Temperature dependence of Cav1.4 calcium channel gating.

The CACNA1F gene encodes the pore-forming subunit of the L-type Cav1.4 voltage-gated calcium channel (VGCC) and plays a central role in tonic vesicular release at photoreceptor ribbon synapses. The main objective of this study was to examine the effects of temperature on human Cav1.4 cDNA clone VGCCs. With 20 mM Ba2+ as charge carrier, increasing the temperature from 23 degrees C to 37 degrees C increases whole-cell conductance, shifts the voltage-dependence of activation to more hyperpolarized voltages, and accelerates the degree of recovery from inactivation over a given time, but does not significantly alter the half-inactivation potential (Vh). The window current for Cav1.4 was also shifted to more hyperpolarized voltages, observable from approximately -35 mV to +20 mV at 37 degrees C in 20 mM Ba2+. Several comparable results were observed when characterizing Cav1.2 at temperatures ranging from 23 degrees C to 37 degrees C. However, one difference between Cav1.4 and Cav1.2 was the temperature dependence of voltage-dependent inactivation kinetics. Increasing temperature from 23 degrees C to 37 degrees C accelerates Cav1.4 inactivation kinetics approximately 50-fold, whereas Cav1.2 only accelerates approximately 10-fold over the same temperature range. The time constant of inactivation (tauh) temperature coefficient (Q10) was 18.8 for Cav1.4 over a temperature range of 23 degrees to 33 degrees C (corresponding to an activation energy Ea=221 kJ/mol), compared with Cav1.2 with a Q10 of 3 (Ea=90 kJ/mol) recorded under identical conditions. In addition, Cav1.4 was also tested using 2 mM Ca2+ as a charge carrier and similar changes in current-voltage Boltzmann parameters and gating kinetics were observed. Hence, despite the accelerated inactivation kinetics of Cav1.4 channels observed at near physiological temperatures the window current is preserved and could allow for tonic glutamate release from photoreceptors in the retina during dark adapted conditions.