Nitric Oxide Modulation of ETB Receptor-induced Vasopressin Release By Rat and Mouse Hypothalamo-neurohypophysial Explants

Endothelin (ET) peptides stimulate 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 pharmacologic approach in hypothalamo-neurohypophysial explants from rats, and an alternative strategy using explants from mice with an inactivating mutation of neuronal NOS (nNOS-/-) 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 to 10 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 ± 63 (rat) and 422 ± 112% basal/explant/h (mouse). Sodium nitroprusside (SNP, 0.1 mM) suppressed maximal AVP release to basal values. Nnitro-L-arginine methyl ester (L-NAME, 0.1 μM), which did not itself stimulate AVP secretion, more than doubled the response to 1 pM IRL 1620, from 136 ± 28 to 295 ± 49% basal/explant/h (p < 0.05) by rat explants. Explants from wild type mice responded similarly. Explants from nNOS-/mice had higher basal AVP secretory rate in response to 1 pM IRL 1620: 271 ± 48 compared with 150 ± 24% basal/explant/h (p < 0.05) from wild type murine explants. In the nNOS-/-, SNP suppressed stimulated release, and L-NAME exerted no additional stimulatory effect: 243 ± 38% basal/explant/h. Thus, nitric oxide inhibits the AVP secretory response induced by ETB receptor activation within the hypothalamo-neurohypophysial 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.