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.