Direct blood pressure measurements in rats during abrupt exposure to, and removal from, cold air

When male rats, each bearing an indwelling femoral arterial cannula, were exposed abruptly to cold (5 degrees C), systolic, diastolic, and mean blood pressures increased within 1 hr to levels significantly above those measured prior to exposure to cold. Blood pressures reached maximal levels within 2 hr of exposure to cold and remained elevated during the next seven days. Heart rates, measured at the same time, responded similarly. When cannulated rats that had been exposed to cold for four weeks were removed abruptly to 26 degrees C, systolic, diastolic, and mean blood pressures decreased nearly linearly for 5 hr. By two days after removal from cold, blood pressures stabilized at levels that were still elevated above those observed prior to exposure to cold. In contrast, heart rate returned to precold exposure level by the 13th day after removal from cold. The mechanisms responsible for the elevation of blood pressure during abrupt exposure of rats to cold and the decrease in blood pressure after removal from cold remain to be elucidated.