β-adrenergic action on wild-type and KPQ mutant human cardiac Na+ channels: shift in gating but no change in Ca2+: Na+ selectivity.

Objective: Prior studies of the modulation of the Na+ current by sympathetic stimulation have yielded controversial results. Separation of the Na+ and Ca2+ currents poses a problem in myocyte preparations. The gating of cloned Na+ channels is different in oocytes compared with mammalian expression systems. We have examined the sympathetic modulation of the α-subunit of the wild-type human cardiac Na+ channel (hH1) and the long QT-associated mutant, ΔKPQ, expressed in human embryonic kidney cells. Methods: Stable cell lines of hH1 and ΔKPQ were established in human embryonic kidney cells. Whole-cell and single-channel currents were measured with the patch–clamp technique. Sympathetic stimulation was effected by exposure to isoproterenol or 8-bromo-cAMP. Na+ channel activation and inactivation were determined using standard voltage clamp protocols. Ca2+: Na+ permeability ratio was determined under bi-ionic conditions. Results: We observed a qualitatively different effect of sympathetic stimulation on the cardiac Na+ current from that reported in frog oocytes: activation and inactivation kinetics were shifted to more negative potentials. This shift was similar for both hH1 and ΔKPQ. [ΔV0.5 for inactivation: 8.3±1.7 mV, p<0.001 (hH1); 6.8±0.9 mV, p<0.001 (ΔKPQ)]. Increased rate of closed-state inactivation contributed to the shifting of the inactivation–voltage relationship. Open-state inactivation was not affected as mean open times were unchanged. Reversal potential measurement in hH1 suggested a low Ca2+: Na+ permeability ratio of 0.017, uninfluenced by sympathetic stimulation. In ΔKPQ, the size of the persistent relative to the peak current was increased with 8-bromo-cAMP from 3.0±0.7% to 4.3±0.6% (p=0.056). Conclusions: Sympathetic stimulation exerts multiple effects on the gating of hH1. Similar effects are also seen in ΔKPQ which may increase arrhythmia susceptibility in long QT syndrome by modifying the Na+ channel contribution to the action potential. [ABSTRACT FROM PUBLISHER]