Neurohypophyseal peptides in hypothyroid rats: plasma levels and kidney response.

A degree of impairment of water excretion may be associated with hypothyroidism. The involvement of vasopressin has been suggested, but its role continues to be debated because of lack of studies where vasopressin was directly assayed. In this study, water excretion was assessed and arginine vasopressin was directly measured in unanesthetized and nonstressed normal and thyroidectomized rats at a basal state and after water loading. Following water load, both groups decreased their plasma osmolality. Plasma vasopressin demonstrated an elevated level in hypothyroid rats compared to control (2.04 and 1.04 μU/ml), respectively, at baseline and (1.32 and 0.68 μU/ml), respectively, after water loading. There was a significant and similar correlation between plasma vasopressin and plasma osmolality in both groups. The regression lines for urine osmolality and plasma vasopressin of the two groups were parallel but with a significantly greater vasopressin level for the hypothyroid rats at any given urine osmolality. This suggested that circulating plasma vasopressin was less active in hypothyroid than control rats; a hypothesis that was tested by measuring the responsiveness of renal medullary adenylate cyclase to vasopressin. Both basal and vasopressin stimulated cyclic AMP levels were less in hypothyroid than control rats. Thus, these studies demonstrate that hypothyroidism in the rat was associated with an elevated plasma vasopressin that did not appear to be fully effective in inducing an antidiuresis. Factors other than vasopressin may be more important in the water imbalance of hypothyroidism.