Sensitization of the cardiac Na channel to alpha1-adrenergic stimulation by inhalation anesthetics: evidence for distinct modulatory pathways.

BACKGROUND: Alpha1-adrenergic receptor stimulation has been shown to inhibit cardiac Na+ current (INa). Furthermore, some form of synergistic interaction of alpha1-adrenergic effects on INa in combination with volatile anesthetics has been reported. In this study, the authors investigated the possible role of G proteins and protein kinase C in the effects of halothane and isoflurane in the absence and presence of alpha1-adrenergic stimulation on the cardiac INa. METHODS: The standard whole-cell configuration of the patch-clamp technique was used. INa was elicited by depolarizing test pulses from a holding potential of -80 mV in reduced Na+ solution (10 mM). The experiments were conducted on ventricular myocytes enzymatically isolated from adult guinea pig hearts. RESULTS: The inhibitory effect of halothane (1.2 mM) and isoflurane (1 mM) on peak INa was significantly diminished in the presence of guanosine 5'-O-[2-thiodiphosphate (GDPbetaS). In myocytes pretreated with pertussis toxin (PTX), the potency of halothane was significantly enhanced, but the isoflurane effect was unchanged. In the presence of the protein kinase C (PKC) inhibitor bisindolylmaleimide (BIS), the effect of halothane was unchanged. In contrast, the effect of isoflurane on INa in the presence of BIS was significantly enhanced. The positive interaction between methoxamine and halothane was evident in the presence of G protein and PKC inhibitors. In contrast, the effect of methoxamine with isoflurane was additive in the presence of GDPbetaS or BIS. CONCLUSIONS: Different second messenger systems are involved in the regulation of cardiac Na+ current by volatile anesthetics. The effect of halothane involves a complex interaction with G proteins but is independent of regulation by PKC. In contrast, PKC is involved in the modulation of cardiac INa by isoflurane. In addition, non-PTX-sensitive G proteins may contribute to the effects of isoflurane. The positive interaction between methoxamine and anesthetics are independent of G proteins and PKC for halothane. In the case of isoflurane, the positive interaction with methoxamine is coupled to PTX-insensitive G proteins and PKC.