CHARACTERIZATION OF K-ATP CHANNELS IN INTACT MAMMALIAN SKELETAL-MUSCLE FIBERS

1 The aim of this study was to characterize the K-ATP channel of intac t rat skeletal muscle (rat flexor digitorum brevis muscle). Changes in membrane currents were recorded with two-electrode voltage-clamp of w hole fibres. 2 The K-ATP channel openers, levcromakalim and pinacidil (10-400 mu M), caused a concentration-dependent increase in whole-cell chord conductance (up to approximately 1.5 mScm(-2)). The activated c urrent had a weak inwardly rectifying current-voltage relation, a reve rsal potential near E-K and nanomolar sensitivity to glibenclamide - c haracteristic of a K-ATP channel current. Concentration-effect analysi s revealed that levcromakalim and pinacidil were not particularly pote nt (EC50 similar to 186 mu M, similar to 30 mu M, respectively), but d iazoxide was completely inactive. 3 The ability of both classical K-AT P channel inhibitors (glibenclamide, tolbutamide, glipizide and 5-hydr oxydecanoic acid) and a number of structurally related glibenclamide a nalogues to antagonize the levcromakalim-induced current was determine d. Glibenclamide was the most potent compound with an IC50 of approxim ately 5 nM. However, the non-sulphonylurea (but cardioactive) compound 5-hydroxydecanoic acid was inactive in this preparation. 4 Regression analysis showed that the glibencramide analogues used have a similar rank order of potency to that observed previously in vascular smooth m uscle and cerebral tissue. However, two compounds (glipizide and DK13) were found to have unexpectedly low potency in skeletal muscle. 5 The se experiments revealed K-ATP channels of skeletal muscle to be at lea st 10x more sensitive to glibenclamide than previously found; this may be because of the requirement for an intact intracellular environment for the full effect of sulphonylureas to be realised. Pharmacological ly, K-ATP channels of mammalian skeletal muscle appear to resemble mos t closely K-ATP channels of cardiac myocytes.