Factors Affecting Cold-Induced Hypertension in Rats

Abstract

A 3- to 4-week exposure of rats to a cold environment (5 ± 2°C) induces hypertension, including elevation of systolic, diastolic, and mean blood pressures and cardiac (left ventricular) hypertrophy. The studies described here were designed to investigate some factors affecting both the magnitude and the time course for development of cold-induced hypertension. The objective of the first study was to determine whether there was an ambient temperature at which the cold-induced elevation of blood pressure did not occur. The objective of the second experiment was to determine whether body weight at the time of exposure to cold affected the magnitude and time course for development of hypertension. To assess the first objective, male rats were housed in a chamber whose temperature was maintained at 5 ± 2°C while others were housed in an identical chamber at 9 ± 2°C. After 7 days of exposure to cold, the rats exposed to the colder temperature had a significant elevation of blood pressure (140 ± 2 mm Hg) compared with the group maintained at 9°C (122 ± 3 mm Hg). The rats exposed to 9°C had no significant elevation of systolic blood pressure at either 27 or 40 days after initiation of exposure to cold. At the latter time, the temperature in the second chamber was reduced to 5 ± 2°C. By the 25th day of exposure to this ambient temperature, the rats had a significant increase in systolic blood pressure above their levels at 9°C. Thus, there appears to be a threshold ambient temperature for elevation of blood pressure during exposure to cold. That temperature appears to lie somewhere between 5 and 9°C. The second objective was assessed by placing rats varying in weight from approximately 250 to 430 g in air at 5°C. There was a highly significant direct relationship (r = 0.96) between body weight at the time of introduction to cold and the number of days required to increase systolic blood pressure by 10 mm Hg above pre-cold exposure level. The third objective was to make an initial assessment of potential differences among strains of rats with respect to development of cold-induced hypertension. To this end, rats of the Fischer 344 strain were used. Systolic blood pressures of these rats also increased during chronic exposure to cold. Thus, both ambient temperature and body weight are important factors in the induction of hypertension in cold-treated rats. Furthermore, the development of cold-induced hypertension does not appear to be the function of a specific strain of rats. Hence, chronic exposure to cold appears to be a promising model for the induction of hypertension in rats without the need for surgical intervention, excessive doses of hormones, or genetic manipulation.