Mechanism of cold diuresis in the rat

The effect of cold exposure (CE) on renal water excretion has not been clearly delineated. Conscious rats were exposed to decreased ambient temperature (15 degrees C). Forty-five minutes of CE resulted in reversible increases in urine flow and decreases in urine osmolality. The diuresis was not due to a diminished response to vasopressin (VP), as the antidiuresis associated with 500 microU of Pitressin given to water-diuresing rats was comparable at 15 and 30 degrees C. To determine whether the diuresis was due to intrarenal factors, glomerular filtration rate, renal blood flow, sodium excretion, and osmolar clearances were measured and found to be equivalent during control and cold conditions. To determine whether the observed diuresis was due to suppression of endogenous VP, VP-free Brattleboro rats undergoing a constant VP infusion were cold exposed. In these rats, CE was not associated with a change in either urine flow or urinary osmolality. This antidiuretic hormone-mediated mechanism was corroborated by a decrease in immunoassayable VP levels. To determine the mechanism whereby CE suppresses endogenous VP, plasma osmolality and hemodynamic parameters were measured. Although CE was not associated with a change in plasma osmolality, it did result in a significant increase in both mean arterial pressure and cardiac index. Pretreatment of rats with 6-hydroxydopamine prevented both the increase in mean arterial pressure and cold diuresis. We conclude that the diuresis observed upon exposure to 15 degrees C results from nonosmotic suppression of endogenous VP, as a consequence of the increase in mean arterial pressure.