Development of hypertension in rats during chronic exposure to cold

Resting systolic, diastolic, and mean blood pressures (MBP), as well as heart rates, of unanesthetized, unrestrained, cold-acclimated (CA, 4 wk, 6 degrees C) rats were measured by direct arterial cannula and compared with those of controls maintained at 25 degrees C. Exposure to cold increased all these measurements significantly. Mean heart weight of CA rats was also increased significantly above that of controls. The responsiveness of MBP and heart rate to administration of the beta-adrenergic agonist, isoproterenol (3, 5, and 8 micrograms/kg ip), to unanesthetized, unrestrained, CA rats during exposure to air at 6 degrees C was similar to, and possibly less than, that of warm-acclimated (WA) rats measured at 25 degrees C. Acute administration of the alpha-adrenergic agonist, phenylephrine (100 micrograms/kg ip), to CA rats while in air at 6 degrees C induced less of a change in MBP from pretreatment level than was observed in WA rats. However, no differences were observed between groups when changes in heart rate from pretreatment level were compared. A similar statement may be made for a higher dose of phenylephrine (150 micrograms/kg ip), although MBP were elevated to higher levels in both groups with the higher dose. Abrupt exposure of WA rats to cold (6 degrees C) resulted in a sharp increase in heart rate and a more gradual increase in MBP over a period of 1 h. Removal of CA rats from 6 to 25 degrees C resulted in a gradual decrease in heart rate with no significant change in MBP during the ensuing hour.(ABSTRACT TRUNCATED AT 250 WORDS)