Osmotic and hypovolemic thirst in spontaneously hypertensive rats.

Osmotic- and hypovolemic-induced water intake as well as urinary excretion of sodium and other solutes were compared in spontaneously hypertensive (SHR) and normotensive Wistar (NWR) rats. No significant differences were found between SHR and NWR in water intake and urinary excretion under conditions of euhydration and in response to 24 hr water deprivation. Administration of osmotic load (0.5 ml.100 g-1 body weight of 10% NaCl IV) elicited: (1) higher intake of water (by 1.77 +/- 0.60 ml.100 g-1 body weight), (2) lower urinary excretion of sodium and other solutes, and (3) greater retention of fluid in SHR than in NWR. Reduction of blood volume by amount equivalent to 1.8% of body weight decreased arterial blood pressure by 31.9 +/- 3.5 and 10.8 +/- 1.7 mmHg in SHR and NWR, respectively. In spite of significantly greater hypotension, bleeding elicited in SHR smaller elevation of water intake than in NWR (1.39 +/- 0.25 vs. 2.14 +/- 0.49 ml.100 g-1 body weight). The data provide evidence for existence of significant differences in the control of body fluid balance between SHR and NWR. It is suggested that hyperdipsia elicited by administration of the hyperosmotic load in SHR does not result from primary hyperresponsiveness of the thirst system to osmotic/sodium stimulation but is rather secondary to osmotic load retention.