Na/Ca exchange and Na/K-ATPase function are equally concentrated in transverse tubules of rat ventricular myocytes

Formamide-induced detubulation of rat ventricular myocytes was used to investigate the functional distribution of the Na/Ca exchanger (NCX) and Na/K-ATPase between the t-tubules and external sarcolemma. Detubulation resulted in a 32% decrease in cell capacitance, whereas cell volume was unchanged. Thus, the surface-to-volume ratio was used to assess the success of detubulation. NCX current (I-NCX) and Na/K pump current (I-pump) were recorded using whole-cell patch clamp, as Cd-sensitive and K-activated currents, respectively. Both inward and outward I-NCX density was significantly reduced by similar to40% in detubulated cells. I-NCX density at 0 mV decreased from 0.19 +/- 0.03 to 0.10 +/- 0.03 pA/pF upon detubulation. I-pump density was also lower in detubulated myocytes over the range of voltages (-50 to 1100 mV) and internal [Na] ([Na](i)) investigated (7-22 mM). At [Na](i) = 10 mM and -20 mV, I-pump density was reduced by 39% in detubulated myocytes (0.28 +/- 0.02 vs. 0.17 +/- 0.03 pA/pF), but the apparent K-m for [Na](i) was unchanged (16.9 +/- 0.4 vs. 17.0 +/- 0.3 mM). These results indicate that although the t-tubules represent only similar to32% of the total sarcolemma, they contribute similar to60% to the total I-NCX and I-pump. Thus, the functional density of NCX and Na/K pump in the t-tubules is 3-3.5-fold higher than in the external sarcolemma.