Circannual changes in renin concentration, plasma electrolytes, and osmolality in the freshwater turtle

Seasonal changes in heart rate, blood pressure, plasma and renal renin concentrations, plasma electrolyte concentrations, and osmolality were studied in freshwater turtles (Amyda japonica) exposed to the natural environment. Seasonal changes in heart rate and blood pressure showed a marked increase in summer and a decrease in winter, which correlated directly with that of an environmental temperature. Two peaks in plasma renin concentration were observed in November and May and correlated with the preparation for hibernation and arousal periods. Renal renin content exhibited a marked seasonal variation characterized mainly by an increase in summer, fall, and winter and a decrease in spring. Plasma sodium concentration and osmolality were the highest in summer, the lowest in winter, and intermediate during spring and fall. A change in plasma osmolality correlated closely with that of temperature. However, no marked seasonal variations in the concentrations of plasma potassium, chloride, and calcium were observed. On the basis of these data, we suggest that a circannual rhythm of renin concentration may be related to hibernation. We also suggest that seasonal changes in sodium concentration and osmolality relate closely to environmental temperature, although the mechanisms are not clear.