Osmoregulatory function in summer and winter acclimatized painted turtles, Chrysemys picta.

Abstract

The painted turtle, Chrysemys picta, is an air breathing ectotherm that hibernates submerged in fresh water. It faces a hypoosmotic environment and the persistent problem of influx of water and efflux of electrolytes across epithelial surfaces. Aquatically hibernating turtles in a dormant hypometabolic state face these osmotic challenges at a time when the capacity for energy utilization is very low. The purpose of this research is to examine the osmoregulatory function of summer- and winter-acclimatized painted turtles. Painted turtles in southeast Michigan hibernated at sites that never froze but became anoxic for extended periods during the winter. Arousal from hibernation in spring was most closely correlated with reversal of the vertical thermal gradient. Exposure to cold led to an increase in body mass in fasted turtles. Total body water (TBW) expressed as the percent of body mass was greater in winter- than summer-acclimatized turtles and increased with decreasing temperature. Thus it appeared an accumulation of environmental water was responsible for the increase in body mass. 83% of the increase in TBW was accounted for by expansion of the extracellular fluid space. Winter acclimatization resulted in a decrease in plasma sodium concentration. Winter animals showed a decrease in water influx compared with summer animals, but could not be explained by a change in skin permeability. Urine flow (V) and glomerular filtration rate (GFR) increased with temperature. Neither GFR nor V underwent seasonal acclimatization. The inulin U/P ratio also increased with temperature but showed no seasonal acclimatization. Thus, temperature affected both glomerular and tubular function. Experimental hydration elicited a marked increase in GFR and V. In hydrated turtles, V and GFR increased with temperature, but did not undergo acclimatization. Inulin U/P did not change with hydration indicating that hydration affects glomerular rather than tubular function. Winter acclimatization and temperature had profound effects on osmoregulatory function of painted turtles. However, winter animals limited water accumulation and body fluid dilution by (1) reducing the rate of water influx, (2) maintaining urine flow, and (3) increasing GFR and urine flow when water is accumulated. These adjustments may permit long-term submergence at low temperature during hibernation.