Effects of temperature on calcium current of bullfrog sympathetic neurons.

1. The temperature dependence of whole-cell calcium current was studied in bullfrog sympathetic neurons. 2. The factor by which the peak calcium current increased upon a 10 degrees C increase in temperature (Q10) was 1.6 +/- 0.1 (mean +/- S.E.M., n = 9), for a change from 20 to 30 degrees C. 3. Activation and deactivation were more rapid at a higher temperature. The Q10 was approximately 7 in the middle of the voltage range (near -10 mV), where the kinetics were slowest. Time constants were less temperature dependent at more positive or negative voltages (Q10 approximately 2 at -70 mV). 4. Near -10 mV, activation and deactivation were associated with a large enthalpy and a large positive entropy change. Deactivation at -70 mV reflected a smaller enthalpy change, and almost no change in entropy. 5. Activation is only slightly more temperature dependent than deactivation, when both are measured at the same voltage. 6. The peak current shifts slightly (approximately 5 mV) to more negative voltages upon a change from 20 to 30 degrees C. 7. Inactivation has a Q10 of approximately 2 at -10 mV. 8. Changes in the kinetics of activation or inactivation could be observed during recording at a constant temperature. These changes were generally small, especially for activation kinetics, and could be distinguished from temperature-dependent changes. 9. The changes in entropy and Q10 with voltage suggest that the rate-limiting steps for activation and deactivation are different at extreme voltages vs. the middle of the voltage range.