Studies of Cerebral Osmoreceptors in Anesthetized Dogs: The Effect of Intravenous and Intracarotid Infusion of Hyper-Osmolar Sodium Chloride Solutions during Sustained Water Diuresis

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Abstract
The function of the suggested hypothalamic osmoreceptors was investigated in dogs during light chloralose anesthesia. The dogs were subjected to an i.v. load of 40 ml/kg body weight (bw) of a hypo-osmolar solution of NaCl and glucose. This degree of hydration was kept constant by a specially constructed servo system based on the weight of the animal. During water diuresis the renal free water clearance remained essentially constant for several hours (mean about 0.2 ml/kg bw .cntdot. min). Renal Na excretion was low (mean 0.82 .mu.mol/kg bw .cntdot. min) and decreased continuously throughout the experimental period. I.v. infusion of hyperosmolar NaCl solution (1.33 mmol/kg bw in 30 min) was followed by prolonged parallel increases in free water clearance and Na excretion, without any detectable change in the excretion of osmoles and K. The renal response to bilateral infusion of hyper-osmolar NaCl (1.33 mmol/kg bw in 30 min) into the common carotid arteries was identical to the response to i.v. infusion. The estimated increase in the osmolality of the carotid blood was 2.2%. In 7 out of 8 experiments intracarotid infusion of NaCl (1.33 mmol/kg bw in 8 min) did not elicit any reduction in free water clearance. On the contrary, an increase was similar to that obtained after i.v. infusion. The estimated increase in the osmolality of the carotid blood was 8.4%. The present results question the validity of the currently held view that hypothalamic osmoreceptors play an important role in the control of the osmolality of plasma.