Controls of vasopressin secretion and thirst: similarities and dissimilarities in signals

RATS HAVE TWO PROMINENT RESPONSES TO DEHYDRATION: secretion of the antidiuretic hormone, vasopressin (VP), and thirst. Many signals directly affect both responses. These include two excitatory signals associated with increased plasma osmolality (pOsm) and two inhibitory signals associated with decreased pOsm (one each from cerebral and visceral osmoreceptors). In addition, both VP secretion and thirst during hypovolemia are stimulated by at least two signals, one neural (from cardiac baroreceptors) and one humoral (i.e., angiotensin II). In contrast to these evident similarities, there are definite dissimilarities in the signals for VP secretion and thirst. For example, acute hypervolemia inhibits VP secretion but not thirst, whereas acute hypertension inhibits thirst but not VP secretion. Thirst and VP secretion in rats each are stimulated by acute arterial hypotension, albeit not by the same signals; water intake is mediated by activation of the renin-angiotensin system but not by a neural signal from arterial baroreceptors, whereas the reverse may be true for the stimulation of VP secretion. These dissimilarities indicate that VP secretion and thirst in rats are neither invariably linked nor controlled by the same mechanisms.