Relationship between steady-state activation and availability of cardiac sodium channel: evidence of uncoupling

The coupling between steady-state activation and availability from inactivation was characterized for the cardiac Na⁺ channel. To evaluate this coupling, we plotted the relationship between the conductance and availability curve midpoint potentials measured in 92 rat ventricular cardiomyocytes and applied a correlation analysis. We found a high correlation between the midpoints (correlation coefficient = 0.86, slope = 0.95) within the availability midpoint potential range positive to -100 mV. In contrast, the midpoints were not correlated in the myocytes (37 of 92 cells) having mid point potential negative to -100 mV, indicating an uncoupling between activation and availability. Received 1 October 1997; received after revision 28 October 1997; accepted 13 November 1997