily, at 09.00 and 16.30, when the yields of the two halves of the udder were recorded. During the experiments, which were in the morning, the goat was on a milking stand, with its head and limbs restricted. Each animal was subjected to three series of experiments, in each of which one substance was tested over the whole range of concentrations. Injections of increasing doses of test substances were given every 10 min. After the last injection, with the same time interval, one half of the udder was milked by hand, and after removal of the teat catheter the second half of the udder was also milked. Recording PD was carried out by means of non-polarizing AgCl electrodes in physiological saline-agar. Joining the electrodes to milk and blood was achieved using polyethene catheters, one in the vena tibialis anterior, and the other, of special design, in the cistern of one half of the udder. The absence of an asymmetric potential between the electrodes was tested before the experiment started. The signal was recorded on a KSP recorder with an accuracy of ±0-2 mV. To record the pressure in the mammary gland (IP), a transistor transducer was used with a sensitivity of ~ 1 mV/0'67 kPa (5 mmHg) and an accuracy of +0-03 kPa (±0-2 minHg). The transducer was placed level with the middle of the teat and joined to the milk via a separate catheter in the teat canal of the mammary gland, which was also used for PD recording. The physiologically active substances (1 ml) were injected into the vena jugularis, via a polyethene catheter; for control injections the same volume of physiological saline was used. The following were tested: oxytocin (Gedeon Richter, Hungarj'), the /?-agonist isoproterenol and the a-agonist phenylephrine (Mesaton, Russia). The latent period of the reaction was determined as the time between administration of the test substance and the appearance of changes in recorded characteristics. For IP, the initial value and the magnitude and duration of any rise were recorded. For PD, the initial value, the amplitude and duration of any reaction, and the rates of increase and decrease of PD were recorded. An additional measure of PD change was provided by the relative coefficient (K), the ratio of the rates of increase and decrease. The results are presented as means with their SD.
RESULTS
The electrode in contact with milk in the teat was negative with respect to that in contact with blood. The initial values of PD and IP in the udders of different goats, and for the same animal in different experiments, were different. The mean value of PD was 24-6 (SD 0-6) mV (range 14-38 mV, n = 75). The mean value of IP was 3-32 (SD 0-08) kPa (24-9 + 0-6 mmHg) with a range of 0-21-5-1 kPa (16-38 mmHg, n = 25). Response to hand milking Hand milking the free teat of the goat's udder resulted in rises in PD (i.e. the milk electrode became increasingly negative) and IP (Fig. 1). The latent periods of these reactions were 28-0 (SD 2-7) s (n = 25) and 35-0 (SD 2-8) s respectively. The average PD changes were: amplitude 7-9 (SD 1-1) mV, reaction duration 257 (SD 27) s. The rate of increase due to hand milking was 0-45 (SD 0-09) mV/s, the rate of decrease
Transepithelial PD in goat mammary gland
471
(a) 10 mV
10 mmHg (1-33 kPa) (b) 50 s i—i
Fig. 1. Changes in (a) the transepithelial potential difference and (b) intramammary pressure during hand milking of the goat.
Oxytocin injection, mU
10 mV
10 mmHg (1-33 kPa)
Fig. 2. Changes in (a) the transepithelial potential difference and (6) intramanimary pressure in the goat in response to i.v. injections of increasing doses of oxytocin (from top to bottom, as indicated). j., Time of injection.
0-034 (SD 0-004) mV/s, with K 13-2. The mean rise in IP was 0-72 (SD 0-08) kPa (5-4 (SD 0-6) mmHg), its duration 195 (SD 19) s. Response to oxytocin Administration of oxytocin (5-100 mU i.v.) caused PD to become increasingly negative, and this was accompanied by a rise in IP (Fig. 2). The latent period of reaction was 20-6 (SD 0-6) s (n = 55). The response of PD to oxytocin appeared to be dose dependent (Fig. 3). The rate of change in PD in response to oxytocin injections calculated from all the recorded reactions was 0-30 (SD 0-003) mV/s, the rate of decrease 0-04 (SD 0-01) raV/s (n = 53), with K 7-5. (In two cases, changes in PD were too small to measure.) Using the highest dose of oxytocin it was possible to record the maximum reaction of 14-8 raV, which lasted 415 s. Practically concurrent with 19-2
472
N. P. ALEKSEEV AND OTHERS (a)
004 003 • 002 _ 001 -
+15
(b)
CD
O)
c CO
S +10
c
o 004 -i
CD
g> 003 f 0-02 8 001 -
•a c
I
+5 •
004 -i 003 002 001 -
s
3 2 1 - log (oxytocin concn, mU)
50
25 20 15 Oxytocin concn, ml)
10
(reciprocal scale)
Fig. 3. Dose dependence of changes in the transepithelial potential difference in the mammary gland of three different goats (1, 2, 3) in response to oxytocin injections expressed (a) as the absolute change and (b) as the reciprocal of the percentage change.
Initial pressure kPa
(a) .
10 mV
4-5
2-5
10 mmHg (1-33 kPa)
50 s Fig. 4. Changes in (a) the transepithelial potential difference and (b) intramammary pressure in the goat in response to i.v. injections of oxytocin (20 mU) at initial intramammary pressures of 4-5 kPa (34 mmHg) and 2-5 kPa (19 mmHg). [, Time of injection.
PD change, a rise in IP was observed, the amplitude of which increased with the dose of oxytocin. The latent period for IP with oxytocin was 17-9 (SD 0-7) s (n = 45). In ten cases, mainly involving low oxytocin concentrations, PD increase was not accompanied by a rise in IP. The maximum rise in IP in one goat at the maximum dose of oxytocin was 2-8 kPa (21 mmHg). In one experiment it was noted that at an initial IP in the gland of 4-5 kPa (34 mmHg) changes in PD were opposite to those found after oxytocin injections. Removing part of the milk (110 ml) via a catheter (the usual volume of milk obtained after the experiment was 560 ml) resulted in a lowering of IP to 2-5 kPa (19 mmHg) and restored the normal pattern of PD changes (Fig. 4a). Removing part of the milk from the gland may lower the superfluous 'stretching' pressure at the alveolar level, since the amplitude of IP changes after oxytocin injection decreased ~ 2-fold (Fig.
Transepithelial PD in goat mammary gland
(a)
473
(b)
10 mmHg (1-33 kPa)
10 mV
50 s
I
Fig. 5. Changes in (a) the transepithelial potential difference and (6) intramammary pressure in the goat in response to i.v. injections of 50 fig noradrenaline (upper trace) or 50 /ig adrenaline (lower trace). J,, Time of noradrenaline injection; f, time of adrenaline injection.
46). However, in other experiments removing part of the milk from the experimental half of the gland contributed to a rise in amplitude of the electrical reactions recorded in response to oxytocin administration. Responses to adrenaline and noradrenaline After noradrenaline and adrenaline injections two-phase PD changes were recorded, comprising comparatively fast and slow components and increases and decreases in PD. After injecting noradrenaline a rapid and a slow growth in PD were recorded (Fig. 5a). Increasing the noradrenaline dose from 10 to 50/ig caused an increase in the initial rapid PD rise from 1-6 to 6-8 mV (prolonging it from 15 to 20 s) and in the secondary slow rise from 1-2 to 4-0 mV (prolonging it from 275 to 490 s). Concurrently with these PD changes, short-term IP increases were recorded (Fig. 56). As the adrenaline dose increased from 10 to 50 fig, the amplitude of the IP change increased from 0-33 to 1-84 kPa (2-5 to 13-8 mmHg), and it was prolonged from 25 to 95 s. After adrenaline injections an initial fall and a subsequent rapid rise in PD took place, followed by a slow secondary rise (Fig. 5a). The initial fall in PD varied within the range 1-2-2-0 mV, lasted 20-30 s and did not seem to depend on the concentration of adrenaline. With increasing adrenaline dose (10-50 fig), the rise in PD increased from 1-6 to 6-0 mV, with a stable duration of 15 s. However, the increase in the slow secondary rise in PD with increasing adrenaline dose was less pronounced, from 2-4 to 3-6 mV, its duration increasing from 380 to 550 s. At the same time, IP changes rose from 1-13 to 2-20 kPa (8-5-16-5 mmHg), and its duration from 40 to 80 s (Fig. 56). Response to phenylephrine After injecting phenylephrine in doses of 50-300 fig, both two-phase and onephase changes in PD were recorded, with a latent period of 13-3 (SD 0-7) s (n = 35). With the two-phase response, which was observed in 15 cases out of 35, a fall preceded the rise (Fig. 6). The largest amplitude of the PD fall was 2 mV, and the
474
N. P. ALEKSEEV AND OTHERS Phenylephrine injection, fig (to) 10 mmHg (1-33 kPa)
10 mV
50 s
Fig. 6. Changes in (a) the transcpithelial potential difference and (6) intramammary pressure in the goat in response to i.v. injections of increasing doses of phenylephrine (from top to bottom, as indicated). I, Time of injection.
Isoproterenol injection, ng
Y 10 mV
0-5
ir
50 s
Pig. 7. Changes in the transepithelial potential difference in the goat in response to i.v. injections of increasing doses of isoproterenol (from top to bottom, as indicated). | . Time of injection.
highest amplitude of the PD rise was 8 mV. The duration of the PD fall was always short (5-50 s) compared with the growth phase (250-280 s). Concurrently with these PD changes, short-term reversible rises in I P were recorded (Fig. 6) with a latent period of 13-8 (SD 0-4) s (n = 33). The largest rise in IP (2-0 kPa, 15 mmHg) was recorded after the 150-200 /tg phenylephrine dose. The duration of these IP rises was shorter than that of PD changes. The rates of PD increase (0-29 (SD 0-04) raV/s) and decrease (0-035 (SD 0-004) mV/s) and their ratio {K = 8-2, n = 32) were similar to values obtained with oxytocin. However, the pattern of change was quite different:
Transepithelial PD in goat mammary gland
475
(b) 007 • 005 •
003 ' (a) j*
+20 -
r
ence,
E
Printed in Great Britain
469
Transepithelial potential difference in the goat mammary gland and its change during hand milking, and administration of oxytocin and catecholamines BY NIKOLAJ P. ALEKSEEV, ALEXANDER G. MARKOV AND YURI A. TOLKUNOV Laboratory of Physiology of Secretory Processes, Institute of Physiology, St Petersburg University, University Embankment 7/9, St Petersburg 199034, Russian Federation {Received 21 October 1991 and accepted for"publication21 May 1992) SUMMARY. The reaction of secretory epithelium and myoepithelial cells in the alveoli to hand milking and i.v. injection of oxytocin and catecholamines was studied in lactating goats. The reaction of secretory cells was assessed by changes in the transepithelial (blood—milk) potential difference (PD), and the contractile reaction of myoepithelial cells by the growth of intramammary pressure (IP). The initial value of PD was 24-6 + 06 raV, that of IP 3-32 + 0-08 kPa (24-9 + 0-6 mmHg). Milking and oxytocin administration caused a rise in PD and an increase in IP. After noradrenaline and adrenaline injections two-phase PD changes and a short-term rise in IP were recorded. Isoproterenol, a /?-agonist, caused a rise in PD but did not affect IP. Phenylephrine, an a-agonist, caused two-phase and one-phase changes in PD. Simultaneous!}7, a rise in IP was recorded. The results show that the reaction of the mammary gland to the substances administered is complex. Myoepithelial and secretory cells respond differently to short-term rises in the level of mediators and hormones in the blood.
To investigate the mechanisms of secretion and milk ejection it is necessary to know the changes taking place in the secretory epithelium of the mammary gland in response to a physiological stimulus (e.g. milking and suckling) and also to the biologically active substances involved in the regulation of milk production and ejection. In microelectrode experiments performed on the mammary glands of mice it was found that the membrane potential of the secretory cells responded differently in amplitude and sign to the administration of substances stimulating or inhibiting milk ejection. The transepithelial potential difference (PD), the potential difference between the intra-alveolar fluid (milk) and the intercellular fluid or blood, appeared to be closely connected with the membrane potential of secretory cells and to change similarly under the action of chemical and physical factors acting upon the alveoli (Tolkunov, 1989a, b, 1990). Unfortunately, problems with methods make it impossible to perform microelectrode studies in the alveoli of the goat's or cow's mammary gland. However, PD recording in goats or cows presents no difficulty (Linzell & Peaker, 1971a, b). It should be noted that the presence of PD in the goat's mammary gland is due to ionic processes in the alveolar secretory cells (Peaker, 1977; Blatchford & Peaker, 1988). The aim of the present study was to investigate changes in transepithelial PD during hand milking, and also after administration of oxytocin and catecholamines. 11)
DARS9
470
N. P. ALEKSEEV AND OTHERS MATERIALS AND METHODS
The experiments were conducted on three goats of 40-50 kg in established lactation. The three goats were kept under similar conditions and milked twice daily, at 09.00 and 16.30, when the yields of the two halves of the udder were recorded. During the experiments, which were in the morning, the goat was on a milking stand, with its head and limbs restricted. Each animal was subjected to three series of experiments, in each of which one substance was tested over the whole range of concentrations. Injections of increasing doses of test substances were given every 10 min. After the last injection, with the same time interval, one half of the udder was milked by hand, and after removal of the teat catheter the second half of the udder was also milked. Recording PD was carried out by means of non-polarizing AgCl electrodes in physiological saline-agar. Joining the electrodes to milk and blood was achieved using polyethene catheters, one in the vena tibialis anterior, and the other, of special design, in the cistern of one half of the udder. The absence of an asymmetric potential between the electrodes was tested before the experiment started. The signal was recorded on a KSP recorder with an accuracy of ±0-2 mV. To record the pressure in the mammary gland (IP), a transistor transducer was used with a sensitivity of ~ 1 mV/0'67 kPa (5 mmHg) and an accuracy of +0-03 kPa (±0-2 minHg). The transducer was placed level with the middle of the teat and joined to the milk via a separate catheter in the teat canal of the mammary gland, which was also used for PD recording. The physiologically active substances (1 ml) were injected into the vena jugularis, via a polyethene catheter; for control injections the same volume of physiological saline was used. The following were tested: oxytocin (Gedeon Richter, Hungarj'), the /?-agonist isoproterenol and the a-agonist phenylephrine (Mesaton, Russia). The latent period of the reaction was determined as the time between administration of the test substance and the appearance of changes in recorded characteristics. For IP, the initial value and the magnitude and duration of any rise were recorded. For PD, the initial value, the amplitude and duration of any reaction, and the rates of increase and decrease of PD were recorded. An additional measure of PD change was provided by the relative coefficient (K), the ratio of the rates of increase and decrease. The results are presented as means with their SD.
RESULTS
The electrode in contact with milk in the teat was negative with respect to that in contact with blood. The initial values of PD and IP in the udders of different goats, and for the same animal in different experiments, were different. The mean value of PD was 24-6 (SD 0-6) mV (range 14-38 mV, n = 75). The mean value of IP was 3-32 (SD 0-08) kPa (24-9 + 0-6 mmHg) with a range of 0-21-5-1 kPa (16-38 mmHg, n = 25). Response to hand milking Hand milking the free teat of the goat's udder resulted in rises in PD (i.e. the milk electrode became increasingly negative) and IP (Fig. 1). The latent periods of these reactions were 28-0 (SD 2-7) s (n = 25) and 35-0 (SD 2-8) s respectively. The average PD changes were: amplitude 7-9 (SD 1-1) mV, reaction duration 257 (SD 27) s. The rate of increase due to hand milking was 0-45 (SD 0-09) mV/s, the rate of decrease
Transepithelial PD in goat mammary gland
471
(a) 10 mV
10 mmHg (1-33 kPa) (b) 50 s i—i
Fig. 1. Changes in (a) the transepithelial potential difference and (b) intramammary pressure during hand milking of the goat.
Oxytocin injection, mU
10 mV
10 mmHg (1-33 kPa)
Fig. 2. Changes in (a) the transepithelial potential difference and (6) intramanimary pressure in the goat in response to i.v. injections of increasing doses of oxytocin (from top to bottom, as indicated). j., Time of injection.
0-034 (SD 0-004) mV/s, with K 13-2. The mean rise in IP was 0-72 (SD 0-08) kPa (5-4 (SD 0-6) mmHg), its duration 195 (SD 19) s. Response to oxytocin Administration of oxytocin (5-100 mU i.v.) caused PD to become increasingly negative, and this was accompanied by a rise in IP (Fig. 2). The latent period of reaction was 20-6 (SD 0-6) s (n = 55). The response of PD to oxytocin appeared to be dose dependent (Fig. 3). The rate of change in PD in response to oxytocin injections calculated from all the recorded reactions was 0-30 (SD 0-003) mV/s, the rate of decrease 0-04 (SD 0-01) raV/s (n = 53), with K 7-5. (In two cases, changes in PD were too small to measure.) Using the highest dose of oxytocin it was possible to record the maximum reaction of 14-8 raV, which lasted 415 s. Practically concurrent with 19-2
472
N. P. ALEKSEEV AND OTHERS (a)
004 003 • 002 _ 001 -
+15
(b)
CD
O)
c CO
S +10
c
o 004 -i
CD
g> 003 f 0-02 8 001 -
•a c
I
+5 •
004 -i 003 002 001 -
s
3 2 1 - log (oxytocin concn, mU)
50
25 20 15 Oxytocin concn, ml)
10
(reciprocal scale)
Fig. 3. Dose dependence of changes in the transepithelial potential difference in the mammary gland of three different goats (1, 2, 3) in response to oxytocin injections expressed (a) as the absolute change and (b) as the reciprocal of the percentage change.
Initial pressure kPa
(a) .
10 mV
4-5
2-5
10 mmHg (1-33 kPa)
50 s Fig. 4. Changes in (a) the transepithelial potential difference and (b) intramammary pressure in the goat in response to i.v. injections of oxytocin (20 mU) at initial intramammary pressures of 4-5 kPa (34 mmHg) and 2-5 kPa (19 mmHg). [, Time of injection.
PD change, a rise in IP was observed, the amplitude of which increased with the dose of oxytocin. The latent period for IP with oxytocin was 17-9 (SD 0-7) s (n = 45). In ten cases, mainly involving low oxytocin concentrations, PD increase was not accompanied by a rise in IP. The maximum rise in IP in one goat at the maximum dose of oxytocin was 2-8 kPa (21 mmHg). In one experiment it was noted that at an initial IP in the gland of 4-5 kPa (34 mmHg) changes in PD were opposite to those found after oxytocin injections. Removing part of the milk (110 ml) via a catheter (the usual volume of milk obtained after the experiment was 560 ml) resulted in a lowering of IP to 2-5 kPa (19 mmHg) and restored the normal pattern of PD changes (Fig. 4a). Removing part of the milk from the gland may lower the superfluous 'stretching' pressure at the alveolar level, since the amplitude of IP changes after oxytocin injection decreased ~ 2-fold (Fig.
Transepithelial PD in goat mammary gland
(a)
473
(b)
10 mmHg (1-33 kPa)
10 mV
50 s
I
Fig. 5. Changes in (a) the transepithelial potential difference and (6) intramammary pressure in the goat in response to i.v. injections of 50 fig noradrenaline (upper trace) or 50 /ig adrenaline (lower trace). J,, Time of noradrenaline injection; f, time of adrenaline injection.
46). However, in other experiments removing part of the milk from the experimental half of the gland contributed to a rise in amplitude of the electrical reactions recorded in response to oxytocin administration. Responses to adrenaline and noradrenaline After noradrenaline and adrenaline injections two-phase PD changes were recorded, comprising comparatively fast and slow components and increases and decreases in PD. After injecting noradrenaline a rapid and a slow growth in PD were recorded (Fig. 5a). Increasing the noradrenaline dose from 10 to 50/ig caused an increase in the initial rapid PD rise from 1-6 to 6-8 mV (prolonging it from 15 to 20 s) and in the secondary slow rise from 1-2 to 4-0 mV (prolonging it from 275 to 490 s). Concurrently with these PD changes, short-term IP increases were recorded (Fig. 56). As the adrenaline dose increased from 10 to 50 fig, the amplitude of the IP change increased from 0-33 to 1-84 kPa (2-5 to 13-8 mmHg), and it was prolonged from 25 to 95 s. After adrenaline injections an initial fall and a subsequent rapid rise in PD took place, followed by a slow secondary rise (Fig. 5a). The initial fall in PD varied within the range 1-2-2-0 mV, lasted 20-30 s and did not seem to depend on the concentration of adrenaline. With increasing adrenaline dose (10-50 fig), the rise in PD increased from 1-6 to 6-0 mV, with a stable duration of 15 s. However, the increase in the slow secondary rise in PD with increasing adrenaline dose was less pronounced, from 2-4 to 3-6 mV, its duration increasing from 380 to 550 s. At the same time, IP changes rose from 1-13 to 2-20 kPa (8-5-16-5 mmHg), and its duration from 40 to 80 s (Fig. 56). Response to phenylephrine After injecting phenylephrine in doses of 50-300 fig, both two-phase and onephase changes in PD were recorded, with a latent period of 13-3 (SD 0-7) s (n = 35). With the two-phase response, which was observed in 15 cases out of 35, a fall preceded the rise (Fig. 6). The largest amplitude of the PD fall was 2 mV, and the
474
N. P. ALEKSEEV AND OTHERS Phenylephrine injection, fig (to) 10 mmHg (1-33 kPa)
10 mV
50 s
Fig. 6. Changes in (a) the transcpithelial potential difference and (6) intramammary pressure in the goat in response to i.v. injections of increasing doses of phenylephrine (from top to bottom, as indicated). I, Time of injection.
Isoproterenol injection, ng
Y 10 mV
0-5
ir
50 s
Pig. 7. Changes in the transepithelial potential difference in the goat in response to i.v. injections of increasing doses of isoproterenol (from top to bottom, as indicated). | . Time of injection.
highest amplitude of the PD rise was 8 mV. The duration of the PD fall was always short (5-50 s) compared with the growth phase (250-280 s). Concurrently with these PD changes, short-term reversible rises in I P were recorded (Fig. 6) with a latent period of 13-8 (SD 0-4) s (n = 33). The largest rise in IP (2-0 kPa, 15 mmHg) was recorded after the 150-200 /tg phenylephrine dose. The duration of these IP rises was shorter than that of PD changes. The rates of PD increase (0-29 (SD 0-04) raV/s) and decrease (0-035 (SD 0-004) mV/s) and their ratio {K = 8-2, n = 32) were similar to values obtained with oxytocin. However, the pattern of change was quite different:
Transepithelial PD in goat mammary gland
475
(b) 007 • 005 •
003 ' (a) j*
+20 -
r
ence,
E
