Thermoregulatory and Cardiac Responses of Infant Spontaneously Hypertensive and Wistar-Kyoto Rats to Cold Exposure

Cardiovascular function during cold exposure is dependent on effective thermoregulation. This dependence is particularly apparent in infants. For example, we have previously demonstrated that in infant rats during cold exposure, cardiac rate is directly related to their ability to produce heat endogenously. The primary source of endogenous heat production for infant rats is brown adipose tissue (BAT). Because of the dependence of cardiac rate on effective thermoregulation in the cold and because hypertension in spontaneously hypertensive rats (SHR) is influenced by the preweanling environment, in this study we examined the thermoregulatory and cardiac rate responses of infant SHR and Wistar-Kyoto rats (WKY) to varying levels of cold exposure. In experiment 1, 7- to 8-day-old SHR and WKY were acclimated at a thermoneutral air temperature (35 degrees C) and then exposed to successive decreases in ambient temperature (30.5 degrees C, 26.5 degrees C, 23 degrees C, and 17 degrees C) while thermal and metabolic measures were recorded. Although both strains increased BAT thermogenesis and oxygen consumption in response to cold exposure, SHR cooled more than WKY and exhibited lower levels of oxygen consumption at the lowest air temperatures. Experiment 2 was identical to experiment 1 except that cardiac rate was also measured. Again, SHR exhibited substantial thermoregulatory deficits compared with WKY; in addition, they were less able than WKY to maintain cardiac rate at the 2 lowest air temperatures tested. Finally, in experiment 3, infant SHR exhibited diminished BAT thermogenesis in response to a range of doses of a selective beta3-adrenoceptor agonist. We hypothesize that long-term thermoregulatory deficits during the early postnatal period influence cardiovascular function and contribute to the development of hypertension in SHR.