Responses of water and salt regulating hormones during acute cold exposure in the rat

Abstract

Soon after exposure to cold, humans and some animals experience a dilute diuresis, which is often accompanied by a natriuresis. Plasma vasopressin has been reported to decrease, possibly due to a central shunting of blood from peripheral vasoconstriction, and subsequent activation of the "Gauer-Henry reflex". The sodium regulating hormones atrial natriuretic factor (ANF), and the renin-aldosterone system have not been measured acutely in the cold. The present study used the conscious, chronically instrumented rat to assess plasma vasopressin, aldosterone, renin, ANF, and urinary vasopressin during acute cold exposures of 1 hour at 13° C., or time control at 26° C.. Urine flow, osmolality, electrolyte excretion rates, and creatinine, osmotic, and free water clearances were measured at 10 minute intervals. Hematocrit, plasma electrolytes and osmolality, and all hormones were obtained using a donor replacement protocol at baseline, at 20, and 60 minutes of cold exposure, and at 1 hour recovery. Rectal temperature, heart rate, and mean arterial blood pressure were monitored, and to evaluate the degree of cold-induced thoracic engorgement, central venous pressure was measured in one series of experiments. In all series of experiments, urine flow increased to double or more baseline values by 20 minutes of cold exposure (p < .05). By 60 minutes, in the cold, however, flow had returned toward baseline, and was different neither from baseline nor from time control. The free water component of the diuresis followed the same pattern, while urinary osmolality decreased biphasically as well. Osmotic clearance significantly increased in all series, and creatinine clearance was unchanged. Plasma vasopressin was significantly reduced compared to baseline at 20 minutes of cold exposure in all three series in which it was measured. Values had returned to baseline levels by 60 minutes in the cold. Mean arterial blood pressure and heart rate increased significantly in the cold (p < .01), while all but one experimental series showed no change in plasma osmolality, and in no series was there a change in rectal temperature. Mean central venous pressure was unaffected by cold exposure, although systolic and pulsatile central venous pressure increased. Cold was without effect on sodium excretion, with both control and cold-exposed groups significantly increasing their rate of excretion. Similarly, there was no change in plasma ANF, although both renin and aldosterone increased progressively during cold exposure. These results suggest that there is no consistent natriuresis in the rat during acute cold exposure, and that an increase in arterial blood pressure may be the driving force behind the observed reductions in plasma vasopressin.