Na+ currents are required for efficient excitation-contraction coupling in rabbit ventricular myocytes: a possible contribution of neuronal Na+ channels.

Ca2+ transients were activated in rabbit ventricular cells by a sequence of action potential shaped voltage clamps. After activating a series of control transients, Na+ currents (INa) were inactivated with a ramp from -80 to -40 mV (1.5 s) prior to the action potential clamp. The transients were detected with the calcium indicator Fluo-4 and an epifluorescence system. With zero Na+ in the pipette INa inactivation produced a decline in the SR Ca2+ release flux (measured as the maximum rate of rise of the transient) of 27 ± 4% (n = 9, P < 0.001) and a peak amplitude reduction of 10 ± 3% (n = 9, P < 0.05). With 5 mm Na+ in the pipette the reduction in release flux was greater (34 ± 4%, n = 4, P < 0.05). The ramp effectively inactivates INa without changing ICa, and there was no significant change in the transmembrane Ca2+ flux after the inactivation of INa. We next evoked action potentials under current clamp. TTX at 100 nm, which selectively blocks neuronal isoforms of Na+ channels, produced a decline in SR Ca2+ release flux of 35 ± 3% (n = 6, P < 0.001) and transient amplitude of 12 ± 2% (n = 6, P < 0.05). This effect was similar to the effect of INa inactivation on release flux. We conclude that a TTX-sensitive INa is essential for efficient triggering of SR Ca2+ release. We propose that neuronal Na+ channels residing within couplons activate sufficient reverse Na+-Ca2+ exchanger (NCX) to prime the junctional cleft with Ca2+. The results can be explained if non-linearities in excitation-contraction coupling mechanisms modify the coupling fidelity of ICa, which is known to be low at positive potentials.