Temperature-dependence of L-type Ca(2+) channel current in atrial myocytes from rainbow trout

Rainbow trout, Oncorhynchus mykiss, inhabit eurythermal environments and must therefore be able to cope with changes in environmental temperature. As ectotherms, their heart is required to maintain cardiac function over a range of ambient water temperatures. This raises important questions concerning the temperature-dependence of cardiac ion channel function in fish hearts, in particular, the channels involved in Ca(2+) transport. Thus, we studied the effects of acute, physiologically relevant temperature changes on the density and kinetics of the L-type Ca(2+) channel current (I(Ca)) in rainbow trout atrial myocytes using the whole-cell patch-clamp technique. Myocytes from fish acclimated to 14 degrees C were first tested at 14 degrees C, then at 21 degrees C and finally at 7 degrees C. Using a square-pulse voltage-clamp in the first series of experiments, the peak density of I(Ca) increased (Q(10)=1.9) as temperature was increased from 14 to 21 degrees C and decreased (Q(10)=2.1) as temperature was decreased from 14 to 7 degrees C. In contrast to current density, the charge carried by I(Ca) was inversely related to temperature as a result of changes in the kinetic properties of the channel; both the fast (tau(f)) and slow (tau(s)) components of inactivation were slower at 7 degrees C than at 14 and 21 degrees C. Action potentials were recorded at the three test temperatures and then used as voltage-clamp stimulus waveforms to reassess I(Ca) in a second series of experiments. While the temperature-dependency of I(Ca) was similar to that found with the square-pulse voltage-clamp, the charge carried by I(Ca) was temperature-independent. These results show that the temperature-dependency of I(Ca) in rainbow trout is in the lower range of that reported in mammals and, although this could have profound effects on Ca(2+) delivery to the myofilaments, the temperature-induced modifications in the action potential may help to maintain a fairly constant Ca(2+) delivery during an acute temperature change in rainbow trout.