Regulation of cardiac pacemaker current If in excised membranes from sinoatrial node cells

We have proposed that voltage-gated ionic channels are regulated by receptor-guanine-nucleotide binding (G) protein pathways that are membrane delimited (direct) or cytoplasmic (indirect). Since indirect pathways are known to couple muscarinic and beta-adrenergic receptors to hyperpolarization-activated current (If) channels, we tested our hypothesis that direct pathways might also be present by measuring If currents in rabbit sinoatrial node cells using the gigaseal patch clamp method. Whole cell If currents were increased by the beta-agonist isoproterenol (Iso) and decreased by the muscarinic agonist carbachol (Carb) at rates that were similar to the rate at which carbachol increased muscarinic K(+)-acetylcholine channel (K+[ACh]) current in the same cells. K+[ACh] currents are directly gated by a G protein, and the results therefore supported our hypothesis. Much stronger support came after we established for the first time that If could be recorded from excised inside out membrane patches in the absence of cytoplasmic substrates. Under these conditions Iso increased and Carb decreased cell-free patch If provided guanosine 5'-triphosphate and Mg2+ were present. Thus G proteins couple autonomic receptors to If within the plasma membrane, probably by a direct action.