Effects of temperature on human L-type cardiac Ca 2+ channels expressed in Xenopus oocytes

Temperature normally affects peak L-type Ca2+ channel (CaCh) current with a temperature coefficient (Q10) of between 1.8 and 3.5; in cardiomyocytes attenuating protein kinase A activity increases Q10 whilst activating it lowers Q10. We examine temperature effects using cloned human cardiac CaChs expressed in Xenopus oocytes. Peak inward currents (IBa) through expressed CaChs (i.e. α1Cα2/δaβ1b) exhibited a Q10 of 5.8±0.4 when examined between 15 and 25°C. The nifedipine-sensitive IBa exhibited a higher Q10 of 8.7±0.5, whilst the nifedipine-insensitive IBa exhibited Q10 of 3.7±0.3. Current/voltage (I/V) relationships shifted to negative potentials on warming. Using instead a different CaCh β subunit isoform, β2c, gave rise to an IBa similar to those expressed using β1b. We utilized a carboxyl deletion mutant, α1C-Δ1633, to determine the temperature sensitivity of the pore moiety in the absence of auxiliary subunits; IBa through this channel exhibited a Q10 of 9.3±0.3. However, the Q10 for macroscopic conductance was reduced compared to that of heteromeric channels; decreasing from 5.0 (i.e. α1Cα2/δaβ1b) and 3.9 (i.e. α1Cα2/δaβ2c) to 2.4 (α1C-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