Divergent estimates of ratio between Na+-Ca2+ current densities in t-tubular and surface membranes of rat cardiomyocytes.

The ratio between Na+-Ca2+ exchange current densities in t-tubular and surface membranes of rat ventricular cardiomyocytes (JNaCa-ratio) estimated from electrophysiological data published to date yields strikingly different values between 1.7 and nearly 40. Possible reasons of such divergence were analysed by Monte-Carlo simulations assuming either normal or log-normal distribution of measured data. The confidence intervals CI95 of the mean JNaCa-ratios computed from the reported data showed an overlap of values between 1 and 3 and between 0.3 and 4.3 in the case of normal and log-normal distribution, respectively. Further analysis revealed that the high published values likely result from a large scatter of data due to transmural differences in JNaCa, dispersion of cell membrane capacitances and variability in incomplete detubulation. Taking into account the asymmetric distribution of measured data, reduction of mean current densities after detubulation, and substantially smaller CI95 of lower values of the mean JNaCa-ratio, the values between 1.6 and 3.2 may be considered as the most accurate estimates. This implicates that 40 to 60% of Na+-Ca2+ exchanger is located at the t-tubular membrane of adult rat cardiomyocytes.