Inhibition of outward K⁺ currents, and the consequent delayed repolarisation of the cardiac action potential, have been suggested to contribute to the positive inotropic effect of α-adrenergic and endothelin-receptor stimulation (Fedida & Bouchard, 1992; Damron et al. 1993). Since α-adrenergic and endothelin receptors share many common signal transduction pathways, it might be expected that stimulation of these receptors would affect a similar cardiac K⁺ current. We have recently shown that endothelin-1 (ET-1) inhibits a steady-state, non-inactivating K⁺ current in rat ventricular myocytes, I_ss (James et al. 2001). In the present study, we have used the permeablised-patch whole-cell patch-clamp recording technique to examine the effects of the α-adrenergic receptor agonist, phenylephrine, on K⁺ currents in isolated rat ventricular myocytes.

Ventricular myocytes were isolated by collagenase perfusion of hearts excised under pentobarbitone anaesthesia (150 mg kg^-1 I.P.) from adult male Wistar rats according to UK legislation. Pipettes were filled with a Hepes-buffered K⁺-rich solution containing 225 mg ml^-1 amphotericin B. Isolated cells were superfused with a Hepes-buffered Tyrode solution containing 1 mM Ca²⁺ (pH 7.35) at 35 °C. Na⁺ and Ca²⁺ currents were removed using short pre-pulses to -40 mV and 3 µM nifedipine, respectively. β-Adrenergic receptors were blocked by inclusion of 1 µM atenolol in the superfusion solution. Data are presented as means ± S.E.M. and P < 0.05 in a Student’s t test was considered statistically significant. Depolarising pulses from the holding potential of -80 mV activated outward currents, which activated rapidly to a peak (I_pk) and subsequently inactivated to a sustained level at the end of the pulse. The effect of phenylephrine on the voltage-dependent inactivation of the currents was examined using a two-pulse protocol in which 1 s conditioning pulses from -100 to +5 mV were given immediately prior to a 1 s pulse to +40 mV. The voltage-dependent inactivation by conditioning pulses positive to -80 mV of I_pk elicited on depolarisation to +40 mV was described by a Boltzmann equation:

I_pk = I_to/(1 + exp ((V_p – V_0.5)/V_s )) + I_ss,

where I_to is the maximal value of the inactivating component and I_ss is the non-inactivating component of I_pk, V_p is the conditioning pulse potential, V_0.5 is the voltage of half-maximal inactivation and V_s is a slope factor. Phenylephrine (PE; 10 µM) reduced I_ss by 26 % from 798 ± 56 to 589 ± 49 pA (n = 7, P < 0.02), but was without significant effect on V_0.5 (control: -45.9 ± 3.1 mV; PE: -43.4 ± 2.3 mV). Mean I_to was also reduced (from 1472 ± 327 to 1213 ± 187 pA). However, in contrast to the response ofI_ss to PE, that of I_to showed marked heterogeneity and did not achieve statistical significance. Thus, similar to ET-1, phenylephrine inhibits a non-inactivating steady-state outward current in rat ventricular myocytes. Inhibition of the steady-state outward current may cause prolongation of the cardiac action potential, and thereby contribute indirectly to the positive inotropic effect of α-adrenergic receptor stimulation.Financial support from the British Heart Foundation is gratefully acknowledged.

Damron, D.S., Van Wagoner, D.R., Moravec, C.S. & Bond, M. (1993). J. Biol. Chem. 268, 27335-27344.

Fedida, D. & Bouchard, R.A. (1992). Circ. Res. 71, 673-688.

James, A.F., Ramsey, J.E., Reynolds, A.M., Hendry, B.M. & Shattock, M.J. (2001). Biochem. Biophys. Res. Comm. 284, 1048-1055.