To assess the possible role of circulating enkephalins in the osmotic release of vasopressin, Met5 (M-Enk)- or Leu5-enkephalin (L-Enk) dissolved in 0.9 or 10% NaCl was infused to either anesthetized or conscious dogs at a dose of 3.5 nM.kg-1.min-1 for 15 min. Intravenous infusion of M-Enk or L-Enk produced 150- to 200-fold increases in their plasma levels, and the elevated levels were maintained during the infusion. Although blood pressure (BP) in anesthetized dogs decreased significantly during L-Enk infusion, BP in conscious dogs was unaffected by both enkephalins. Neither M-Enk nor L-Enk infusion affected plasma vasopressin concentration (PAVP) in these anesthetized and conscious dogs that were not osmotically stimulated. PAVP in conscious time control dogs increased significantly after start of 10% NaCl infusion in spite of an increase in BP. M-Enk or L-Enk significantly blunted the increase in PAVP induced by 10% NaCl infusion. This attenuation was not accompanied by any significant changes in plasma sodium concentration and BP compared with those of time control dogs. Thus increase in plasma enkephalins attenuates the osmotic release of vasopressin.
Animals Arginine Vasopressin