EFFECTS OF CROMAKALIM ON THE MEMBRANE POTASSIUM PERMEABILITY OF FROG SKELETAL-MUSCLE INVITRO

1 The effects of the potassium channel opener, cromakalim, and its active enantiomer, lemakalim, have been investigated in frog skeletal muscle. 2 Cromakalim (30-300-mu-M) increased Rb-86 efflux from muscles loaded with the isotope, hyperpolarized the fibres and reduced membrane resistance. 3 These effects were inhibited by the sulphonylureas, glibenclamide and tolbutamide. The IC50 for glibenclamide inhibition of Rb-86 efflux was ca. 8 nM. 4 Phentolamine (300-mu-m) (which blocks responses to cromakalim in smooth muscle and inhibits ATP-sensitive K+ channels in pancreatic beta-cells) had no effect on the reduction in membrane resistance caused by 100-mu-M lemakalim. 5 Diazoxide (600-mu-M) had no effect on Rb-86 efflux. 6 The similarities of the K+ channel activated by cromakalim in frog skeletal muscle to the channel acted on in smooth muscle and to the ATP-sensitive K+ channel of beta-cells are discussed.