Effects of temperature and pH on cardiac myofilament Ca2+ sensitivity in rat and ground squirrel.

Chemically skinned papillary muscles from active and hibernating ground squirrels were used to determine whether the enhanced cardiac contractility observed in hibernation is due to a change in myofilament Ca2+ sensitivity. A similar preparation from rats was used to reflect the changes in a nonhibernator. When examined at pH 7.00 in all three groups and under physiological pH with varying temperatures in the ground squirrels, the calcium concentration at which muscle tension is at 50% maximum (pCa2+50) decreased significantly (P < 0.05) with decreasing temperature (25, 15, and 5 degrees C). When hibernating and active ground squirrels were compared, no significant difference in pCa2+50 was observed at 25 degrees C; however, the values at 15 and 5 degrees C were significantly higher (P < 0.05) in the hibernating squirrels. The results indicate that cardiac myofilament Ca2+ sensitivity decreases significantly at low temperature in both active and hibernating ground squirrels; however, the higher Ca2+ sensitivity in the hibernating squirrels at 15 and 5 degrees C could partially contribute to the enhanced cardiac contractility typically seen during hibernation.