DIFFERENTIAL TIME-COURSE FOR DESENSITIZATION TO MUSCARINIC EFFECTS ON K+ AND CA2+ CHANNELS

The time course of muscarinic effects on K and Ca currents was investigated at 22-24 degrees C in guinea-pig atrial myocytes, using the whole-cell voltage clamp. At a holding potential of -40 or -50 mV, short exposures to 100 mu M acetylcholine (ACh) or carbachol (CCh) reproducibly induced outward K currents (I-K,I-ACh). During long exposures to these agonists, I-K,I-ACh faded with time. In cells not dialysed with guanosine triphosphate (GTP), I-K,I-ACh could dissipate completely following 15-20 min of agonist exposure. After agonist washout, lost sensitivity was not recovered. In cells dialysed with GTP (0.2-1 mM), I-K,I-ACh still faded but normal sensitivity to agonists was restored with washout. Fade of I-K,I-ACh was not prevented by intracellular heparin or dextran, excluding the involvement of either beta-adrenergic or muscarinic receptor kinase. I-K,I-ACh induced by bethanechol or adenosine also faded, and subsequent CCh application after washout revealed a diminished response. Intracellular guanosine-5'-o-(3-thiotriphosphate (GTP(gamma)S) induced I-K,I-ACh which faded and subsequent exposure to CCh was without effect. Equally, after full desensitization with CCh, GTP(gamma)S failed to induced I-K,I-ACh. The Ca current (I-Ca) was activated by voltage steps to O mV and increased with 1-3 mu M isoproterenol. This increase could be reversed by CCh, even when I-K,I-ACh had completely faded. Prolonged muscarinic agonist exposure sometimes also caused fade of the effect on I-Ca, which always occurred after loss of I-K,I-ACh. The results show that desensitization is heterologous and may involve the guanine nucleotide-binding (G) protein. The differential desensitization to the effects on I-K,I-ACh and I-Ca suggests the involvement of two different signalling pathways in the muscarinic control of K and Ca channels.