Localization of K+ channels in the T-tubules of cardiomyocytes as suggested by the parallel decay of membrane capacitance, IK1 and IKATP during culture and by delayed IK1 response to barium

Adult ventricular myocytes lose T-tubules over few days in culture, which causes the loss of about 60% of the cell membrane capacitance (Cm) (Mitcheson et al., 1996). In this study, we have measured, in whole-cell voltage-clamped rabbit right ventricular myocytes at 0, 1, 2 and 3-5 days of culture (nine to 20 myocytes at each age) in a defined Dulbecco's modified Eagle's medium, the value of Cm and the magnitudes of the background inward rectifier current (IK1) and of the 2,4-dinitrophenol-induced ATP-sensitive potassium current (IKATP). Cm, IK1 and IKATP, all had decreased significantly by 51, 83 and 88%, respectively after 4 days of culture. Analysis using a single exponential decay function of time gave time constants of, 2.6 +/- 0.2, 2.2 +/- 0.5 and 2.4 +/- 0.4 days, respectively. Linear regressions of IK1 and IKATP versus Cm had regression coefficients of 0.93 and 0.98, respectively. These observations are consistent with a strong link of the decay of IK1 and IKATP currents to that of Cm. Furthermore, the time course of changes in IK1 when an external blocker (100 mu M BaCl2) was applied and washed by local perfusion (95% change in 50 ms) agrees with a model including a diffusion lime constant of 300 ms. This value is consistent with the known kinetics of diffusion of divalent cations in the T-tubules, Taken together, these results could be explained by the localization of a major part of the IK1 and IKATP currents of ventricular cardiomyocytes in the T-tubules. As a consequence, transient accumulation of K+ ions in cardiac T-tubules may take place and modulate excitation-contraction coupling. (C) 1999 Academic Press.