Title: Slow diffusion of K<sup>+</sup> in the T tubules of rat cardiomyocytes
Abstract: Cardiomyocyte contractility is regulated by the extracellular K + concentration ([K + ] o ). Potassium dynamics in the T tubules during the excitation-contraction cycle depends on the diffusion rate of K + , but this rate is not known. Detubulation of rat cardiomyocytes was induced by osmotic shock using formamide, which separated the surface membrane from the T tubules. Changes in current and membrane potential in voltage-clamped (−80 mV) and current-clamped control and detubulated cardiomyocytes were compared during rapid switches between 5.4 and 8.1 mM [K + ] o , and the results were simulated in a mathematical model. In the voltage-clamp experiments, the current changed significantly slower in control than in detubulated cardiomyocytes during the switch from 5.4 to 8.1 mM [K + ] o , as indicated by the times to achieve 25, 50, 90, and 95% of the new steady-state current [control (ms) t 25 = 98 ± 12, t 50 = 206 ± 20, t 90 = 570 ± 72, t 95 = 666 ± 92; detubulated t 25 = 61 ± 11, t 50 = 142 ± 17, t 90 = 352 ± 52, t 95 = 420 ± 69]. These time points were not significantly different either during the 8.1 to 5.4 mM [K + ] o switch or in current-clamped cardiomyocytes switching from 5.4 to 8.1 mM [K + ] o . Mathematical simulation of the difference current between control and detubulated cardiomyocytes gave a t-tubular diffusion rate for K + of ∼85 μm 2 /s. We conclude that the diffusion of K + in the T tubules is so slow that they constitute a functional compartment. This might play a key role in local regulation of the action potential, and thus in the regulation of cardiomyocyte contractility.