THE ROLE OF ANTIDIURETIC-HORMONE IN COLD-INDUCED DIURESIS IN THE ANESTHETIZED RAT

The aim of this study was to investigate whether the increased diuresi s in consequence of hypothermia is due to a depression of the hypothal amic release of antidiuretic hormone (ADH). The plasma concentration o f antidiuretic hormone and the effect of intravenous (i.v.) administra tion of 65 ng kg(-1) desmopressin (selective V-2-receptor agonist) wer e determined in the anaesthetized rat. In spite oi a 50% (P < 0.001) d ecrease in glomerular filtration rate, urine flow increased sixfold (P < 0.01) and urine sodium excretion increased sevenfold (P < 0.05), wh ereas urine osmolality decreased (P < 0.001). At the same time plasma antidiuretic hormone decreased from 7.5 +/- 1.1 to 3.8 +/- 0.4 pg mL(- 1) (P = 0.01). After injection of desmopressin urine flow was complete ly restored, whereas urine osmolality and sodium excretion were only p artially normalized Since tubular conservation of water and fractional water reabsorption decreased during hypothermia, the diuresis must ha ve resulted from an augmented loss of water. This is further supported by the fact that osmolal excretion was not influenced either by hypot hermia or by desmopressin. it is concluded that the diuresis in conseq uence to hypothermia is due both to a decrease in the release of ADH a nd to a reduction of renal medullary hypertonicity.