NO synthase is present in magnocellular neurons of supraoptic and paraventricular nuclei as well as in the posterior pituitary gland and may participate in control of vasopressin secretion. To test this possibility, experiments were performed in conscious, chronically prepared rabbits to determine the effect of NO synthesis inhibition with NG-nitro-L-arginine methyl ester hydrochloride (L-NAME) on basal vasopressin secretion and vasopressin responses to increased plasma osmolality (hypertonic saline infusion; P osm) and decreased blood pressure (nitroprusside infusion). L-NAME infusion (0.5 mg.kg-1 x min-1 i.v.) increased mean arterial pressure [MAP; 82.6 +/- 3.4 to 93.0 +/- 3.0 mmHg (P < 0.02)], decreased heart rate [HR; 242 +/- 12 to 209 +/- 9 beats/min (P < 0.02)], decreased plasma renin activity [PRA; 3.1 +/- 0.6 to 2.0 +/- 0.6 ng.ml-.2 h-1 (P < 0.001)], and increased plasma vasopressin concentration [P AVP; 2.2 +/- 0.3 to 4.5 +/- 1.0 pg/ml (P < 0.05)]. P(osm) did not change. Hypertonic saline infusion did not change MAP or HR but decreased PRA [4.3 +/- 0.8 to 0.9 +/- 0.2 ng.ml-1 x 2 h-1 (P < 0.01)], increased P(osm) [284 +/- 1 to 305 +/- 2 mosmol/kg H2O (P < 0.001)], and increased PAVP [2.8 +/- 0.3 to 12.7 +/- 2.7 pg/ml (P < 0.01)].(ABSTRACT TRUNCATED AT 250 WORDS)

The results indicated that inhibition of endogenous oxytocin affects the endocrinology of the ovulatory cycle in women, and strongly suggest that oxytocin has a role in the physiological processes of LH regulation.

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