Single exposure to testosterone in adulthood rapidly induces regularity in the growth hormone release process.

The neonatal gonadal steroid milieu is known to be important in imprinting the striking sexual dimorphism of growth hormone (GH) secretion; however, the influence of the sex steroids on GH control in adult life and their mechanism/site of action are largely unknown. In the present study, we tested the hypothesis that testosterone (T) subserves the gender-specific regularity of the GH release process in adulthood. The approximate entropy statistic (ApEn) was used to quantify the degree of regularity of GH release patterns over time. Eighteen hours after a single subcutaneous injection of 1 mg T, both sham-operated and ovariectomized (OVX) female adult rats displayed plasma GH profiles that were strikingly similar to the regular male-like ultradian rhythm of GH secretion. The highest ApEn values, denoting greater disorderliness of GH secretion, were observed in the ovary-intact group, and T injection significantly (P < 0.001) reduced this irregularity whether or not the ovaries were present. Serial intravenous injections of GH-releasing hormone (GHRH) caused a similar increase in plasma GH levels in sham-operated females independently of time of administration. In contrast, female rats administered T exhibited a male-like intermittent pattern of GH responsiveness to GHRH, the latter known to be due to the cyclic release of endogenous somatostatin. These results demonstrate that acute exposure to T during adult life can rapidly and profoundly "masculinize" GH pulse-generating circuits in the female rat. Our findings suggest that the enhanced orderliness characteristic of the GH release process in males, compared with females, is regulated by T. We postulate that this T-induced regularity is mediated at the level of the hypothalamus by inducing regularity in somatostatin secretion, which in turn governs overall GH periodicity.

Cellular distribution of G protein Go alpha in pituitary lactotrophs: effects of dopamine.

Membrane-bound GTP-binding (G) proteins mediate signal transduction in a variety of cell systems. The exact mechanisms of G proteins action are still under investigation but they appear to involve effectors located in the plasma membrane as well as in other parts of the cell. With this study, we investigated the cellular and ultrastructural localization of G protein subunits, and particularly of Go alpha, in normal rat anterior pituitaries and in estrone-induced rat adenomatous lactotrophs. We also evaluated the effects of Go alpha cellular redistribution in rat adenomatous lactotrophs following short-term exposure to dopamine (DA). Using the Protein A-gold (PAG) methodology, Go alpha was found to be present in the cysternae of the endoplasmic reticulum of normal pituitary cells and of adenomatous lactotrophs. In the latter, Go alpha could be co-localized with prolactin (PRL). By immunoblots, using specific antisera, significant amounts of Go alpha and Gs42 alpha, together with smaller amounts of Gi alpha, Gs47 alpha and G beta were found to be present in the uncontaminated supernatant fraction of adenomatous lactotrophs. Unexpectedly, exposure of the cells to DA induced a rapid and short-lived decrease in the cytosolic fraction of Go alpha and G beta associated with a decrease of PRL release. Since cytosolic Go alpha can be ADP-ribosylated by pertussis toxin (PT) and is therefore in a heterotrimeric form, our data suggest that the soluble Go protein may play a role during lactotrophs' exposure to an inhibitor of PRL release, perhaps through its relocalization after being internalized with the D2 receptor or by being used for interaction with intracellular and/or membrane-bound effectors.

MMQ cells: a model for evaluating the role of G proteins in the modulation of prolactin release.

It is well known that dopamine (DA) inhibits while vasoactive intestinal peptide (VIP) and angiotensin II (ANG II) stimulate prolactin (PRL) release from normal anterior pituitary lactotrophs; however, elucidation of the intracellular mechanisms involved in these effects has been hindered by the cellular heterogeneity of the anterior pituitary. MMQ cells, isolated from the PRL-secreting rat pituitary tumor 7315a is an interesting model since they only secrete PRL. In order to determine whether and which GTP-binding (G) proteins are involved in the modulation of cyclic 3',5'-adenosine monophosphate (cAMP) accumulation and phospholipids turnover and eventually PRL release, we have performed studies with MMQ cells. For this purpose, the levels of various G proteins (alpha o, alpha s, alpha i, alpha q and beta) and their mRNAs, measured by Western and Northern blots respectively, were correlated with intracellular cAMP accumulation in response to DA, VIP or DA plus VIP, and with inositol phosphates (IPx) formation in response to ANG II, DA or DA plus ANG II. This study shows that, when compared to normal pituitary tissue, the levels of alpha o, alpha o2 and alpha i3 were significantly decreased in MMQ cells; those of alpha o1, alpha i (alpha i1 + alpha i2), alpha s42 and alpha q were very low or undetectable while those of alpha s47 and beta were normal. DA was unable to inhibit basal PRL release and cAMP accumulation. VIP increased both cAMP accumulation and PRL release, while cAMP accumulation elicited by VIP could be suppressed by DA. BAY K 8644-induced PRL release also could be suppressed by DA.(ABSTRACT TRUNCATED AT 250 WORDS)

Short-term adult exposure to estradiol feminizes the male pattern of spontaneous and growth hormone-releasing factor-stimulated growth hormone secretion in the rat.

Experimental evidence indicates that the neonatal gonadal steroid environment is an important determinant of the sexual dimorphism of GH secretion and body growth. However, the influence of the sex steroids in GH control during adult life and their mechanism/site of action are largely unknown. In the present study we examined the effects of adult gonadectomy (GNX) and short term adult exposure to 17 beta-estradiol (E2) on both spontaneous and GRF-stimulated GH release in free-moving adult male rats. The rate of body weight gain was also monitored. GNX (3 weeks postoperatively) resulted in a 2-fold reduction in GH pulse amplitude compared to that in sham-operated control rats, but did not significantly alter the GH nadir or the interpeak interval. Exposure to E2 (sc implants) for 4 days markedly disrupted the spontaneous GH secretory profile of both sham-operated and GNX rats; E2-treated animals exhibited a striking elevation (4- to 20-fold) of GH trough levels and a significant decrease in GH interpeak interval, remarkably similar to the typical female rat GH secretory profile. The augmentation in both GH nadir and GH pulse frequency was evident as early as 12 h after a single sc injection of E2 valerate. In sham and GNX rats bearing control implants, the GH response to 1 micrograms rat GRF-(1-29)NH2, iv, was significantly greater when GRF was administered at peak (1100 h) than at trough (1300 h) times of GH secretion; the latter is known to be due to antagonism by the cyclical increased release of endogenous somatostatin (SRIF) in the male rat. Treatment with E2 abolished this time-dependent difference in both groups and produced a regular pattern of GH responsiveness to GRF similar to that typically observed in the female rat, thus suggesting that E2 has altered the pattern of hypothalamic SRIF secretion from a cyclic to a more continuous mode of release. Chronic exposure to E2 for 2 weeks resulted in an almost 6-fold inhibition of the rate of body weight gain in sham-operated male rats to levels comparable to those in normal adult female rats. Taken together, these results demonstrate that short term exposure to E2 during adult life can profoundly feminize the male pattern of spontaneous and GRF-stimulated GH secretion, as well as rate of somatic growth.(ABSTRACT TRUNCATED AT 400 WORDS)

Sexual dimorphism of somatostatin and growth hormone-releasing factor signaling in the control of pulsatile growth hormone secretion in the rat.

A striking sexual dimorphism exists in the pattern of GH secretion and rate of somatic growth; however, the mechanism(s) mediating this sex difference is unknown. To elucidate the physiological roles of the hypothalamic neuropeptides, somatostatin (SRIF) and GRF, and their interrelation, in generating the sexually dimorphic GH secretory pattern we examined: 1) GH responsiveness to exogenous GRF and 2) the effects of immunoneutralization of endogenous SRIF and GRF on GH secretory dynamics, in free-moving male and female rats. In males, the GH response to 1 microgram rat(r)GRF(1-29)NH2 iv was significantly greater at peak compared to trough times of GH secretion (925.2 +/- 250.8 vs. 95.6 +/- 27.8 ng/ml; P less than 0.02), the latter known to be due to antagonization by the cyclic increased release of endogenous SRIF. In contrast, females failed to exhibit a time-dependent difference in GH responsiveness to GRF. Passive immunization with a specific antiserum to SRIF in males resulted in significant elevation of GH nadir levels but had no effect on GH peak amplitude. In contrast, immunoneutralization of endogenous SRIF in females caused a marked augmentation of plasma GH levels at all time points; there was a significant increase in GH peak amplitude (171.3 +/- 39.9 vs. 67.5 +/- 11.3 ng/ml; P less than 0.05), GH nadir (18.3 +/- 2.7 vs. 5.8 +/- 1.1 ng/ml; P less than 0.01) and mean 6-h plasma GH level (78.7 +/- 4.1 vs. 33.1 +/- 5.8 ng/ml; P less than 0.001), compared to normal sheep serum-treated controls. These results indicate that the pattern of hypothalamic SRIF secretion in females does not follow the male-like ultradian rhythm. Passive immunization with a specific antiserum to GRF obliterated spontaneous GH pulses in both sexes. Moreover, in females, anti-GRF serum attenuated GH nadir levels (4.3 +/- 1.7 vs. 21.4 +/- 3.5 ng/ml; P less than 0.01) indicating a physiological role for GRF in maintaining the elevated basal GH level of females, in addition to its important role in generating the episodic GH pulses. Taken together, these findings provide support for the hypothesis that, in female rats, the pattern of hypothalamic SRIF secretion into hypophyseal portal blood is continuous, rather than cyclical, as in the male; whereas in the case of GRF secretion, in addition to steady-state release which occurs at a higher level in females than males, there is also episodic GRF bursting which does not follow a specific rhythm, as in the male.(ABSTRACT TRUNCATED AT 400 WORDS)

Paradoxical enhancement of pituitary growth hormone (GH) responsiveness to GH-releasing factor in the face of high somatostatin tone.

Pituitary GH secretion is regulated by a delicate interplay between stimulatory (GRF) and inhibitory [somatostatin (SRIF)] hypothalamic hormones, although the nature of the GRF/SRIF interaction remains to be elucidated. In the present study, we documented a significant elevation of plasma SRIF-like immunoreactivity in 72-h fasted rats compared to that in fed controls (129.0 +/- 17.9 vs. 38.2 +/- 5.8 pg/ml; P less than 0.01) and used this model of high SRIF tone to further delineate the interrelation between GRF and SRIF in physiological regulation of pulsatile GH secretion. We examined pituitary GH responsiveness to GRF, both in vivo and in vitro, after 72-h exposure to nutritional deprivation and high SRIF secretion. In vivo, GRF-induced GH release was markedly enhanced in the face of high circulating SRIF; freely moving, starved rats released 4- to 8-fold more GH than fed controls in response to rat GRF iv. In vitro, both basal and human GRF-induced GH release were augmented 2- to 4-fold in perifused dispersed anterior pituitary cells of starved rats compared to those in fed controls, and this enhanced responsiveness persisted in the presence of 10(-9) M SRIF. These results demonstrate that SRIF not only inhibits GH secretion stimulated by GRF, but that under different temporal conditions SRIF may act in a paradoxically positive manner to sensitize pituitary GH responsiveness to GRF. Such a cooperative interaction of the two peptides may be necessary to optimize pulsatile GH release. Our findings provide support for the hypothesis that the temporal patterning of hypothalamic GRF/SRIF signals to pituitary somatotrophs may be the major determinant for pulsatile GH secretion and, ultimately, body growth.

Effects of intracellular glucopenia on pulsatile growth hormone secretion: mediation in part by somatostatin.

We examined the effects of 2-deoxy-D-glucose (2DG)-induced intracellular glucoprivation on GH, insulin, and glucose secretory dynamics in freely moving rats bearing chronic intracerebroventricular and intracardiac venous cannulae. Intravenous administration of 2DG (400 mg/kg) caused a severe suppression in amplitude and duration of spontaneous GH surges; plasma GH levels remained significantly depressed for at least 5 h in the presence of marked hyperglycemia. Plasma insulin levels were unchanged. Central administration of a low dose of 2DG (8 mg/10 microliters) also markedly attenuated GH pulse amplitude and raised plasma glucose levels, but this low dose was without effect when injected peripherally, suggesting a central site of action. To elucidate the mechanism(s) mediating the GH suppression response to insufficient glucose we assessed the involvement of the two hypothalamic GH-regulatory peptides, somatostatin (SRIF) and GH-releasing factor (GRF). Passive immunization of 2DG-treated rats with a specific SRIF antiserum (AS) caused an initial surge of GH release and significant elevation of both trough and mean 6-h plasma GH levels, compared to 2DG-normal sheep serum controls. However, SRIF AS failed to restore the amplitude of GH pulses to normal levels. Administration of three iv boluses of human GRF (10 micrograms), at 90-min intervals, to 2DG-treated rats resulted in GH release which was variable and time dependent; the magnitude of the first response (1 h after 2DG injection) was significantly less than that of the other two. Immunoneutralization with SRIF AS eliminated this difference and significantly enhanced human GRF-induced GH release. These results demonstrate that intracellular glucopenia is a potent inhibitor of pulsatile GH secretion in the rat and that this response is mediated, in part, by an increase in SRIF release. While the present findings are compatible with the hypothesis that glucoprivation induces an acute SRIF release, only a partial role for SRIF is indicated in this response. The longer lasting suppression of GH pulses observed after glucose deprivation may be due to decreased output of GRF from the hypothalamus.