Role of antidiuretic hormone in fasting-induced natriuresis in conscious rats.

Urinary sodium output increases in the first several days of fasting both in humans and rabbits. The major causal factor of the fasting-induced natriuresis was proposed to be glucagon (1), but the mechanism remains obscure. We have recently observed that fasting for one to four days resulted in the negative sodium balance in rats provided a low salt diet and saline (2). The fasting-induced natriuresis in the rat revealed two aspects, one dependent on the level of sodium intake, and the other not (3). The latter seems to be largely concerned with the negative value of sodium balance in the lower level of sodium intake maintained relatively constant. On the other hand, it was indicated in the clearance experiments using the method of i.v. infusion of hypertonic saline in anesthetized rats that sodium excretion was markedly increased by fasting for one to five days (4). This may relate to the sodium intake-dependent natriuresis of fasting in conscious rats, since the total sodium intake seen with i.v. administration was much larger than the amount ingested orally by rats (16.4 mEq/ day as compared with 1.68+0.05 mEq/day). The present study was undertaken to elucidate the possible resemblance of the cause of the fasting-induced natriuresis between conscious and anesthetized rats using the forced p.o. loading method of 2 sodium chloride in conscious rats. Furthermore, the possible involvement of antidiuretic hormone (ADH) in the natriuresis of fasting under water or solute diuresis was evaluated. Female Wistar rats (Clea, Japan) were maintained on a CE-2 diet (Clea, Japan) and water ad libitum. The basal experimental conditions were the same as described in the previous report (3) except that a regular diet was provided in place of a low sodium diet. Only food was withheld at 9 a.m. in one group of rats, in which the mean body weight was equal to that in the other group of rats without deprivation of food. On the next day, each rat was individually housed in a metabolism cage, and ingested 2 % (w/v) sodium chloride solution via an oral-gastric tube in a volume of 2 ml/animal at intervals of 1 hr. The urine was collected over a 2 hr period counted from the first hr after the first loading. Urine flow gradually increased and was approximately constant in the 3-5 hr period afterward, in which the following determinations were made. Urinary sodium and potassium concentrations and osmolality were …