Effects of temperature on human L-type cardiac Ca²⁺ channels expressed in Xenopus oocytes

Abstract

 Temperature normally affects peak L-type Ca²⁺ channel (CaCh) current with a temperature coefficient (Q ₁₀) of between 1.8 and 3.5; in cardiomyocytes attenuating protein kinase A activity increases Q ₁₀ whilst activating it lowers Q ₁₀. We examine temperature effects using cloned human cardiac CaChs expressed in Xenopus oocytes. Peak inward currents (I _Ba) through expressed CaChs (i.e. α_1Cα₂/δ_aβ_1b) exhibited a Q ₁₀ of 5.8±0.4 when examined between 15 and 25°C. The nifedipine-sensitive I _Ba exhibited a higher Q ₁₀ of 8.7±0.5, whilst the nifedipine-insensitive I _Ba exhibited Q ₁₀ of 3.7±0.3. Current/voltage (I/V) relationships shifted to negative potentials on warming. Using instead a different CaCh β subunit isoform, β_2c, gave rise to an I _Ba similar to those expressed using β_1b. We utilized a carboxyl deletion mutant, α_1C-Δ1633, to determine the temperature sensitivity of the pore moiety in the absence of auxiliary subunits; I _Ba through this channel exhibited a Q ₁₀ of 9.3±0.3. However, the Q ₁₀ for macroscopic conductance was reduced compared to that of heteromeric channels; decreasing from 5.0 (i.e. α_1Cα₂/δ_aβ_1b) and 3.9 (i.e. α_1Cα₂/δ_aβ_2c) to 2.4 (α_1C-Δ1633). These observations differ markedly from those made in studies of cardiomyocytes, and suggest that enhanced sensitivity may depend on the membrane environment, channel assembly or other regulatory factors.