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* Department of Physiology, Loyola University Chicago Stritch School of Medicine, Maywood, Illinois; and School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom
Correspondence: Address reprint requests to Donald M. Bers, PhD, Dept. of Physiology, Loyola University Chicago, 2160 South First Ave., Maywood, IL 60153. Tel.: 708-216-1018; Fax: 708-216-6308; E-mail: dbers{at}lumc.edu.
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 (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 0 mV 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 +100 mV) and internal [Na] ([Na]i) investigated (7–22 mM). At [Na]i = 10 mM and -20 mV, 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 ± 0.3 mM). 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.
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