Characterization of KATP channels in intact mammalian skeletal muscle fibres

1. The aim of this study was to characterize the K_ATP channel of intact rat skeletal muscle (rat flexor digitorum brevis muscle). Changes in membrane currents were recorded with two-electrode voltage-clamp of whole fibres.
2. The K_ATP channel openers, levcromakalim and pinacidil (10–400 μM), caused a concentration-dependent increase in whole-cell chord conductance (up to approximately 1.5 mScm⁻²). The activated current had a weak inwardly rectifying current-voltage relation, a reversal potential near E_K and nanomolar sensitivity to glibenclamide – characteristic of a K_ATP channel current. Concentration-effect analysis revealed that levcromakalim and pinacidil were not particularly potent (EC₅₀ ∼186 μM, ∼30 μM, respectively), but diazoxide was completely inactive.
3. The ability of both classical K_ATP channel inhibitors (glibenclamide, tolbutamide, glipizide and 5-hydroxydecanoic acid) and a number of structurally related glibenclamide analogues to antagonize the levcromakalim-induced current was determined. Glibenclamide was the most potent compound with an IC₅₀ of approximately 5 nM. However, the non-sulphonylurea (but cardioactive) compound 5-hydroxydecanoic acid was inactive in this preparation.
4. Regression analysis showed that the glibenclamide analogues used have a similar rank order of potency to that observed previously in vascular smooth muscle and cerebral tissue. However, two compounds (glipizide and DK13) were found to have unexpectedly low potency in skeletal muscle.
5. These experiments revealed K_ATP channels of skeletal muscle to be at least 10× 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. Pharmacologically, K_ATP channels of mammalian skeletal muscle appear to resemble most closely K_ATP channels of cardiac myocytes.