Effect of Ca2+- and K+-channel blockers on ACTH-stimulated steroidogenesis of isolated adrenal glands of adult and old rats

Ca2+ channel blocker verapamil (0.2 mM) and K+ channel blocker 2-aminopyridine (1.0 mM; 5.0 mM and 10.0 mM) have been studied for their effect on ACTH-stimulated steroidogenesis in isolated adrenal glands (IAG) of the adult (6-7 months) and old (26-28 months) male rats. The administration of verapamil produced a similar, about 2-fold decrease in ACTH-stimulated secretion of 11-OCS in the IAG of both animal groups. 2-aminopyridine, administered in several concentrations, produced no effect whatsoever on ACTH-stimulated secretion of 11-OCS in the IAG of the old rats. However, when administered in concentrations of 5.0 mM and 10.0 mM this agent decreased significantly the ACTH-stimulated steroidogenesis in the IAG of the adult animals. It has been suggested that plasma membrane permeability of z. fasciculata adrenocorticocytes to K+ ions decreases with age.     

Effects of β-adrenergic receptor agonists on drinking and arterial blood pressure in young and old rats

These experiments examined water-drinking and arterial blood pressure responses to β-adrenergic receptor activation in young (4 mo), "middle-aged" adult (12 mo), and old (29 mo) male rats of the Brown-Norway strain. We used isoproterenol to simultaneously activate β(1)- and β(2)-adrenergic receptors, salbutamol to selectively activate β(2)-adrenergic receptors, and the combination of isoproterenol and the β(2)-adrenergic receptor antagonist ICI 118,551 to stimulate only β(1)-adrenergic receptors. Animals received one of the drug treatments, and water drinking was measured for 90 min. About 1 wk later, animals received the same drug treatment for measurement of arterial blood pressure responses for 90 min. In some rats, levels of renin and aldosterone secretion in response to isoproterenol or salbutamol were measured in additional tests. Old and middle-aged rats drank significantly less after isoproterenol than did young rats and also had greater reductions in arterial blood pressure. Old and middle-aged rats drank significantly less after salbutamol than did young rats, although reductions in arterial blood pressure were equivalent across the ages. The β(2)-adrenergic antagonist ICI 118,551 abolished drinking after isoproterenol and prevented most of the observed hypotension. Renin secretion after isoproterenol and salbutamol was greater in young rats than in middle-aged rats, and wholly absent in old rats. Aldosterone secretion was reduced in old rats compared with young and middle-aged rats after treatment with isoproterenol, but not after treatment with salbutamol. In conclusion, there are age-related differences in β-adrenergic receptor-mediated drinking that can be explained only in part by age-related differences in renin secretion after β-adrenergic receptor stimulation.     

Differences in the accessibility of the beta-adrenergic receptor in isolated hepatocytes from foetal and adult rats.

Beta-receptor number (measured by [3H]-CGP 12 177 binding) and beta-adrenergic response (measured by isoproterenol stimulated glucose liberation and isoproterenol stimulated adenylate cyclase activity) were compared in hepatocytes isolated from foetal (on day 22 of gestation), adult female and adult male rats. Beta-receptor numbers in crude membrane preparations of hepatocytes from adult female and adult male rats were found to be nearly equal (15.5 and 15.1 fmol/mg), but in crude membrane preparations of foetal rats beta-adrenergic receptor number was significantly higher (34.3 fmol/mg). Determination of number of beta-adrenergic surface receptors of intact hepatocytes showed relative high values in foetal rats (about 22,000/cell) and adult female rats (about 20,000/cell), but in male rats the number was less (about 6500/cell). Glucose liberation was stimulated by isoproterenol to the same extent in hepatocytes isolated from adult female and foetal rats (about 150% over basal), whereas no effect was found in hepatocytes isolated from adult male rats. Dose-response curves showed that in foetal rat hepatocytes glucose release was already increased by 10(-8) M isoproterenol, whereas in female rat hepatocytes at least 10(-6) M isoproterenol was required. Adenylate cyclase was stimulated by isoproterenol in lysates of hepatocytes from adult female rats by about 180% and from foetal rats by about 250%. No effects were observed using lysates of hepatocytes from adult male rats. We interpret the observed differences of beta-adrenergic responses between adult female and male rats as being primarily caused by different accessibility of the beta-receptor to the beta-agonist isoproterenol in intact hepatocytes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Developmental alterations in guanine nucleotide regulation of the beta-adrenergic receptor-adenylate cyclase system of skeletal muscle

The postnatal development of skeletal muscle is accompanied by an increased capacity for glycogenolysis and anaerobic glycolysis. In the present study, regulatory features of cAMP synthesis were examined in neonatal and adult rabbit sarcolemmal membranes. Adult sarcolemma exhibited a 3-, 6-, and 10-fold greater adenylate cyclase activity than neonate for basal, NaF, and isoproterenol plus GTP, respectively. The Km for activation by isoproterenol was 1.4 X 10(-8) M and 6 X 10(-8) M for GTP. The number of beta-receptors was similar (0.9-1.2 pmol/mg). 10 microM GTP shifted isoproterenol EC50 from 1 X 10(-8) M to 1 X 10(-7) M in adult; neonatal agonist affinity was unaffected by GTP. Cholera toxin stimulated adenylate cyclase activity 2-fold and catalyzed 32P ribosylation of a Mr = 42,000 peptide in adult sarcolemma; both activities were low or absent in neonate. Isoproterenol-stimulated GTPase activity was elevated 4-fold in adult compared to neonatal sarcolemma. Mn2+ ion-stimulated basal activity, an indicator of catalytic function of adenylate cyclase, was also elevated in adult. Together, these findings suggest that the development of catecholamine-sensitive cAMP synthesis in muscle is governed by the coordinate expression of the regulatory and catalytic proteins of adenylate cyclase, but not the beta-receptor.     

Age-related characteristics of the effect of insulin on the membrane potential of cells of the zona fasciculata of the adrenal cortex

The value of the membrane potential of adrenocorticocytes (ACC) in zona fasciculata of isolated adrenal (IA) cortex and the activity of Na, K-ATPase of IA homogenate is determined in adult (6-7 months) and old (26-28 months) male Wistar rats. No significant age differences are found in the above indices. Administration of insulin in vivo (1.6 U/kg) and in vitro (0.1 U/ml) induced hyperpolarization of ACC plasmic membranes. Insulin increases the activity of Na, K-ATPase of IA in animals of both age groups. Ouabain and actinomycin D blocks the insulin-induced hyperpolarization of ACC, while 2-aminopyridine has no effect on this process. Insulin effect on the MP of IA ACC is less pronounced in old vs. adult animals.     

Mechanisms of insulin inhibition of ACTH-stimulated steroid secretion by cultured bovine adrenocortical cells.

Results of previous studies indicated that insulin at levels comparable to those in humans during hyperinsulinemia decreased ACTH-stimulated cortisol and androstenedione secretion by bovine adrenal fasciculata-reticularis cells in primary culture. In the present studies this inhibitory action was examined further by comparing the effects of insulin on ACTH-stimulated corticosteroid secretion with its effects on 8-(4-chlorophenylthio)-cAMP (cpt-cAMP), forskolin- and [5val]angiotensin II (Ang II)-stimulated corticosteroid secretion. Effects on corticosteroid secretion were correlated with effects on cAMP accumulation and rates of cAMP production. Monolayers were incubated for 24 h in the absence or presence of each agonist alone or in combination with insulin. Insulin (1.7 x 10(-9) or 17.5 x 10(-9) M) caused about a 50% decrease in cortisol and androstenedione secretion in response to ACTH (10(-11) or 10(-8) M). Insulin also decreased ACTH-stimulated aldosterone secretion by cultured glomerulosa cells. Cpt-cAMP (10(-4) or 10(-3) M)-stimulated increases in cortisol and androstenedione secretion were inhibited by insulin, but to a lesser extent than those in response to ACTH. The inhibition of cpt-cAMP-stimulated steroid secretion was not related to increased degradation of the cyclic nucleotide. Increases in cortisol and androstenedione secretion caused by a submaximal concentration (10(-6) M) of forskolin were decreased 50-70% by insulin. In contrast, insulin failed to significantly affect cortisol or androstenedione secretion caused by a maximal concentration (10(-5) M) of forskolin. The secretory responses to Ang II (10(-8) M) were also unaffected by insulin. The effect of insulin to inhibit ACTH-stimulated steroid secretion was accompanied by a reduction in cAMP accumulation as well as an apparent inhibition of adenylate cyclase activation. These data indicate that the effect of insulin to attenuate ACTH-stimulated corticosteroid secretion results from both an inhibition of ACTH-stimulated adenylate cyclase activity and an antagonism of the intracellular actions of cAMP.     

Developmental differences in L-type calcium current of human atrial myocytes

We investigated differences in L-type Ca2+ current (ICa) between infant (INF, 1-12 mo old), young adult (YAD, 14-18 yr old), and older adult (AD) myocytes from biopsies of right atrial appendages. Basal ICa was smaller in INF myocytes (1.2 +/- 0.1 pA/pF, n = 29, 6 +/- 1 mo old, 11 patients) than in YAD (2.5 +/- 0.2 pA/pF, n = 20, 16 +/- 1 yr old, 5 patients) or AD (2.6 +/- 0.3 pA/pF, n = 19, 66 +/- 3 yr old, 9 patients) myocytes (P < 0.05). Maximal ICa produced by isoproterenol (Iso) was similar in INF, YAD, and AD cells: 8.4 +/- 1.1, 9.6 +/- 1.0, and 9.2 +/- 1.3 pA/pF, respectively. Efficacy (Emax) was larger in INF (607 +/- 50%) than for YAD (371 +/- 29%) or AD (455 +/- 12%) myocytes. Potency (EC50) was 8- to 10-fold higher in AD (0.82 +/- 0.09 nM) or YAD (0.41 +/- 0.14 nM) than in INF (7.6 +/- 3.5 nM) myocytes. Protein levels were similar for Gialpha2 but much greater for Gialpha3 in INF than in AD or YAD atrial tissue. When Gialpha3 activity was inhibited by inclusion of a Gialpha3 COOH-terminal decapeptide in the pipette, basal ICa and the response to 10 nM Iso were increased in INF, but not in YAD, cells. We propose that basal ICa and the response to low-dose beta-adrenergic stimulation are inhibited in INF (but not YAD or AD) cells as a result of constitutive inhibitory effects of Gialpha3.     

Corticosteroid regulation of Na+ and K+ transport in the rat distal colon during postnatal development

To study the role of corticosteroids in the regulation of colonic electrogenic amiloride-sensitive Na+ absorption (ISCNa) and barium-sensitive K+ secretion (ISCK) during development, we investigated suckling (10-day old), weanling (25-day old) and adult (90-day old) adrenalectomized rats after they had received aldosterone, dexamethasone or corticosterone. Adrenalectomy reduced markedly ISCNa in suckling rats and completely inhibited ISCNa in weanling animals; the ISCNa was absent in intact adult rats. The doses of aldosterone, corticosterone and dexamethasone estimated to be equivalent to the endogenous production rate of aldosterone and corticosterone restored ISCNa after 1 day in both suckling and weanling rats. Compared with aldosterone, glucocorticoids produced a greater increase in ISCNa. Concurrent spironolactone treatment (a mineralocorticoid antagonist) completely prevented the effect of aldosterone but had no effect in dexamethasone-treated rats. The glucocorticoid antagonist RU 38 486 inhibited the dexamethasone-induction of ISCNa but had no effect on aldosterone. The response to corticosteroids, measured as the increase of ISCNa, declined from suckling to adult rats. In contrast to ISCNa, the same time of treatment and the same doses of corticosteroids did not influence ISCK. ISCK was stimulated only after chronic treatment (4 days). These findings suggest that, in the distal colon of young rats, (1) both corticosteroids may regulate amiloride-sensitive Na+ absorption and barium-sensitive K+ secretion, (2) different receptors mediate the colonic effects of glucocorticoids and mineralocorticoids, (3) immature rats are more sensitive to corticosteroids than adult animals, and (4) the acute effect of corticosteroids is an increase in Na+ absorption which is followed by delayed stimulation of K+ secretion.     

Differential modulation of ACTH-stimulated cortisol and androstenedione secretion by insulin

Results of previous clinical studies suggested counter regulatory actions between insulin and DHEA(S). The present studies were performed using primary monolayer cultures of bovine fasciculata-reticularis cells to test the hypothesis that insulin directly affects adrenal androgen secretion. Although having no independent effect, insulin exhibited complex time- and concentration-specific actions on ACTH-stimulated secretion of both C21 (cortisol) and C19 (androstenedione) corticosteroids. In the presence of low concentrations (0.05-0.1 nM) of ACTH, cortisol secretion during a 2 h incubation was about 2-fold greater in the presence than in the absence of insulin (0.01-100 ng/ml). In the presence of a maximal concentration (10 nM) of ACTH, on the other hand, cortisol secretion was not affected by insulin at concentrations less than or equal to 0.1 ng/ml, but was decreased at higher insulin concentrations. ACTH-stimulated androstenedione secretion was not significantly affected by insulin during a short-term (2 h) incubation. During a prolonged (24 h) incubation, insulin produced a concentration-dependent inhibition of ACTH-stimulated cortisol secretion. At an insulin concentration of 100 ng/ml, ACTH (10 nM)-stimulated cortisol secretion declined to a level only 30% of that produced by ACTH alone. In contrast, insulin exhibited biphasic effects on the secretion of androstenedione by cells maintained in the presence of ACTH for 24 h; an effect that was most dramatic in the presence of a maximal concentration of ACTH. At an insulin concentration of 0.1 ng/ml, androstenedione secretion by cells maintained in the presence of 10 nM ACTH was increased approximately 2.5-fold. At higher concentrations of insulin, ACTH-stimulated androstenedione secretion was inhibited to an extent comparable to that in cortisol secretion. The effects of insulin on ACTH-stimulated cortisol and androstenedione secretion could not be accounted for by changes in steroid degradation or a loss in 11 beta-hydroxylase activity. These results indicate that insulin interacts with ACTH to modulate the secretion of both C21 and C19 corticosteroids and that physiological concentrations (less than or equal to 1 ng/ml) of insulin may have a long-term effect to enhance selectively adrenal androgen secretion. These data are consistent with a servo mechanism between insulin and DHEA(S) in vivo and indicate that the correlations observed clinically result, at least in part, from a direct action of insulin to modulate the rate of adrenal androgen production.     

The met-enkephalin analog FK 33-824 and naloxone do not directly influence cortisol secretion by cultured human adrenocortical cells

Systemic administration of the enkephalin analog FK 33.824 was previously shown to inhibit ACTH secretion in man. In this study, the direct action of this analog on cortisol release was studied. The enkephalin analog (1 uM and 10 uM) did not influence basal or ACTH-stimulated cortisol production by cultured isolated adrenocortical cells prepared from the hyperplastic adrenal glands from three patients with Cushing's disease. Naloxone (10 uM) had also no direct effect on cortisol release. It is concluded that the met-enkephalin analog used in this study and naloxone do affect the hypothalamo-pituitary-adrenal axis via a central effect.     

Effect of indomethacin on bombesin-like immunoreactivity, somatostatin and gastrin secretion from rat stomach

In the present study the effect of indomethacin-induced prostaglandin deficiency was examined on the release of bombesin-like immunoreactivity (BLI), a putative peptidergic neurotransmitter, from the isolated perfused rat stomach. In addition, gastrin and somatostatin (SLI) secretion was determined. Pretreatment of rats with indomethacin (2 mg/kg X h) resulted in a 3-fold increase of basal BLI secretion. In response to acetylcholine (2 X 10(-6) M) BLI rose from 2,000 to 4,000 pg/min, whereas in controls BLI increased from 400 to 1,400 pg/min. While absolute values for BLI secretion were higher in indomethacin-treated stomachs the relative increase above baseline was lower (100 vs. 250%). In control rats the increase in BLI secretion in response to acetylcholine was abolished when the acidity in the gastric lumen was increased from pH 7 to pH 2. After indomethacin, however, the stimulatory effect of acetylcholine during luminal pH 7 and pH 2 was identical. The decrease of SLI by acetylcholine at luminal pH 7 was abolished in indomethacin-treated stomachs in response to 10(-6) M acetylcholine, and 2 X 10(-6) M had even a stimulatory effect on SLI secretion. Indomethacin pretreatment reduced gastrin secretion at luminal pH 7. These data demonstrate that endogenous prostaglandins exert an inhibitory tone on basal and stimulated BLI and stimulated SLI secretion in the rat stomach. It is suggested that endogenous prostaglandins also inhibit the release of a peptidergic neurotransmitter, similar to their effect on the classical neurotransmitters acetylcholine and norepinephrine.     

介绍一种LHRH调节LH分泌的离体模型—分散垂体细胞灌流技术

成年雄性 SD 大鼠断头后分离出垂体前叶(anterior pituitary,AP)。用胰蛋白酶消化和机械分散制备 AP 细胞(成活率大于95%)。分散的细胞悬液与生物凝胶混合后装上灌流柱,然后用 M199溶液连续灌流24h 以上。每间隔1~h 给予一次6min 的 LHRH 脉冲式刺激。细胞在此灌流过程中有一个稳定的基础 LH 分泌水平。LHRH 刺激能迅速引起 LH 分泌。对同一剂量 LHRH 的多次刺激可产生相同的 LH 脉冲。在一定的 LHRH 浓度范围内(1×10~(-10)_1×10~(-7)mol/L),LH 分泌与 LHRH 的剂量-效应曲线呈线性。实验结果表明,连续灌流分散的 AP 细胞的技术,优于单层细胞培养和组织块灌流等其他方法,是一种较为理想的研究LHRH 调节 LH 分泌机理的体外模型。     

Effect of iopanoic acid on basal and thyrotropin-stimulated thyroid hormone levels in suckling rat pups

We proposed that basal and thyrotropin (TSH)-stimulated thyroid hormone levels of rat pups would be altered in the presence of iopanoic acid (IA), a radiographic contrast agent which competitively inhibits T4-to-T3 conversion, and that the nature of these changes would further depend upon the route of TSH administration in a manner distinct from that reported in adults. To test this hypothesis, litters from 24 Sprague-Dawley female rats were adjusted to 8 pups each. On day 5, 80 pups received IA (2.5 mg/100 g body weight) injections. On day 8, control and IA pups were further subdivided, and given bovine TSH (bTSH) either by subcutaneous injection or by intragastric gavage (to simulate milk-borne TSH intake), and then sacrificed 0, 1.5, or 3 hours later. We found significantly higher T4 and reverse-T3 (rT3) levels in IA-treated pups, but IA had no effect on basal or TSH-stimulated T3 levels attained, regardless of route of bTSH administration or time post-treatment. Our data demonstrate that the effects of IA on T4 and rT3 levels in the immature rat are comparable to those observed in adult rats and humans, but that the marked depression of T3 levels found in IA-treated adults does not occur in the 8-day old rat pup. We speculate that the IA-treated suckling pup's ability to sustain normal basal T3 levels and generate elevated T3 concentrations in response to TSH stimulation may reflect the activity during development of a T4-5'-deiodinase relatively resistant to competitive inhibition by this drug.     

Influence of cholecystokinin on hypothalamic-stalk median-eminence-extract stimulation of ACTH output from isolated pituitary cells

The influence of cholecystokinin 33 (CCK33) on CRF-like stimulation of ACTH output was tested in vitro using isolated pituitary cells. ACTH was assayed using isolated adrenal cell preparations. The CRF-like material was contained in a crude acetic-acid extract of hypothalamic stalk median eminence (HSME). CCK33, in doses of 1 U, 10(-3) U, and 10(-6) U/ml cell suspension had no influence on basal or ACTH-stimulated corticosterone output from isolated adrenal cells. Isolated pituitary cells responded in a dose-related fashion of HSME extract, however, the absolute response to a given dose of HSME extract varied according to the basal (non-stimulated) output of a particular cell preparation. CCK33, in the dose range tested, had no influence on basal ACTH output. In contrast, 10(-3) U/ml oc CCK33, which corresponds to a concentration of 8 X 10(-11) M, significantly inhibited the output of ACTH from isolated pituitary cells stimulated by 0.2 equivalents of HSME. Higher concentrations of CCK33 had a variable effect. We conclude that cholecystokinin may have a role in the regulation of HSME-stimulated ACTH output from the pituitary.     

Pertussis toxin enhances the beta-adrenergic and blocks the alpha 2-adrenergic regulation of renin secretion in renal cortical slices

The adrenergic regulation of renin secretion was studied in renal cortical slices from control and pertussis toxin-treated rats. Pertussis toxin was used to study the role of adenylate cyclase in the control of renin release. It was observed that isoproterenol and epinephrine stimulated renin secretion and that clonidine decreased both basal and isoproterenol-stimulated renin secretion in the control group. Pertussis toxin: a) increased significantly basal renin secretion, b) displaced to the left the concentration-response curve for isoproterenol and epinephrine and magnified the response to epinephrine and c) abolished the inhibitory effect of clonidine on renin secretion. This work confirms our previous results obtained in vivo and suggests a direct effect of pertussis toxin on the cells that secrete renin.     

Enkephalinergic control of somatostatin secretion from the perfused rat stomach

A role for the enkephalins in the regulation of gastric somatostatin (SLI) secretion has been investigated in an isolated perfused rat stomach model. Both methionine- and leucine-enkephalins caused a dose-dependent inhibition of gastric inhibitory polypeptide (GIP) stimulated SLI secretion. Leu-enkephalin was one order of magnitude less potent than met-enkephalin: 50% inhibition by met-enkephalin was at 4 X 10(-9) M and with leu-enkephalin 3.5 X 10(-8) M. Naloxone (100 nM) had no effect on basal secretion but blocked the inhibitory action of met-enkephalin (1 nM or 1 microM). Vagal stimulation (7 V, 10 Hz, 5 ms) inhibited GIP-stimulated SLI release. Administration of naloxone partially reversed this inhibition, suggesting that endogenous opioids were at least partially responsible for vagally induced inhibition. A number of possible pathways by which endogenous enkephalins may modulate SLI release have been proposed.     

Calcitonin secretion during postnatal development in the rat: beta-adrenergic agents and vitamin D3 metabolites

The possible contribution of catecholamines and vitamin D3 metabolites to the high plasma calcitonin (CT) levels in suckling baby rats is unknown. So, in vivo and in vitro (using a perifusion system) effects of beta-adrenergic agents and vitamin D3 metabolites on CT release were studied in the rat during the postnatal development. In 13-day-old rats, the increase in plasma CT levels induced by isoproterenol injection (0.1 micrograms/kg b.w.) was inhibited by a previous administration of propranolol. A significant decrease in plasma CT levels was observed after propranolol injection in baby rats (0.68 +/- 0.05 ng/ml vs. 0.93 +/- 0.01 ng/ml). A daily injection of 1,25-dihydroxycholecalciferol (1,25-(OH)2D3; 25 pmoles/rat/day during 4 days) induced a marked rise in plasma calcium (16.1 +/- 0.2 mg/dl), and a great decrease in thyroidal CT contents (approximately 70% of control values) in 13-day-old rats while no change was noted with 24,25-dihydroxycholecalciferol (24,25-(OH)2D3). A negative correlation between plasma calcium and thyroidal CT stores was found in suckling and in weaning rats treated with different doses of 1,25-(OH)2D3, suggesting an indirect effect of 1,25-(OH)2D3 on CT secretion. The mobilization of the thyroidal CT content was greater in weaning than in suckling rats in response to a given hypercalcemia. In vitro, 5 X 10(-5) M isoproterenol induced a rapid increase in CT secretion rate while 1,25-(OH)2D3 inhibited the rise in CT release induced by 3.0 mM calcium.(ABSTRACT TRUNCATED AT 250 WORDS)     

Aging and gender affect the response of thyrotropin (TSH) to gastrin releasing peptide (GRP) in rats

We had previously shown that GRP acts directly at the pituitary gland inhibiting basal and TRH-stimulated TSH secretion in adult male rats. In this study we showed a gender dimorphism in this response of old animals pituitaries to GRP. In both female and male young adult animals, GRP-incubated pituitaries showed approximately 50% less basal and TRH-stimulated TSH secretion to the medium, without affecting the pituitary content of TSH. However, GRP did not have any significant effect upon TSH secretion in old male rats, but the old female showed the same degree of response to GRP as the young adult female rat, regarding basal and TRH-stimulated TSH secretion, while the TSH pituitary content after GRP incubation was higher than that of the young female group. Our data suggest a loss of thyrotrope responsiveness to GRP in aged male rats that could contribute to the decrease in TSH pituitary stores leading to lower basal and TRH-stimulated TSH secretion. Meanwhile, the preservation of GRP responsiveness could help in the relative maintenance of these parameters in the old female rat.     

Intratesticular injection of [D-Met2-Pro5]enkephalinamide suppresses testosterone secretion of the testis of immature rat

The possible role of enkephalin in the local control of testicular function was studied in neonatal rats. 5- and 10-day old hemicastrated rats were treated intratesticularly with an enkephalin analog [D-Met2-Pro5]enkephalinamide. In 5-day-old rats local injection of different doses (0.1-0.3 micrograms/testis) of the peptide suppressed basal testosterone secretion in vitro in a dose-dependent manner 2 h posttreatment. Intratesticular administration of naloxone prior to enkephalin treatment prevented the decrease in basal testosterone production induced by the opioid agonist. In 10-day-old animals intratesticular injection of 1.0 and 3.0 micrograms/testis of enkephalinamide reduced serum testosterone concentration and basal testosterone secretion in vitro. Systemic injection of the peptide produced no change in steroidogenesis. These results suggest that enkephalins might be among the intratesticular factors regulating Leydig cell functions.     

Age-related alterations in pulsatile luteinizing hormone release: effects of long-term ovariectomy, repeated pregnancies and naloxone

Aging of the female reproductive system may be regulated by changes at the hypothalamic, pituitary, and ovarian levels. Long-term ovariectomy (LT-OVX) and/or multiple pregnancies delay age-related deterioration of several parameters of reproductive potential in rodents. We tested whether long-term suppression of cyclic ovarian hormone release that is normally associated with the 4- to 5-day estrous cycle decelerates age-related decreases in the frequency of luteinizing hormone (LH) pulses to assess whether hormonal milieu influences the rate of aging of the pulse generator. We determined the percentage of rats exhibiting pulsatile LH secretion, mean LH levels, and amplitude and frequency of LH pulses in seven groups of ovariectomized (OVX) rats. Young (3-4 mo), middle-aged (8-10 mo), and old (18-22 mo) virgin rats, ovariectomized 4 wk (4WK-OVX) prior to experimentation, were used to determine the effect of age. The effect of long-term ovarian hormone deprivation was tested by ovariectomizing rats at 2-3 mo of age and using them when they were middle-aged (8-10 months) or old (18-22 mo). The effect of deprivation of cyclic increases in ovarian hormones associated with repeated estrous cycles was tested by using retired breeder (RB) rats that had been ovariectomized 4 wk prior to experimentation. Each rat was implanted with a right atrial cannula and bled the next day at 10-min intervals for 3 h.(ABSTRACT TRUNCATED AT 250 WORDS)