Adrenergic mediation of vasopressin secretion in newborn pigs

Effects of inhibition of α₁- (prazosin) and α₂- (yohimbine) adrenoreceptors on plasma lysine vasopressin concentration during normotension and hemorrhagic hypotension were studied in unanesthetized newborn pigs. During the normotensive period, treatment with prazosin and yohimbine both increased plasma lysine vasopressin concentration (vehicle = 3.4 ± 1.21 μU/ml; prazosin = 35.8 ± 10.8 μU/ml; and yohimbine = 20.1 ± 9.5 μU/ml). Prazosin caused a decline in arterial pressure (vehicle = 60 ± 6 mmHg; prazosin = 49 ± 5 mmHg), which may account for the increase in plasma lysine vasopressin concentration, whereas yohimbine increased arterial pressure (73 ± 3 mmHg). On hemorrhage to equivalent arterial pressure, plasma lysine vasopressin concentration increased to similar levels in vehicle- (110.3 ± 28.7 μU/ml) and prazosin-treated (92.6 ± 14.2 μU/ml) piglets. In contrast, on hemorrhage of yohimbine-treated piglets, the increase in plasma lysine vasopressin concentration was augmented remarkably (527.0 ± 103.2 μU/ml) in comparison to the other groups. We conclude that, in unanesthetized newborn pigs, treatment with the α₂- adrenoceptor antagonist yohimbine increased plasma lysine vasopressin concentration and markedly accentuated the vasopressin response to hemorrhage. An α₂-adrenergic receptor-mediated mechanisms appears to be an important inhibitory component in the vasopressin secretory system of the newborn pig.