Mediation by nitric oxide of LH-RH-stimulated gonadotropin secretions in human subjects

L-Arginine alleviates the testosterone reduction in heat-treated mice by upregulating LH secretion, the testicular antioxidant system and expression of steroidogenesis-related genes

High temperature can reduce testes function, leading to decreased testosterone secretion. Dietary l-arginine (l-Arg) supplementation improves the semen quality and libido of boars. The present study investigated whether l-Arg could enhance the production of testosterone in mice exposed to high ambient temperature. Twenty-four 6-week-old male ICR mice were randomly divided into three groups: a control group, a heat-treated (HT) group and a group subjected to heat treatment plus 2mg kg-1 l-Arg (HT+Arg). l-Arg was administered to mice by oral gavage for 18 consecutive days, after which the HT and HT+Arg groups were placed into an incubator at 40°C for 30min every day for 5 days. Serum testosterone and LH concentrations were significantly increased in the HT+Arg compared with HT group, as was catalase, total superoxide dismutase and glutathione peroxidase activity and the expression of steroidogenesis-related genes steroidogenic acute regulatory protein (Star), steroidogenic factor-1 (Sf1), 17β-hydroxysteroid dehydrogenase 3 (Hsd17b3) and 17α-hydroxylase/17,20-lyase (Cyp17a1) in the testes. These results demonstrate that l-Arg can alleviate testosterone reductions in heat-treated mice by upregulating LH secretion, enhancing the antioxidant system and increasing the expression of testosterone synthesis-related genes.

Nitric oxide and guanylate cyclase signalling are differentially involved in gonadotrophin (LH) release responses to two endogenous GnRHs from goldfish pituitary cells

Nitric oxide synthase (NOS) immunoreactivity is present in goldfish gonadotrophs. The present study investigated whether two native goldfish gonadotrophin-releasing hormones (GnRHs), salmon (s)GnRH and chicken (c)GnRH-II, use NOS/nitric oxide (NO) and soluble guanylate cyclase (sGC)/cyclic (c)GMP/protein kinase G (PKG) signalling to stimulate maturational gonadotrophin [teleost gonadotrophin-II, luteinising hormone (LH)] release. In cell column perifusion experiments with dispersed goldfish pituitary cells, the application of three NOS inhibitors (aminoguanidine hemisulphate, 1400W and 7-nitroindazole) and two NO scavengers [2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) and rutin hydrate] reduced sGnRH-elicited, but not cGnRH-II-induced, LH increases. The NO donor sodium nitroprusside (SNP) increased NO production in goldfish pituitary cells in static incubation. SNP-stimulated LH release in column perifusion was attenuated by PTIO and the sGC inhibitor 1H-(1,2,4)oxadiazolo[4,3-a]quinoxalin-1-oneon (ODQ), and additive to responses elicited by cGnRH-II, but not sGnRH. ODQ and the PKG inhibitor KT5823 decreased sGnRH- and cGnRH-II-stimulated LH release. Similarly, the LH response to dibutyryl cGMP was reduced by KT5823. These results indicate that, although only sGnRH uses the NOS/NO pathway to stimulate LH release, both GnRHs utilise sGC/PKG to increase LH secretion.

Effect of oxytocin on nitric oxide activity controlling gonadotropin secretion in humans

BACKGROUND

Previously described inhibitory effects of the nitric oxide synthase (NOS) inhibitor L-NAME on luteinizing hormone-releasing hormone (LH-RH)-induced LH and follicle stimulating hormone (FSH) secretion in humans suggested modulation by nitric oxide (NO) of the gonadotropin-releasing action of LH-RH.

DESIGN

In order to establish whether oxytocin (OT) participates in this regulatory mechanism, 10 normal men were treated with LH-RH (100 micro g as an i.v. bolus) given alone or in the presence of L-NAME (40 micro g kg-1 injected plus 50 micro g kg-1 infused i.v. for 60 min), OT (2 IU injected plus 4 IU infused i.v. for 60 min) or a combination of both drugs.

RESULTS

The administration of OT was unable to change the gonadotropin responses to LH-RH. In contrast, L-NAME significantly reduced both FSH and LH increments induced by LH-RH. When L-NAME was given in the presence of OT, the LH and FSH responses to LH-RH were similar to those observed after the administration of LH-RH alone.

CONCLUSION

These data suggest antagonistic actions of OT and L-NAME in the control of NOS activity in regulation of gonadotropin secretion induced by LH-RH.

Nitric oxide regulation of gonadotrophin secretion in prepubertal heifers1

Mechanisms responsible for the pulsatile release of gonadotrophin secretion in prepubertal heifers are not fully known. We have shown that an excitatory amino acid agonist, N-Methyl-D,L-aspartic acid (NMA), induces an immediate release of luteinizing hormone (LH) and follicle stimulating hormone (FSH) in prepubertal heifers. Nitric oxide (NO) has also emerged as an important regulator of LH release in rats. This study was designed to test the role of NO in the regulation of gonadotrophin release as well as the possible mediation by NO of the effects of NMA and gonadotrophin releasing hormone (GnRH) on gonadotrophin secretion in heifer calves. In experiment 1, four groups of five prepubertal heifers (33 weeks old) received one of the following treatments: (1); N-G-nitro-L-arginine methyl ester (L-NAME, a NO synthase inhibitor, 35 mg/kg, i.v., once); (2) NMA (4.7 mg/kg, i.v., once); (3) L-NAME+NMA (as above); and (4) Vehicle (saline, i.v.). All heifers in all groups were also challenged with a bolus injection of GnRH (10 ng/kg, i.v., once). Blood samples were collected every 15 min for 10 h. L-NAME was injected after the first blood sample, NMA after 2 h and GnRH after 6 h of blood sampling. Administration of L-NAME alone, suppressed the spontaneous pulses of LH (P<0.04). Heifers in the NMA group responded with a significantly greater LH release than did the heifers in the L-NAME+NMA group (P<0.05). Following the GnRH challenge, heifer calves treated with L-NAME or NMA had higher LH pulse responses than the controls (P<0.05). In a second experiment, four groups of five heifer calves (34 weeks old) were given one of the following treatments: (1) L-NAME (as above); (2) L-arginine, a NO precursor (ARG, 100 mg/kg/h, i.v. drip infused for 6 h starting 2 h after first blood sample was taken); (3) L-NAME+ARG (as above); and (4) Vehicle (saline i.v. bolus and drip for 6 h). Blood samples were taken every 10 min for 8 h. Administration of L-NAME suppressed the pulsatile release of LH and FSH (P<0.05). Compared to the control group, infusion of ARG by itself did not change the pattern of LH secretion (P>0.05); however, in heifers given L-NAME, ARG restored a normal pattern of LH pulses, similar to the control values (P>0.05). It was therefore concluded that NO is involved in the regulation of LH, and possibly FSH, secretion and that NO may mediate, at least in part, the stimulatory effects of NMA on LH, and to some extent FSH, release. The responses to GnRH led us to suggest that NO may have inhibitory effects on the pituitary and NMA may have increased pituitary sensitivity to GnRH.

In vitro nitric oxide effects on basal and gonadotropin-releasing hormone-induced gonadotropin secretion by pituitary gland of male crested newt (Triturus carnifex) during the annual reproductive cycle

The objective of this study was to test the possible nitric oxide (NO) involvement in pituitary gonadotropin secretion in the male crested newt, Triturus carnifex. Pituitaries were incubated in vitro with medium alone, GnRH, NO donor (NOd, sodium nitroprusside), NO synthase inhibitor (NOSi, Nomega-nitro-L-arginine methyl ester), cGMP analogue (cGMPa, 8-bromo-cGMP), soluble guanylate cyclase inhibitor (sGCi, cystamine), GnRH plus NOSi, GnRH plus sGCi, and NOd plus sGCi during the annual reproductive cycle: pre-reproduction, reproduction (noncourtship and courtship), and the refractory, recovery, and estivation periods. To determine pituitary gonadotropin secretion indirectly, newt testes were superfused in vitro with preincubated pituitaries, and androgen release was determined. NO synthase (NOS) activity and cGMP levels were assessed in the preincubated pituitaries. Medium alone- and GnRH-preincubated pituitary increased androgen secretion during pre-reproduction, noncourtship, courtship, and recovery; the GnRH-induced increase was higher than the medium alone-induced increase during pre-reproduction, noncourtship, and recovery. NOd and cGMPa increased androgens in all reproductive phases considered except courtship; the NOd- and cGMP-induced increase was higher than the medium alone-induced increase during pre-reproduction, noncourtship, and recovery. NOS activity was highest during courtship and lowest during the refractory and estivation periods. GnRH increased NOS activity during pre-reproduction, noncourtship, and recovery. Cyclic GMP levels were highest during courtship and lowest during the refractory period and estivation. GnRH increased cGMP levels during pre-reproduction, noncourtship, and recovery, while NOd did so during all reproductive phases considered. These results suggest that basal and GnRH-induced gonadotropin secretion are up-regulated by NO in the pituitary gland of the male Triturus carnifex.

Effect of nitric oxide on basal and TRH-or metoclopramide-stimulated prolactin release in normal men. volpi@ipruniv.cce.unipr.it

In order to establish whether nitric oxide (NO) is involved in the regulation of basal and/or TRH- or metoclopramide (MCP)-stimulated PRL secretion, normal male subjects were treated i.v. with the NO-synthase (NOS) inhibitor N-nitro-L-arginine methyl ester (L-NAME) (40 mg/kg injected plus 50 mg/kg infused over 60 min) in basal conditions (N.7 subjects) or just before the PRL releasing hormone TRH (20 or 200 microg iv) (N.7 subjects) or the antidopaminergic agent MCP (1 or 10 mg iv) (N.7 subjects). In control experiments, subjects received normal saline instead of L-NAME. The administration of L-NAME modified neither the basal secretion of PRL, nor the PRL release induced by TRH (20 or 200 microg) or MCP (1 or 10 mg). These data suggest that in humans, NO is not involved in the control of PRL release at the anterior pituitary level.

Involvement of nitric oxide in vasoactive intestinal peptide-stimulated prolactin secretion in normal men

To establish whether nitric-oxide (NO) participates in the regulation of prolactin (PRL) secretion in humans in basal conditions and/or under stimulation with vasoactive intestinal polypeptide (VIP), seven normal men were treated with a placebo (normal saline) or the NO synthase (NOS) inhibitor L-NAME (40 microg/kg injected plus 50 microg/kg infused intravenously over 60 minutes), which in previous studies has been found able to modify other pituitary hormone secretions. Experiments were performed either in basal conditions or during stimulation of PRL secretion with an intravenous infusion of VIP (4 pmol/kg min over 60 minutes). The administration of L-NAME was unable to change the basal secretion of PRL. In contrast, L-NAME significantly enhanced the PRL increase induced by VIP. These data argue against an involvement of NO in regulation of basal PRL secretion. In contrast, the stimulatory effect of L-NAME on VIP-induced PRL secretion suggests that NO exerts an inhibitory control of the PRL response to VIP.

Nitric oxide mediates gonadotropin-releasing hormone effects on frog pituitary

We studied the possible role of nitric oxide (NO) in GnRH-induced gonadotropin secretion in the female water frog, Rana esculenta. During pre-reproduction, pre-ovulation, ovulation, post-ovulation, refractory, recovery and hibernation, pituitaries were incubated with medium-alone, GnRH, NO donor (NOd), NO synthase inhibitor (NOSi), cyclic GMP analogue (cGMPa), soluble guanylate cyclase inhibitor (sGCi), GnRH plus NOSi, GnRH plus sGCi, and NOd plus sGCi. Because antisera raised against gonadotropins are not available for this species, we measured these hormones indirectly through their effects on ovarian progesterone secretion. The ovaries were superfused with the pituitaries pre-incubated as reported above. In addition, NOS activity and cGMP levels were determined in the pre-incubated pituitaries. Those pre-incubated with medium-alone and with GnRH increased progesterone secretion during pre-reproduction, pre-ovulation, ovulation and recovery; the increase induced by GnRH was higher than that induced by medium-alone during pre-reproduction, pre-ovulation and recovery. NOd and cGMPa increased progesterone in all considered reproductive phases except ovulation; the increase induced by NOd and cGMP was higher than that induced by medium-alone during pre-reproduction, pre-ovulation and recovery. NOS activity was highest during ovulation and lowest during post-ovulation, refractory and hibernation. GnRH increased NOS activity during pre-reproduction, pre-ovulation and recovery. Cyclic GMP levels were highest during ovulation and lowest during post-ovulation, refractory and hibernation. GnRH increased cGMP levels during pre-reproduction, pre-ovulation and recovery, NOd during all considered reproductive phases. These results suggest that NO mediates basal and GnRH-induced gonadotropin secretion in female Rana esculenta.

Influence of nitric oxide on hypoglycemia--or angiotensin II-stimulated ACTH and GH secretion in normal men

In order to establish whether nitric oxide (NO) is involved in the regulation of ACTH and/or GH secretion, normal male subjects were treated i.v. with the NO-synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME) (40 micrograms/kg injected plus 50 micrograms/kg infused over 60 min) in basal conditions and/or during stimulation with insulin (0.15 IU/kg body weight in an i.v. bolus) to induce hypoglycemia (ITT) or ASP 1 ILE-5 angiotensin II (ANG II) (increasing doses of 4, 8 and 16 ng/kg/min, each dose for 20 min). The administration of L-NAME neither changed the basal secretion of ACTH and GH nor modified the hormonal responses to ANG II stimulation. Also the GH response during ITT remained unchanged in the presence of L-NAME. In contrast, the ACTH response to hypoglycemia was significantly higher when L-NAME was administered. These data suggest that in normal men NO has a negative effect on ACTH secretion, but not GH secretion, in response to hypoglycemia. Furthermore, our results argue against a role of NO in the control of basal and ANG II-stimulated ACTH and GH secretions.