Na/Ca Exchange and Na/K-ATPase Function Are Equally Concentrated in Transverse Tubules of Rat Ventricular Myocytes

AbstractFormamide-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 (INCX) and Na/K pump current (Ipump) were recorded using whole-cell patch clamp, as Cd-sensitive and K-activated currents, respectively. Both inward and outward INCX density was significantly reduced by ∼40% in detubulated cells. INCX density at 0mV decreased from 0.19±0.03 to 0.10±0.03 pA/pF upon detubulation. Ipump density was also lower in detubulated myocytes over the range of voltages (−50 to +100mV) and internal [Na] ([Na]i) investigated (7–22mM). At [Na]i=10mM and −20mV, Ipump density was reduced by 39% in detubulated myocytes (0.28±0.02 vs. 0.17±0.03 pA/pF), but the apparent Km for [Na]i was unchanged (16.9±0.4 vs. 17.0±0.3mM). These results indicate that although thet-tubules represent only ∼32% of the total sarcolemma, they contribute ∼60% to the total INCX and Ipump. 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