Transverse tubules (t-tubules) are invaginations of the cardiac cell membrane; Takeuchi et al. (2000) recently used immunolabelling techniques to show that Kv4.2, which underlies rat transient outward K⁺ current (I_TO), is concentrated in the t-tubules. We have used formamide-induced detubulation (Kawai et al. 1999) to investigate the functional localisation of I_TO, delayed rectifier (I_K), and inward rectifier (I_K1) currents.

Subepicardial ventricular myocytes (< 1 mm from the epicardium) were enzymatically isolated from the hearts of adult Wistar rats killed using a Schedule 1 method. I_TO, I_K and I_K1 were measured in control and detubulated cells using the whole-cell patch-clamp technique. To elicit I_TO and I_K, membrane potential was held at -120 mV and 500 ms test pulses to potentials between -30 and +60 mV were applied every 10 s, following a 40 ms pre-pulse to inactivate Na⁺ current. To elicit I_K1, membrane potential was held at -40 mV and 2 s pulses to potentials between -120 and -50 mV were applied every 10 s. All experiments were performed at room temperature.

Formamide treatment significantly reduced cell capacitance (200 ± 7 pF in control cells, n = 37; 160 ± 8 pF in formamide-treated cells, n = 23; P < 0.001, unpaired t test) and Ca²⁺ current (I_Ca) density (at 0 mV: -7.8 ± 1.1 pA pF^-1 in control cells, n = 6; -3.8 ± 0.5 pA pF^-1 in formamide-treated cells, n = 5; P < 0.05, unpaired t test), consistent with detubulation (Kawai et al. 1999). However, the current density of I_TO, I_K and I_K1 was not significantly different in control and formamide-treated cells (Fig. 1).These data suggest that while I_Ca is concentrated in the t-tubules, I_TO, I_K and I_K1 are homogeneously distributed between the t-tubule and surface membranes.This work was supported by the British Heart Foundation and Wellcome Trust.

Figure 1. Mean (± S.E.M.) current density-voltage relationships of ITO (A), IK (B), IK1 (C) and ICa (D) in control ([opencircle]) and formamide-treated ([fullcircle]) cells. n numbers are shown in each panel.

Kawai, M., Hussain, M. & Orchard, C.H. (1999). Am. J. Physiol. 277, H603-609.

Takeuchi, S., Takagishi, Y., Yasui, K., Murata, Y., Toyama, J. & Kodama, I. (2000). J. Mol. Cell. Cardiol. 32, 1361-1369.