TEMPERATURE-DEPENDENCE OF ELECTROPHYSIOLOGICAL PROPERTIES OF GUINEA-PIG AND GROUND-SQUIRREL MYOCYTES

The effects of changing temperature on the electrophysiology of isolated cardiac myocytes of the guinea pig and Richardson's ground squirrel were studied by patch-clamp techniques. In cells from both species, the resting membrane potential declined on cooling from 36 to 12-degrees-C by approximately 6 mV. The duration of the plateau of the action potential in guinea pig cells increased monotonically on cooling. In contrast, the action potential of ground squirrel cells showed a biphasic response, increasing in duration from 36 to 24-degrees-C and then decreasing on cooling from 24 to 12-degrees-C. From voltage-clamp studies, the properties of L-type calcium currents (I(Ca)) on cooling were compared in the two species and were found to be similar: In both cases, I(Ca) decreased in amplitude from approximately 2 nA peak current at 36-degrees-C to < 400 pA at 12-degrees-C. The Q10 of both the maximum amplitude and time to peak for I(Ca) in both species was approximately 1.8. The time for half inactivation had a greater Q10 of 2.5-3. It is concluded that, surprisingly, factors affecting the resting membrane potential and properties of L-type calcium channels are not major contributors to cardiac dysfunction on cooling. Rather, it is sarcomplasmic reticulum calcium release and reuptake that are likely to be the most important cold-sensitive processes.