Effect of constant light and darkness on the circadian rhythms in rats. II. Plasma renin activity and insulin concentration.

Plasma renin activity (PRA), immunoreactive insulin level (IRI), plasma sodium osmolality, and hematocrit were studied in rats decapitated at 08, 16 and 24 h. The animals were subjected either to normal 12/12 h light/dark (LD) cycle, or to constant light (LL) or darkness (DD). Under LD PRA and hematocrit were maximal at 16 h, IRI--at 08 and 24 h, and plasma osmolality--at 08 h. Plasma sodium was devoid of diurnal variations. After 16 days of LL all circadian rhythms studied were eliminated. After 16 days of DD the circadian rhythms of IRI, plasma osmolality and hematocrit were eliminated, while PRA shifted its maximum at 24 h. We concluded that LD cycle is a potent external synchronizer of the circadian rhythms of PRA and IRI in rats. The PRA circadian rhythm is not dependent on diurnal variations in body hydration.