Temperature dependence of Ca v1.4 calcium channel gating

Abstract

The CACNA1F gene encodes the pore-forming subunit of the L-type Ca v1.4 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 Ca v1.4 cDNA clone VGCCs. With 20 mM Ba 2+ as charge carrier, increasing the temperature from 23 °C to 37 °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 (V h). The window current for Ca v1.4 was also shifted to more hyperpolarized voltages, observable from ∼−35 mV to +20 mV at 37 °C in 20 mM Ba 2+. Several comparable results were observed when characterizing Ca v1.2 at temperatures ranging from 23 °C to 37 °C. However, one difference between Ca v1.4 and Ca v1.2 was the temperature dependence of voltage-dependent inactivation kinetics. Increasing temperature from 23 °C to 37 °C accelerates Ca v1.4 inactivation kinetics ≈50-fold, whereas Ca v1.2 only accelerates ≈10-fold over the same temperature range. The time constant of inactivation (τ h) temperature coefficient (Q 10) was 18.8 for Ca v1.4 over a temperature range of 23° to 33 °C (corresponding to an activation energy E a=221 kJ/mol), compared with Ca v1.2 with a Q 10 of 3 (E a=90 kJ/mol) recorded under identical conditions. In addition, Ca v1.4 was also tested using 2 mM Ca 2+ as a charge carrier and similar changes in current–voltage Boltzmann parameters and gating kinetics were observed. Hence, despite the accelerated inactivation kinetics of Ca v1.4 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.