Temperature dependence of macroscopic L-type calcium channel currents in single guinea pig ventricular myocytes

Introduction: Lowering temperature greatly reduces calcium influx through calcium channels. Studies on a number of tissues demonstrate that the peak inward current, I-Ca, exhibits Q(10), values ranging from 1.8 to 3.5; however, it remains unclear which component(s) of calcium channel gating may give rise to this large temperature sensitivity. Components of gating that may affect channel availability include phosphorylation and changes in [Ca2+], processes that vary in pertinence depending on the channel examined. This study addresses this problem by examining the temperature sensitivity (from 34 degrees to 14 degrees C) of cardiac I-Ca under control conditions, during attenuation or activation of protein kinase A (PKA) activity, and when intracellular [Ca2+] has been elevated. Methods and Results: I-Ca was studied using the whole cell configuration of the patch clamp technique. In control, lowering temperature from 34 degrees to 24 degrees C resulted in a shift in the potential for maximum slope (V-a) and the peak current (Y-max) toward more positive membrane potentials. The Q(10) values for the decrease in Y-max and the macroscopic slope conductance (G(max)), which reflects the number of available channels, were 3.15 +/- 0.19 and 2.57 +/- 0.13, respectively. At 0 mV the Ca2+ current decayed biexponentially, and the two time constants (tau(1) and tau(2)) showed Q(10) values of 1.79 +/- 0.21 and 2.06 +/- 0.38, while their contribution to the total current (I-1 and I-2) showed a Q(10) of 5.99 +/- 0.83 and 1.61 +/- 0.22. In myocytes loaded with inhibitors of the PKA cycle sufficient to inhibit the increase of I-Ca to 1 mu M isoprenaline, the Q(10) values for some of the kinetic parameters were increased with the Q(10) for I-1 increasing to 17.06 +/- 3.48. Stimulation of ICa by exposing myocytes to 1 mu M isoprenaline reduced the temperature sensitivity of Y-max G(max) and I-1, yielding respective values of 2.00 +/- 0.18, 1.85 +/- 0.07, and 2.04 +/- 0.15. Raising [Ca2+](i) to enhance Ca-i(2+)-dependent inactivation, while affecting inactivation and activation kinetics, affected temperature sensitivity little compared to control. The Q(10) for time to peak changed little under experimental conditions (2.3 to 2.4). Conclusions: Increasing the phosphorylated states of calcium channels, but not Ca-i(2+)-dependent inactivation, reduces temperature sensitivity of certain gating parameters. The data suggest that the rate of the transitions between the unavailable and also between the various closed states are changed in the opposite direction to that induced by PKA-dependent phosphorylation. Processes, e.g., inhibitory mechanisms, may be involved to maintain channels in unavailable or ''unphosphorylated'' states, and it may be these that contribute to the high Q(10) of macroscopic channel currents.