EFFECT OF HYPOTENSION AND HYPEROSMOLALITY ON VASOPRESSIN AND ACTH RESPONSES TO HYPOGLYCEMIA IN CONSCIOUS DOGS

The purpose of these studies was, first, to determine whether hypertonic saline (HS) infusion or nitro-prusside (NiPr)-induced hypotension augments the vasopressin (AVP) and adrenocorticotropic hormone (ACTH) responses to insulin (Ins)-induced hypoglycemia and, second, to determine whether neurohypophysectomy could attenuate the augmentation. Conscious, male dogs (n = 8) underwent two different types of experiments. In the first, Ins was preceded by either a 30-min infusion of normal saline (control) or HS to raise plasma osmolality and AVP. HS augmented the AVP response but diminished the ACTH response to Ins. In the second group of experiments, Ins was preceded by a controlled decrease in mean arterial pressure using NiPr, which led to an increase in AVP and ACTH. The initial ACTH and AVP response to Ins was augmented by NiPr, but this early augmentation was not sustained. Neurohypophysectomy attenuated the early augmentation of the ACTH response to Ins by NiPr, but did not alter the final ACTH level achieved. We conclude that HS augmented the AVP but inhibited the ACTH response to Ins probably because of expansion of plasma volume. Concomitant hypotension led to an augmentation of the early but not sustained AVP and ACTH response to Ins. Neurohypophysectomy eliminated this augmentation, suggesting a role for AVP from the neural lobe in the early ACTH response to combined hypotension and Ins-induced hypoglycemia.