Gap junctions as modulators of adrenal cortical cell proliferation and steroidogenesis.

Gap junctions are membrane specializations that are composed of connexin protein. The assembly of these proteins into channels between cells allows for the intercellular transfer of regulatory molecules. In the adrenal gland, as in most other tissues, intercellular communication provides the potential for regulation of a number of complex interactive cell processes including differentiation, steroidogenesis, migration, and proliferation. This review is concerned with the regulation of gap junctions and cell function in cortical cells of the adrenal gland and in pathological disorders such as adrenal cancer.

Follicle-stimulating hormone secretion in the female rat: cyclic release is dependent on circulating androgen.

In adult cyclic female rats, intravenous injections of an antiserum to testosterone prevented the continued increase in serum follicle-stimulating hormone (FSH) which normally occurs during the early morning hours of estrus. This treatment did not prevent the initial increases in serum FSH (and luteinizing hormone) which occur during the critical period (1400 to 2000 of proestrus), nor did it interfere with subsequent ovulation. These observations indicate the existence of two separate mechanisms for preovulatory FSH release in the rat and implicate circulating testosterone as the stimulus for continued secretion of FSH during estrus.

N-methyl-D,L-aspartate elicits hypothalamic gonadotropin-releasing hormone release in prepubertal male rhesus monkeys (Macaca mulatta).

In higher primates, the protracted delay from infancy to puberty results from an interruption in hypothalamic GnRH release. To determine whether the quiescent hypothalamic GnRH neurons of the prepubertal macaque are capable of discharging the decapeptide in response to a generalized neural depolarization, an excitatory amino acid analog, N-methyl-D,L-aspartate (NMA), was administered systemically to orchidectomized rhesus monkeys between 13 and 20 months of age. GnRH secretion was estimated indirectly by monitoring changes in circulating LH concentrations after the responsivity of pituitary gonadotropes to GnRH had been greatly facilitated by the chronic intermittent iv infusion of GnRH (0.1 microgram/min for 3 min every hour). The iv bolus administration of increasing doses of NMA (1.5, 4.8, and 15.0 mg/kg BW), 10-14 h after termination of the priming infusion of GnRH, elicited distinct discharges of LH, with magnitudes directly related to the amount of the excitant injected. Administration of a higher dose of NMA (48 mg/kg BW), however, failed to induce further LH release. The finding that pretreatment with a long-acting and potent GnRH receptor antagonist [( AcD2Nal1,4ClPhe2,DTrp3,DArg6,DAla10] GnRH-HOAc) abolished the LH-releasing activity of NMA provides compelling evidence for the view that the action of the neural excitant to induce gonadotropin release was exerted at a suprapituitary level. The additional observation that an N-methyl-D-aspartate receptor antagonist (D,L-2-amino-5-phosphono-valeric acid) blocked the NMA-induced release of GnRH suggests that the amino acid analog interacted with the N-methyl-D-aspartate receptor on neurons that synthesize and/or control the release of the hypothalamic hormone. Most interestingly, three sequential GnRH discharges, with a period and an amplitude apparently similar to those generated by the hypothalamus of the adult, were elicited from the brain of prepubertal monkeys by the sequential administration of three injections of NMA at hourly intervals. Taken together these findings demonstrate that the apparent dormancy of hypothalamic GnRH neurons, which is characteristic of prepubertal development in higher primates and underlies the protracted delay in the onset of puberty in these species, may be readily terminated by application of a generalized neural excitation. Plasma FSH, PRL, GH, and cortisol concentrations were also monitored during the course of some of these experiments, and release of each of these four hormones was observed after the iv injection of NMA (15 mg/kg BW).

Luteinizing hormone in the cat. I. Tonic secretion.

We have used a RIA system for measuring LH in the plasma of domestic cats and characterized the component of LH secretion which is controlled by negative feedback inhibition. Blood samples were collected at 6- to 10-min intervals from animals with chronically indwelling venous cannulae. The inhibitory influence (negative feedback) of gonadal secretion was evidenced by the increased plasma concentrations of LH seen 24 h after castration in 9 animals and within 5 days in all 15 cats. Restoration of negative feedback by either short or long term administration of 17 beta-estradiol reduced LH concentrations to precastration levels. In castrated animals of both sexes, the plasma concentrations of LH fluctuated episodically, with increases occurring at intervals of 20-30 min, presumably a reflection of intermittent periods of LH release. The dynamics of this pattern of LH release were simulated by the iv injection of gonadotropin-releasing hormone. In combination with our additional observation that plasma concentrations of LH decreased rapidly after treatment with four different anesthetic agents, the observations are suggestive of episodic secretion of gonadotropin-releasing hormone in the absence of negative feedback. The mechanisms regulating tonic LH secretion in this reflex ovulator appear to be more sensitive to neural stimuli but qualitatively similar to those previously described for other species in which ovulation occurs spontaneously.

The prepubertal hiatus in gonadotropin secretion in the male rhesus monkey (Macaca mulatta) does not appear to involve endogenous opioid peptide restraint of hypothalamic gonadotropin-releasing hormone release.

To examine the possibility that the prepubertal hiatus in gonadotropin secretion in primates is occasioned by an endogenous opioid peptide (EOP)-dependent suppression of pulsatile GnRH release, the ability of an EOP receptor antagonist, naloxone (NAL), to elicit GnRH release was examined indirectly in the rhesus monkey. For this purpose, six castrated male monkeys, aged 18-24 months, first received an intermitten iv infusion of GnRH (0.1 micrograms/min for 3 min every h) to enhance the responsiveness of the gonadotroph to endogenous GnRH. Acute and chronic blockade of EOP receptors with single bolus injections of NAL at three doses (0.2, 2.0, and 10 mg/kg BW) and a continuous infusion of the antagonist (2 mg/h for 36 h), respectively, failed to elicit significant increments in circulating concentrations of mean LH. In addition, changes in plasma LH concentrations during a chronic intermitten iv infusion of NAL (2 mg/kg BW every 6 h for up to 16 days) were unremarkable. Unequivocal discharges of LH, however, were observed in response to small doses of GnRH (0.3 micrograms/monkey) administered iv after all modes of NAL administration. Taken together, these findings fail to provide evidence for the view that in primates, EOPs underlie the hiatus in pulsatile GnRH release, which in these species is responsible for the quiescence of the pituitary-testicular axis during the greater part of prepubertal development.

A comparison of moment to moment and diurnal changes in circulating inhibin and testosterone concentrations in male rhesus monkeys (Macaca mulatta).

The secretion of inhibin by the testis was studied in the rhesus monkey, a species which exhibits marked episodic and diurnal patterns of testosterone (T) secretion. Inhibin and T were measured by RIA in blood samples drawn every 20 min for 24 h from 5 adult male monkeys. The molecular size of circulating inhibin, estimated by gel chromatography, was approximately 31 kDA. Plasma inhibin levels were undetectable in long term castrates. T was secreted episodically at a frequency of 6.0 +/- 0.9 pulses/24 h. The computer algorithm also identified 4.6 +/- 0.8 inhibin pulses/24 h. Of 30 T pulses among the 5 animals, however, only 7 coincided with low amplitude inhibin secretory bursts. Each animal demonstrated a significant diurnal periodicity in T secretion, with mean maximum concentrations at 0108 h (range, 2100-0640 h). By contrast, there was no significant diurnal rhythm for inhibin in any of the animals. The pulsatile administration of GnRH (0.1 micrograms/min, iv, for 3 min every 3 h) was used to activate the pituitary testicular axis in 6 juvenile monkeys. After 5 weeks of GnRH priming, a pulse of GnRH produced an immediate 4-fold rise in serum LH concentrations, followed within 30-50 min by a 5-fold increase in circulating T levels. FSH levels rose 50%. During the 3-h GnRH interpulse interval, however, there was no change in serum inhibin levels. Two GnRH-treated juvenile monkeys underwent bilateral orchidectomy. In each animal, circulating inhibin levels declined rapidly, with estimated first phase half-lives of 23 and 32 min, respectively. In conclusion, circulating inhibin concentrations in male rhesus monkeys exhibit neither the prominent moment to moment changes nor the circadian pattern characteristic of T secretion in this species. The relatively constant inhibin levels cannot be explained by prolonged metabolic clearance. The data are consistent with the proposal that most of the inhibin in the circulation is released across the apical surface of Sertoli cells into the seminiferous tubular fluid with passage into the rete testis from which it is continuously absorbed. The intermittent LH signal, by contrast, appears to make a minor contribution to the release of inhibin from the primate testis into the circulation.

Evidence that inhibin plays a major role in the regulation of follicle-stimulating hormone secretion in the fully adult male rhesus monkey (Macaca mulatta).

In the juvenile male rhesus monkey in which an adult-like pattern of endocrine activity in the pituitary-testicular axis is imposed by pulsatile stimulation with exogenous GnRH, administration of inhibin antiserum elicits a marked and selective hypersecretion of FSH. This finding suggests that in the monkey, testicular inhibin plays a major role in the postpubertal regulation of this gonadotropin. The purpose of the present study was to confirm this view more directly. To this end, 10 adult male rhesus monkeys were implanted with indwelling venous catheters and housed in specialized cages that permit continuous access to the venous circulation with minimal restraint and without tranquilization. Six of the males received a continuous infusion of an ovine antiserum to the alpha-subunit of human inhibin (iv bolus injection of 2.22 ml/kg BW, followed by a continuous infusion of serum at 0.62 ml/kg BW.24 h), and 4 animals received a similar infusion of control ovine immune serum. The duration of the infusion of the inhibin antiserum ranged from 2.5-7.5 days, and that for the control serum was 7.5 days. The FSH response to immunoneutralization of circulating inhibin was determined by measuring concentrations of this gonadotropin in sequential plasma samples collected between 1900-2300 h on the day before initiation of the anti-serum infusion and, depending on the duration of the infusion, on days 0.5, 1.5, 2.5, 4.5, and 6.5 of antiserum administration. In 5 of the 6 animals that received the inhibin antiserum, a progressive hypersecretion of FSH was observed during the initial 2.5 days of the infusion. This increase in circulating FSH concentration, which reached, by day 2.5 of treatment, a value 2- to 3-fold greater (P less than 0.05) than the pretreatment control level, was not associated with changes in either LH or testosterone levels. Continuation of the infusion of the inhibin antiserum beyond 2.5 days invariably resulted in a marked decline in LH and testosterone secretion, suggesting that the hypophysiotropic drive to the pituitary-testicular axis may have been compromised, presumably by a mechanism related to the infusion of heterologous serum. Infusion of the control immune serum for 2.5 days was not associated with an elevation of circulating FSH concentrations, and changes in circulating concentrations of plasma LH and testosterone were, as expected, unremarkable. Continuation of the infusion of control serum, like that of antiserum, generally resulted in a temporary but precipitous decline in LH and testosterone secretion.(ABSTRACT TRUNCATED AT 400 WORDS)

Gap junction proteins and cell-cell communication in the three functional zones of the adrenal gland.

Mouse and monkey adrenal glands were used to study the relationships between gap junction protein expression, intercellular communication and adrenal zonation. Dye communication patterns were determined by incubating freshly excised and hemisected adrenal glands in Lucifer yellow, a gap junction permeable fluorescent dye. Immunohistochemical techniques were used to localize adrenal gap junction proteins. The combination of these two techniques permitted the correlation of gap junction proteins with dye transfer and hormone responses in specialized regions of the adrenal cortex. Lucifer yellow dye communication was most pronounced in the inner glucocorticoid/androgen-producing regions (zona fasciculata/zona reticularis), but was virtually absent in the outer mainly mineralocorticoid-producing region (zona glomerulosa). This pattern of dye communication was coincident with immunohistochemical localization of the gap junction protein, alpha(1)Cx43. The variations in communication and alpha(1)Cx43 expression within the adrenal cortex are thought to be relevant to normal physiological regulation of the adrenal gland.

Serum concentrations of testosterone, oestrogens, luteinizing hormone and follicle-stimulating hormone in male and female rats during the critical period of neural sexual differentiation.

Untreated male and female rat pups were killed 1--5 days post partum and the serum concentrations of testosterone, oestrogens, LH and FSH were determined by radioimmunoassay. At all five sampling times, the serum concentrations of testosterone in male rats were about three times higher than those in female rats, but serum levels of oestrogens did not differ between the sexes. Serum concentrations of LH and FSH were lower in male than in female pups. In another study, rats were decapitated 1--10 days after birth and serum concentrations of testosterone were determined with a different radioimmunoassay. Again, at all four sampling times, the concentration of testosterone was significantly higher in the male than in the female pups.

Inhibitory action of porcine follicular fluid upon granulosa cell luteinization in vitro: assay and influence of follicular maturation.

Culture medium 199 supplemented with follicular fluid from 1-2 mm antral porcine follicles inhibited spontaneous luteinization of granulosa cells from preovulatory porcine follicles in vitro. Three characteristics of luteinization were inhibited: morphological transformation, progesterone secretion, and accumulation of cyclic AMP in response to LH. The last was inhibited more effectively by culture media containing 50% follicular fluid than by media containing 20% follicular fluid. The inhibitory actions of the follicular fluid were not altered by charcoal or petroleum ether extraction. Follicular fluid from large follicles (6-12 mm) did not exhibit any of these inhibitory actions. These observations may indicate the presence of a luteinization inhibitor in the fluid of small follicles which (1) is lost by the time the follicle reaches the preovulatory stage, or (2) is overcome by a stimulatory agent which may accumulate as the follicle grows.