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 approximately 40% 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 +100 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 thet-tubules represent only approximately 32% of the total sarcolemma, they contribute approximately 60% 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.
Despa
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Department of Physiology, Loyola University Chicago Stritch School of Medicine, Maywood, Illinois 60153, USA.
Brette
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Orchard
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Bers
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D000818:Animals; D002118:Calcium / Q000378:metabolism*; D048429:Cell Size / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D025802:Electric Capacitance; D006352:Heart Ventricles / Q000187:drug effects; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology*; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D012508:Sarcolemma / Q000187:drug effects / Q000502:physiology*; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D014018:Tissue Distribution; D016276:Ventricular Function
D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D002118:Calcium