beta-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 sympatheti c stimulation have yielded controversial results. Separation of the Na+ and Ca2+ currents poses a problem in myocyte preparations. The gating of clone d Na+ channels is different in oocytes compared with mammalian expression s ystems. We have examined the sympathetic modulation of the alpha-subunit of the wild-type human cardiac Na+ channel(hH1) and the long QT-associated mu tant, 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 isopr oterenol or 8-bromo-cAMP. Na+ channel activation and inactivation were dete rmined using standard voltage clamp protocols. Ca2+: Na+ permeability ratio was determined under bi-ionic conditions. Results: We observed a qualitati vely different effect of sympathetic stimulation on the cardiac Na+ current from that reported in frog oocytes: activation and inactivation kinetics w ere shifted to more negative potentials. This shift was similar for both hH 1 and Delta KPQ. [Delta V-0.5 far inactivation: 8.3 +/- 1.7 mV, p<0.001 (hH 1); 6.8 +/- 0.9 mV, p<0.001 (Delta KPQ)]. Increased rate of closed-state in activation contributed to the shifting of the inactivation-voltage relation ship. Open-state inactivation was not affected as mean open times were unch anged. Reversal potential measurement in hH1 suggested a low Ca2+: Na+ perm eability 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. Simi lar effects are also seen in Delta KPQ which may increase arrhythmia suscep tibility in long QT syndrome by modifying the Na+ channel contribution to t he action potential. (C) 1999 Elsevier Science B.V. All rights reserved.