Nitric oxide modulation of ET_B receptor-induced vasopressin release by rat and mouse hypothalamo-neurohypophyseal explants

Abstract

peptides stimulate vasopressin (AVP) secretion via ETB receptors at hypothalamic loci. Nitric oxide modulates the actions of ET in the cardiovascular system and also influences neurotransmission and specifically suppresses firing of magnocellular neurons. The purpose of these studies was to ascertain whether nitric oxide, generated in response to ETB receptor stimulation, buffers the stimulatory effect of ET and suppresses AVP release. Studies were performed using a pharmacological approach in hypothalamo-neurohypophyseal explants from rats, and an alternative strategy using explants from mice with an inactivating mutation of neuronal NOS (nNOS Ϫ/Ϫ ) and their wild-type parent strain. Whole explants in standard culture or only the hypothalamus of compartmentalized explants was exposed to the ETB selective agonist, IRL 1620 (10 Ϫ13 to 10 Ϫ8 M). Rat and wild-type mouse explants displayed similar responses, although absolute basal release rates were higher from murine explants. Maximal AVP release at 0.1 nM IRL 1620 was 311 Ϯ 63 (rat) and 422 Ϯ 112% basal⅐explant Ϫ1 ⅐h Ϫ1 (mouse). Sodium nitroprusside (SNP; 0.1 mM) suppressed maximal AVP release to basal values. N -nitro-L-arginine methyl ester (L-NAME, 0.1 M), which did not itself stimulate AVP secretion, more than doubled the response to 1 pM IRL 1620, from 136 Ϯ 28 to 295 Ϯ 49% basal⅐explant Ϫ1 ⅐h Ϫ1 (P Ͻ 0.05) by rat explants. Explants from wild-type mice responded similarly. Explants from nNOS Ϫ/Ϫ mice had higher basal AVP secretory rate in response to 1 pM IRL 1620: 271 Ϯ 48 compared with 150 Ϯ 24% basal⅐explant Ϫ1 ⅐h Ϫ1 (P Ͻ 0.05) from wild-type murine explants. In the nNOS Ϫ/Ϫ , SNP suppressed stimulated release, and L-NAME exerted no additional stimulatory effect: 243 Ϯ 38% basal⅐explant Ϫ1 ⅐h Ϫ1. Thus nitric oxide inhibits the AVP secretory response induced by ETB receptor activation within the hypothalamo-neurohypophyseal system and is generated primarily by the nNOS isoform. The modulation of AVP secretion by ET and also nitric oxide can take place independently from their effects on cerebral blood flow, systemic hemodynamics, or the arterial baroreflex.endothelin; nitric oxide synthase; neuronal nitric oxide synthase; posterior pituitary THE ENDOTHELINS (ET) ARE PEPTIDES that exhibit potent vasoactive actions. ET peptides also participate in neurotransmission and neuromodulation within the central nervous system (12, 16). All components of the ET system, including both ET-1 and ET-3 isoforms, their respective mRNAs, ET-converting enzyme activity and both ET A and ET B receptor subtypes, exist within nonvascular brain tissues, and specifically in loci involved in vasopressin (AVP) secretion (13,15,17,38,45). Moreover, several studies have implicated ET peptides in the regulation of AVP secretion in vivo (24,27,29,30) and in vitro (23-26, 28, 31, 34).The actions of ET on the vasopressinergic system are complex and depend on the specific ET isoform, as well as the site of action and the receptor subtype involved (26, 41). ET A receptors are located on mag...

Multiple Antioxidants and L-Arginine Modulate Inflammation and Dyslipidemia in Chronic Renal Failure Rats

“…[54] The renoprotective and antihypertensive effect of L-arginine may be related in part to reduction in vascular and renal endothelin-1(ET-1) production through increasing NO release both in the circulation and in the renal tissues. [32,55,56] Improvement of renal NO release may also prevent the development of glomerulosclerosis and the resulting hyperfiltration and proteinuria, which may be in part due to the reduction of glomerular ET-1 production. [57] In fact, NO inhibits the mitogenic response and the expression of extracellular matrix proteins induced by various agonists in mesangial cells, including endothelin-1.…”

Regulation of Blood Pressure and Salt Homeostasis by Endothelin

“…Explants from NOS1 knockout mice display an augmented AVP secretory response to ET B activation that is not further enhanced by l -NAME (636), suggesting that NOS1 rather than NOS3 is the primary source of NO responsible for the suppression of AVP. In experiments where the dominant negative construct of NOS1 was transfected into the paraventricular nucleus of baroreceptor-intact Long Evan rats, the pressor effect of ET-1 microinjected into the subfornical organ was potentiated and had shorter latency consistent with nitritergic suppression of inputs to the paraventricular nucleus (637).…”

Effects of nitric oxide on magnocellular neurons of the supraoptic nucleus involve multiple mechanisms

“…On the contrary, in vitro studies reveal different effects of NO on neurohypophysial hormone secretion. In rodent hypothalamic explants, NO suppressed VP secretion, an effect seen with NO donors SIN-1 and SNP (49,57). L-arginine also reduced VP release in this preparation, an effect reversed and reduced, respectively, by the NOS blocker L-NMMA and the addition of human hemoglobin, an NO scavenger (49).…”