Effects of inhibition of alpha 1-(prazosin) and alpha 2-(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 microU/ml;
prazosin = 35.8 +/- 10.8 microU/ml; and
yohimbine = 20.1 +/- 9.5 microU/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 microU/ml) and
prazosin-treated (92.6 +/- 14.2 microU/ml) piglets. In contrast, on
hemorrhage of
yohimbine-treated piglets, the increase in plasma
lysine vasopressin concentration was augmented remarkably (527.0 +/- 103.2 microU/ml) in comparison to the other groups. We conclude that, in unanesthetized newborn pigs, treatment with the alpha 2-adrenoreceptor antagonist
yohimbine increased plasma
lysine vasopressin concentration and markedly accentuated the
vasopressin response to
hemorrhage. An alpha 2-adrenergic receptor-mediated mechanism appears to be an important inhibitory component in the
vasopressin secretory system of the newborn pig.