Halothane (HAL), isoflurane (ISO) and sevoflurane (SEVO) have a lesser abbreviating effect on action potential duration (APD) in cells isolated from the subepicardium (EPI) than the subendocardium (ENDO) of the rat left ventricle (Rithalia et al. 2001), suggesting differences in anaesthetic-sensitive currents in the two cell types. To investigate the mechanisms underlying the differential transmural effects of these agents on APD we measured L-type Ca²⁺ current (I_Ca), transient outward current (I_to) and steady-state current (I_ss) in EPI and ENDO myocytes.

Hearts were removed from humanely killed male Wistar rats (200-250 g weight) and EPI and ENDO myocytes isolated enzymatically from the left ventricle (Rithalia et al. 2001). Membrane currents (whole-cell patch-clamp) were recorded at 30 °C in the absence and presence of the three anaesthetics (0.6 mM and/or 1 mM) in EPI and ENDO cells (n = 7-11 cells in each group). I_Ca was evoked by 200 ms pulses from -40 to 0 mV and I_to and I_ss by 200 ms pulses from -80 to +80 mV (with 10 mM nifedipine to inhibit I_Ca). Data are presented as means ± S.E.M. P values result from paired or unpaired t tests as appropriate.

I_Ca was not significantly different in EPI (-0.48 ± 0.04 nA, n = 16) and ENDO (-0.50 ± 0.05 nA, n = 13) cells. HAL, ISO and SEVO (0.6 mM) significantly decreased (P < 0.005) I_Ca to a similar extent in EPI and ENDO cells (HAL: -30.2 ± 3.8 vs -24.4 ± 4.0% ISO: -14.5 ± 3.4 vs -16.7 ± 1.4% SEVO: -9.4 ± 1.4 vs -10.1 ± 1.5 %, respectively).

I_to was greater (P < 0.001) in EPI (3.65 ± 0.23 nA, n = 33) than ENDO (0.88 ± 0.06 nA, n = 28) cells. 0.6 mM and 1 mM HAL decreased I_to in EPI cells, by 8.3 ± 1.7 % (P = 0.004) and 17.9 ± 2.1 % (P = 0.002), respectively. ISO (1 mM) and SEVO (1 mM) also significantly decreased I_to in EPI cells (ISO: -13.2 ± 1.9 %, P = 0.005, SEVO: -4.6 ± 1.6 %, P = 0.039). There was no significant effect of HAL, ISO or SEVO on I_to in ENDO cells.

I_ss, measured as the end current in I_to recordings, was not significantly different in EPI (1.94 ± 0.09 nA) and ENDO (1.69 ± 0.10 nA) cells. HAL (0.6 mM) significantly increased I_ss in EPI (+7.2 ± 2.0 %, P = 0.011) and ENDO (+5.8 ± 0.8 %, P < 0.001) cells. 1 mM SEVO also increased I_ss in both EPI and ENDO cells (EPI: +8.6 ± 1.0 %, P < 0.001, ENDO: +8.2 ± 3.1 %, P = 0.024) but 1 mM ISO had no significant effect on I_ss.

Reduction of I_Ca and an increase in I_ss would lead to shortening of APD, whereas inhibition of I_to would prolong APD. In ENDO cells where I_to is essentially absent, blockade of inward current would predominate, leading to a greater anaesthetic-induced shortening of APD than in EPI cell where both I_Ca and I_to are inhibited and I_ss increased.

This work was supported by the British Heart Foundation.

All procedures accord with current UK legislation.