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Effect of oxytocin on plasma testosterone levels in the male macaques (Macaca fascicularis )
CONTRACEPTION
EFFECT OF OXYTOCIN ON PLASMA TESTOSTERONE LEVELS IN THE MALE MACAQUES (Macaca fascicularis) G.J.S. Tan and T.K. Kwan+c Departmentsof Physiologyand +:Biochemistry, Facultyof Medicine,Universityof Malaya,59100 Kuala Lumpur,Malaysia ABSTRACT The effect of oxytocin on testicular function was examined in the adult male long-tailed macaques (Macaca fascicularis). The monkeys were either infused with increasing concentrations of synthetic oxytocin (16-128 m.i.u./min for 3 h) or injected daily for a week with the same hormone (20 i.u., i.v.) and the plasma testosterone levels measured. The results of the present study show that acute infusion or chronic injection of oxytocin does not significantly affect the plasma testosterone levels, suggesting that systemic control of testicular endocrine function by oxytocin may be unimportant.
Submitted for publication July 16, 1987 Accepted for publication September 9, 1987
SEPTEMBER 1987 VOL. 36 NO. 3
359
CONTRACEPTION
INTRODUCTION There is evidence to suggest that oxytocin may affect the process of steroidogenesis [l]. Production of testosterone by cultured rat testicular cells has been shown to be suppressed by oxytocin [z]. However, the physiological role of oxytocin in the control of male testicular function is still uncertain as infusion of this hormone in normal men does not appear to alter the plasma testosterone levels [3]. effect tailed
The present study was thus undertaken to of oxytocin administration in the adult macaques (Macaca fascicularis).
examine the male long-
MATERIALS AND METHODS The effect of oxytocin on testicular function was evaluated by examining the action of an acute infusion or chronic injection of oxytocin on the plasma testosterone levels. Acute
infusion
of
oxytocin
Four adult male monkeys were infused with increasing concentrations of synthetic oxytocin (Syntocinon from Sandoz, Australia Pty Ltd, Sydney, Australia) in saline for 3 h. The doses were 16, 32 and 64 m.i.u./min, respectively, for a period of 30 nin each and 128 m.i.u./min for the remaining 90 min. The infusion rate was 1 ml/h. Blood samples were collected from peripheral veins by venipuncture The controls for immediately before and after the infusion. the experiment consisted of 2 monkeys receiving only saline solution. Chronic
injection
of
oxytocin
Five adult male monkeys were injected daily with 20 i.u. oxytocin intravenously for a week. Blood samples were collected immediately after oxytocin injection on day 1, 4 The controls for the experiand 7 of the injection period. ment consisted of 3 monkeys receiving only saline solution. Blood samples were also taken from the monkeys several weeks before oxytocin administration to establish the normal fluctuations of their plasma testosterone levels. Weekly blood samplings were continued thereafter to see if there was any long-term effect of oxytocin administration on the The plasma samples were then testicular endocrine function. stored at -2O’C before assay. Levels of testosterone were measured in duplicate using a radioimmunoassay kit obtained
SEPTEMBER
1987 VOL. 36 NO. 3
CONTRACEPTION
from Diagnostic Products Corporation, Los Angeles, CaliforInter- and intra-assay coefficients of variation nia, U.S.A. were 9.6 (n=ZO) and 6.6% (n=20), respectively. Statistical analysis of the results was done by student's t-test and analysis of variance (ANOVA). P values greater than 0.01 were not considered significant. RESULTS Acute infusion of oxytocin Although the plasma testosterone levels at the end of oxytocin infusion period were suppressed by 41.1 + 26.4% (+SD)(n=3), they were not significantly different from the controls (t-test) which were suppressed by 51.4% (the mean value from 2 monkeys; the individual value of which did not vary by more than 4%) (Table I). Table I. Effect of oxytocin infusion on plasma testosterone levels in the male Macaca fascicularis
Plasma testosterone Treatment -
Beginning of infusion
levels (ng/nl) End of infusion
Oxytocin" Monkey A Monkey B Monkey C
4.5 7.4 6.0
3.4
13.0 24.0
6.6 11.2
Saline Monkey a Monkey b
y: Infused at increasing concentrations (16 m.i.u./min to 128 m.i.u./min) for 3 h. Monkey D was omitted as its testicular volume was only about a third that of monkey A, B or C. Its plasma testosterone levels were also significantly lower (see Figure 1). The weekly fluctuations in plasma testosterone levels of the monkeys were also not affected by oxytocin infusion (by ANOVA) (Figures 1 and 2).
SEPTEMBER
1987 VOL. 36 NO. 3
361
CONTRACEPTION
0 z” 30-
,,,
2
4
61
x
8I
I
10,
I
12,
0 az
D
20-
IO-
0
2
4
8
8
10
12
WEEKS
Figure 1. Weekly fluctuations in plasma testosterone levels before and after oxytocin (OT) infusion in 4 monkeys (A, B C and D).
362
SEPTEMBER
1987 VOL. 36 NO. 3
CONTRACEPTION
20
S k! 0 a 30 Ill L 0 ZJ w +
20
ia
c 0
c 2
4
I
6
8
10
13
WEEKS
Figure 2. Weekly fluctuations in plasma testosterone levels before and after saline (S) infusion in 2 monkeys (a and b).
SEPTEMBER
1987 VOL. 36 NO. 3
363
CONTRACEPTION
Chronic injection of oxytocin Plasma testosterone levels are shown in Figures 3 and 4. The weekly fluctuations in plasma testosterone levels did not seem to be affected by oxytocin administration. The fluctuations in plasma testosterone levels during the period of oxytocin administration were not significantly different from the levels before treatment; neither were the levels affected after oxytocin administration (by ANOVA). Injection of saline also had no significant effect on the plasma testosterone levels (by ANOVA).
30 20 10 0
30 20 10 0
30 20 10
0 30
1
2
i
2
3
OT '
5
6
7
5.
6.
7
20 10
0 30 20 10
j
i
WEEKS
Figure 3. Weekly fluctuations in plasma testosterone levels before, during and after daily injection of oxytocin (OT) in 5 monkeys.
364
SEPTEMBER
1987 VOL. 36 NO. 3
CONTRACEPTION
0’. 0
1
1
I,
I
2
I,
I
3
4
I,
I
5
I
6
I,
7
WEEKS Figure 4. Weekly fluctuations in plasma testosterone levels before, during and after daily injection of saline (S) in 3 monkeys.
DISCUSSION The testosterone levels measured in our monkeys were within the range reported by Steiner and co-workers [4] on a similar species of monkey. While oxytocin has been shown to suppress the in vitro production of steroid hormones by testicular cellsTZJ,asma testosterone levels were found not to be affected by either infusion or chronic injection of oxytocin. The infusion dosages were similar to that used by Dawood and co-workers [5] where plasma oxytocin levels were found to be significantly elevated by this dosage regimen. Although the levels of oxytocin were not measured, the amount of oxytocin injected (20 i.u.) far exceeded that present in the plasma [6]. The absence of effect of oxytocin may be due to either an increase in gonadotrophic secretion to counteract the action of oxytocin or insufficient uptake by the testis as, in vitro, high levels are needed for oxytocin effect [z].
SEPTEMBER 1987 VOL. 36 NO. 3
365
CONTRACEPTION
Studies by Dawood and co-workers [5] show that elevated levels of oxytocin have no effect on gonadotrophin secretion in man; it is more likely that insufficient uptake nay explain the finding of the present study. Similar findings were reported by Chiodera and co-workers [3] where oxytocin infusion in man also did not affect plasma testosterone levels. In the rat, while oxytocin suppressed progesterone production by the corpus luteum, systemic effect of oxytocin was also unobserved [7]. The present study thus suggests that systemic control of testicular testosterone production by oxytocin may be unimportant physiologically. The presence of oxytocin in rat testis [S] and its suppression of testosterone production [2] in vitro indicate that any moduiation of testicular steroidogenxby oxytocin probably occurs locally.
ACKNOWLEDGEMENT This work was supported by research grants No.F108/84 and No.F234/85 from the University of Malaya.
REFERENCES 1.
Oxytocin Tan, G.J.S., Tweedale, R. and Biggs, J.S.G. may play a role in the control of the human corpus luteum. J Endocrinol 95, 65-70 (1982)
2.
Adashi, E.Y. and Hsueh, A.J.W. Direct inhibition of testicular androgen biosynthesis revealing antigonadal activity of neurohypophyseal hormones. Nature 293, 650652 (1981)
3.
Chiodera, P., Rossi! G., Camellini, L., Pignatti, D., Maffei, M.L., Crottl, G. and Coiro, V. Oxytocin infusion does not alter testosterone levels in normal men. IRCS Med Sci 12, 28 (1984)
4.
Steiner, R.A., Peterson, A.P., Yu, J.Y.L., Conner, H., Gilbert, M. Tel- Penning, B. and Bremner, W.J. Ultradian luteinizing hormone and testosterone rhythms in the adult male nonkey, Macaca fascicularis. -Endocrinology 107, 1489-1493 (198oJ
5.
Dawood, M.Y., Ylikorkala, O., Trivedi, D. and Gupta, I!. Oxytocin levels and disappearance rate and plasma follicle--stimulating hormone and luteinizing hormone after oxytocin infusion in men. J Clin Endocrinol Metab 50, 397-400 (1980)
SEPTEMBER
1987 VOL. 36 NO. 3
CONTRACEPTION
6.
Falconer, J., Mitchell, Robinson, J.S. Plasma the menstrual cycle in mulatta. J Reprod Fert
7.
Effect of oxytocin on the rat corpus Tan, G.J.S. luteum function. Med Sci Res 15, 323 (1987)
8.
ImmunocytochemGuldenaar, S.E.F. and Pickering, B.T. ical evidence for the presence of oxytocin in rat testis. Cell Tiss Res 240, 485-487 (1985)
SEPTEMBER
M.D., Mountford, L.A. and oxytocin concentrations during the rhesus monkey, Macaca 59, 69-72 (1980)
1987 VOL. 36 NO. 3
367
EFFECT OF OXYTOCIN ON PLASMA TESTOSTERONE LEVELS IN THE MALE MACAQUES (Macaca fascicularis) G.J.S. Tan and T.K. Kwan+c Departmentsof Physiologyand +:Biochemistry, Facultyof Medicine,Universityof Malaya,59100 Kuala Lumpur,Malaysia ABSTRACT The effect of oxytocin on testicular function was examined in the adult male long-tailed macaques (Macaca fascicularis). The monkeys were either infused with increasing concentrations of synthetic oxytocin (16-128 m.i.u./min for 3 h) or injected daily for a week with the same hormone (20 i.u., i.v.) and the plasma testosterone levels measured. The results of the present study show that acute infusion or chronic injection of oxytocin does not significantly affect the plasma testosterone levels, suggesting that systemic control of testicular endocrine function by oxytocin may be unimportant.
Submitted for publication July 16, 1987 Accepted for publication September 9, 1987
SEPTEMBER 1987 VOL. 36 NO. 3
359
CONTRACEPTION
INTRODUCTION There is evidence to suggest that oxytocin may affect the process of steroidogenesis [l]. Production of testosterone by cultured rat testicular cells has been shown to be suppressed by oxytocin [z]. However, the physiological role of oxytocin in the control of male testicular function is still uncertain as infusion of this hormone in normal men does not appear to alter the plasma testosterone levels [3]. effect tailed
The present study was thus undertaken to of oxytocin administration in the adult macaques (Macaca fascicularis).
examine the male long-
MATERIALS AND METHODS The effect of oxytocin on testicular function was evaluated by examining the action of an acute infusion or chronic injection of oxytocin on the plasma testosterone levels. Acute
infusion
of
oxytocin
Four adult male monkeys were infused with increasing concentrations of synthetic oxytocin (Syntocinon from Sandoz, Australia Pty Ltd, Sydney, Australia) in saline for 3 h. The doses were 16, 32 and 64 m.i.u./min, respectively, for a period of 30 nin each and 128 m.i.u./min for the remaining 90 min. The infusion rate was 1 ml/h. Blood samples were collected from peripheral veins by venipuncture The controls for immediately before and after the infusion. the experiment consisted of 2 monkeys receiving only saline solution. Chronic
injection
of
oxytocin
Five adult male monkeys were injected daily with 20 i.u. oxytocin intravenously for a week. Blood samples were collected immediately after oxytocin injection on day 1, 4 The controls for the experiand 7 of the injection period. ment consisted of 3 monkeys receiving only saline solution. Blood samples were also taken from the monkeys several weeks before oxytocin administration to establish the normal fluctuations of their plasma testosterone levels. Weekly blood samplings were continued thereafter to see if there was any long-term effect of oxytocin administration on the The plasma samples were then testicular endocrine function. stored at -2O’C before assay. Levels of testosterone were measured in duplicate using a radioimmunoassay kit obtained
SEPTEMBER
1987 VOL. 36 NO. 3
CONTRACEPTION
from Diagnostic Products Corporation, Los Angeles, CaliforInter- and intra-assay coefficients of variation nia, U.S.A. were 9.6 (n=ZO) and 6.6% (n=20), respectively. Statistical analysis of the results was done by student's t-test and analysis of variance (ANOVA). P values greater than 0.01 were not considered significant. RESULTS Acute infusion of oxytocin Although the plasma testosterone levels at the end of oxytocin infusion period were suppressed by 41.1 + 26.4% (+SD)(n=3), they were not significantly different from the controls (t-test) which were suppressed by 51.4% (the mean value from 2 monkeys; the individual value of which did not vary by more than 4%) (Table I). Table I. Effect of oxytocin infusion on plasma testosterone levels in the male Macaca fascicularis
Plasma testosterone Treatment -
Beginning of infusion
levels (ng/nl) End of infusion
Oxytocin" Monkey A Monkey B Monkey C
4.5 7.4 6.0
3.4
13.0 24.0
6.6 11.2
Saline Monkey a Monkey b
y: Infused at increasing concentrations (16 m.i.u./min to 128 m.i.u./min) for 3 h. Monkey D was omitted as its testicular volume was only about a third that of monkey A, B or C. Its plasma testosterone levels were also significantly lower (see Figure 1). The weekly fluctuations in plasma testosterone levels of the monkeys were also not affected by oxytocin infusion (by ANOVA) (Figures 1 and 2).
SEPTEMBER
1987 VOL. 36 NO. 3
361
CONTRACEPTION
0 z” 30-
,,,
2
4
61
x
8I
I
10,
I
12,
0 az
D
20-
IO-
0
2
4
8
8
10
12
WEEKS
Figure 1. Weekly fluctuations in plasma testosterone levels before and after oxytocin (OT) infusion in 4 monkeys (A, B C and D).
362
SEPTEMBER
1987 VOL. 36 NO. 3
CONTRACEPTION
20
S k! 0 a 30 Ill L 0 ZJ w +
20
ia
c 0
c 2
4
I
6
8
10
13
WEEKS
Figure 2. Weekly fluctuations in plasma testosterone levels before and after saline (S) infusion in 2 monkeys (a and b).
SEPTEMBER
1987 VOL. 36 NO. 3
363
CONTRACEPTION
Chronic injection of oxytocin Plasma testosterone levels are shown in Figures 3 and 4. The weekly fluctuations in plasma testosterone levels did not seem to be affected by oxytocin administration. The fluctuations in plasma testosterone levels during the period of oxytocin administration were not significantly different from the levels before treatment; neither were the levels affected after oxytocin administration (by ANOVA). Injection of saline also had no significant effect on the plasma testosterone levels (by ANOVA).
30 20 10 0
30 20 10 0
30 20 10
0 30
1
2
i
2
3
OT '
5
6
7
5.
6.
7
20 10
0 30 20 10
j
i
WEEKS
Figure 3. Weekly fluctuations in plasma testosterone levels before, during and after daily injection of oxytocin (OT) in 5 monkeys.
364
SEPTEMBER
1987 VOL. 36 NO. 3
CONTRACEPTION
0’. 0
1
1
I,
I
2
I,
I
3
4
I,
I
5
I
6
I,
7
WEEKS Figure 4. Weekly fluctuations in plasma testosterone levels before, during and after daily injection of saline (S) in 3 monkeys.
DISCUSSION The testosterone levels measured in our monkeys were within the range reported by Steiner and co-workers [4] on a similar species of monkey. While oxytocin has been shown to suppress the in vitro production of steroid hormones by testicular cellsTZJ,asma testosterone levels were found not to be affected by either infusion or chronic injection of oxytocin. The infusion dosages were similar to that used by Dawood and co-workers [5] where plasma oxytocin levels were found to be significantly elevated by this dosage regimen. Although the levels of oxytocin were not measured, the amount of oxytocin injected (20 i.u.) far exceeded that present in the plasma [6]. The absence of effect of oxytocin may be due to either an increase in gonadotrophic secretion to counteract the action of oxytocin or insufficient uptake by the testis as, in vitro, high levels are needed for oxytocin effect [z].
SEPTEMBER 1987 VOL. 36 NO. 3
365
CONTRACEPTION
Studies by Dawood and co-workers [5] show that elevated levels of oxytocin have no effect on gonadotrophin secretion in man; it is more likely that insufficient uptake nay explain the finding of the present study. Similar findings were reported by Chiodera and co-workers [3] where oxytocin infusion in man also did not affect plasma testosterone levels. In the rat, while oxytocin suppressed progesterone production by the corpus luteum, systemic effect of oxytocin was also unobserved [7]. The present study thus suggests that systemic control of testicular testosterone production by oxytocin may be unimportant physiologically. The presence of oxytocin in rat testis [S] and its suppression of testosterone production [2] in vitro indicate that any moduiation of testicular steroidogenxby oxytocin probably occurs locally.
ACKNOWLEDGEMENT This work was supported by research grants No.F108/84 and No.F234/85 from the University of Malaya.
REFERENCES 1.
Oxytocin Tan, G.J.S., Tweedale, R. and Biggs, J.S.G. may play a role in the control of the human corpus luteum. J Endocrinol 95, 65-70 (1982)
2.
Adashi, E.Y. and Hsueh, A.J.W. Direct inhibition of testicular androgen biosynthesis revealing antigonadal activity of neurohypophyseal hormones. Nature 293, 650652 (1981)
3.
Chiodera, P., Rossi! G., Camellini, L., Pignatti, D., Maffei, M.L., Crottl, G. and Coiro, V. Oxytocin infusion does not alter testosterone levels in normal men. IRCS Med Sci 12, 28 (1984)
4.
Steiner, R.A., Peterson, A.P., Yu, J.Y.L., Conner, H., Gilbert, M. Tel- Penning, B. and Bremner, W.J. Ultradian luteinizing hormone and testosterone rhythms in the adult male nonkey, Macaca fascicularis. -Endocrinology 107, 1489-1493 (198oJ
5.
Dawood, M.Y., Ylikorkala, O., Trivedi, D. and Gupta, I!. Oxytocin levels and disappearance rate and plasma follicle--stimulating hormone and luteinizing hormone after oxytocin infusion in men. J Clin Endocrinol Metab 50, 397-400 (1980)
SEPTEMBER
1987 VOL. 36 NO. 3
CONTRACEPTION
6.
Falconer, J., Mitchell, Robinson, J.S. Plasma the menstrual cycle in mulatta. J Reprod Fert
7.
Effect of oxytocin on the rat corpus Tan, G.J.S. luteum function. Med Sci Res 15, 323 (1987)
8.
ImmunocytochemGuldenaar, S.E.F. and Pickering, B.T. ical evidence for the presence of oxytocin in rat testis. Cell Tiss Res 240, 485-487 (1985)
SEPTEMBER
M.D., Mountford, L.A. and oxytocin concentrations during the rhesus monkey, Macaca 59, 69-72 (1980)
1987 VOL. 36 NO. 3
367