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 alpha-subunit of the wild-type human cardiac Na+ channel(hH1) and the long QT-associated mutant, Delta KPQ, expressed in human embryonic kidney cells. Methods: Stable cell lines of hH1 and Delta KPQ 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 Delta KPQ. [Delta V-0.5 far inactivation: 8.3 +/- 1.7 mV, p<0.001 (hH1); 6.8 +/- 0.9 mV, p<0.001 (Delta KPQ)]. 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 Delta KPQ, 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 Delta KPQ which may increase arrhythmia susceptibility in long QT syndrome by modifying the Na+ channel contribution to the action potential. (C) 1999 Elsevier Science B.V. All rights reserved.