RELEASE OF TEA BLOCKADE OF MAXI-K+ CHANNELS BY ISOPROTERENOL

The outward K+ current induced by step depolarization of freshly dispe rsed myocytes of guinea-pig taenia coli decreased about 80% upon treat ment with 3 mM tetraethylammonium chloride (TEA). Isoproterenol (ISO, 2-5 mu M) restored it to a large extent. This restoration did not occu r in the presence of propranolol (2 mu M). In single-channel recording s from cell-attached parches, the activity of maxi-K+ channel is domin ant. When 3 mM TEA is incorporated in the pipette solution, the domina nt channel-openings observed had much smaller unitary conductance. On the addition of ISO (2 mu M) to the bath solution, but not to the pipe tte solution, K+-channel openings with unitary conductance similar to that without TEA treatment appeared. Cyclic AMP incorporated into the cytoplasm through the pipette was ineffective. These results indicate that ISO release TEA decrease of maxi-K+ channel conductance through s ome intracellular second messenger system other than adenylyl cyclase- protein kinase A system. (C) 1994 Academic Press, Inc.