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 (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 (I(Ba)) through expressed CaChs (i.e. α(1C) α2/δ(a) β(1b)) exhibited a Q10 of 5.8±0.4 when examined between 15 and 25°C. The nifedipine-sensitive I(Ba) exhibited a higher Q10 of 8.7±0.5, whilst the nifedipine-insensitive I(Ba) 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, 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 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. α(1C) α2/δ(a) β(1b)) and 3.9 (i.e. α(1C) α2/δ(a) β(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.