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

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. alpha(1C) alpha(2)/delta(a) beta(1b)) exhibited a Q(10) of 5.8+/-0.4 when examined between 15 and 25 degrees 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 beta subunit isoform, beta(2c), gave rise to an I-Ba similar to those expressed using beta(1b). We utilized a carboxyl deletion mutant, alpha(1C-Delta 1633), 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. alpha(1C) alpha(2)/delta(a) beta(1b)) and 3.9 (i.e. alpha(1C) alpha(2)/delta(a) beta(2c)) to 2.4 (alpha(1C-Delta 1633)). 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.