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 (/_NCX) and Na/K pump current (/ _pump) were recorded using whole-cell patch clamp, as Cd-sensitive and K-activated currents, respectively. Both inward and outward /_NCX density was significantly reduced by ∼40% in detubulated cells. / _NCX density at 0 mV decreased from 0.19 ± 0.03 to 0.10 ± 0.03 pA/pF upon detubulation. /_pump density was also lower in detubulated myocytes over the range of voltages (-50 to + 100 mV) and internal [Na] ([Na]_i) investigated (7-22 mM). At [Na]_i = 10 mM and -20 mV, /_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 ∼32% of the total sarcolemma, they contribute ∼60% to the total /_NCX and /_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.