Time for loss of increased cardiac responsiveness to isoproterenol in cold-acclimated rats after removal from cold.

Female rats exposed to air at 5 +/- 1 degree C for 12 weeks had a greater increase in heart rate in response to s.c. administration of d,l-isoproterenol (8 microgram/kg body weight) than warm-acclimated controls when both were tested in air at 25 +/- 1 degree C. After removal from cold for 24, 48, or 72 h, cold-acclimated rats still showed a greater responsiveness of heart rate to s.c. administration of isoproterenol (8 microgram/kg body weight) when compared with warm-acclimated controls. However, by 96 h after removal from cold, the responsiveness of heart rate to isoproterenol in the cold-treated group no longer differed from that of the warm-acclimated group. Hence, the increased beta-adrenergic responsiveness of heart rate in cold-acclimated rats was lost at some time between 72 and 96 h after removal from cold.