PROTEIN KINASE-INDEPENDENT INHIBITION OF MUSCARINIC K+ CHANNELS BY STAUROSPORINE

Acetylcholine (ACh) binding to atrial muscarinic receptors activates a n inwardly rectifying K+ current (I-K[ACh]) via a pertussis toxin-sens itive GTP-binding protein (G(K)). The muscarinic K+ channel (termed GI RK1) has been cloned, and the nucleotide sequence contains nine consen sus sites for protein kinase C (PKC) phosphorylation (16). Dephosphory lation of the muscarinic K+ channel has been implicated in rapid I-K[A Ch] desensitization in the presence of agonist (13). Staurosporine is a widely used membrane-permeant inhibitor of PKC and other protein kin ases (7), including G protein-coupled receptor kinases. We investigate d the role of phosphorylation in the regulation of I-K[ACh] by examini ng the effect of a variety of protein kinase inhibitors. Staurosporine produced a rapid and reversible dose-dependent decrease in I-K[ACh], activated by either GTP or guanosine 5'-O-(3-thiotriphosphate) (GTP ga mma S). Other PKC inhibitors, including calphostin C and K-252b, were without effect on GTP gamma S-activated I-K[ACh]. In excised patches o f atrial membrane under nonphosphorylating conditions (0 ATP, 1 mM 5'- adenylylimidodiphosphate), staurosporine reversibly reduced muscarinic K+ channel activity without altering single-channel current amplitude . These results suggest that staurosporine inhibits I-K[ACh] by a mech anism independent of intracellular protein kinases.