SULFONYLUREA BINDING IN RAT ISOLATED GLOMERULI - PHARMACOLOGICAL CHARACTERIZATION AND DEPENDENCE ON CELL-METABOLISM AND CYTOSKELETON

The kidney is endowed with ATP-sensitive K+ channels (K-ATP channels) both at the vascular and at the epithelial level. In this study we hav e characterized the binding of the sulphonylurea glibenclamide, the mo st widely used blocker of K-ATP channels, in rat isolated glomeruli. I n metabolically intact glomeruli, H-3-glibenclamide labelled two diffe rent binding components with affinities of 47 +/- 12 nM and 10 +/- 1 m u M and estimated binding capacities of 1.2 +/- 0.1 and 501 +/- 11 pmo l/mg protein, respectively. H-3-glibenclamide binding was inhibited di fferentially by other sulphonylureas (tolbutamide, glibornuride, gliqu idone and glipizide) and benzoic acid analogues such as meglitinide, A Z-DF 265 and UL-DF 9. Sulphonylureas interacted with the high affinity component and, in some cases, also with the low affinity component wh ereas the benzoic acid derivatives inhibited exclusively low affinity glibenclamide binding. Severe metabolic stress affected both component s of glibenclamide binding by shifting high affinity binding to the ri ght and reducing the capacity of the low affinity component. Disruptio n of the cytoskeletal actin filaments by cytochalasin B and D mimicked the effect of metabolic stress on the high affinity component but lef t the low affinity component unchanged. In crude membranes, the affini ty of the first component was again reduced and a major loss of the lo w affinity sites was observed. The data show that the two binding comp onents of glibenclamide binding in rat isolated glomeruli have very di fferent properties. The high affinity component is not recognized by t he benzoic acid derivatives; its affinity is modulated by cell metabol ism and the actin component of the cytoskeleton. The low affinity site s are, in their majority, cytosolic. The function and cellular localiz ation of the high affinity sites are under further study.