Binding of [3H]-P1075, an opener of ATP-sensitive K+ channels, to rat glomerular preparations.
ATP-sensitive K+ channels (KATP channels) in the kidney have been found in the tubular system and in the afferent arteriole. In this study we have examined the binding of [3H]-P1075 ([3H]-N-cyano-N'-(1, 1-dimethylpropyl)-N"-3-pyridylguanidine), a selective opener of KATP channels, in rat glomerular preparations. Equilibrium (saturation, competition) and kinetic experiments indicated that [3H]-P1075 binds to a single class of sites with a dissociation constant of about 3 nM and a maximum binding capacity of 10 fmol mg-1 glomerular protein. The association rate constant of the complex was 6,5 x 10(7) M-1 min-1; dissociation occurred with a half-time of 6.2 min. Specific [3H]-P1075 binding was strongly reduced when the metabolic state of the glomerular preparation was impaired during the preparation procedure or the binding assay or when the preparation was subjected to mild collagenase treatment. In different metabolically competent preparations, the amount of specific [3H]-P1075 binding correlated well with the number of vascular endings adherent to the glomeruli; no specific binding was found in mesangial cells in culture. Specific [3H]-P1075 binding was inhibited by representatives of the different classes of KATP channel openers and by sulphonylurea-type blockers with inhibition constants similar to those obtained in rat aortic rings. It is concluded that rat glomerular preparations possess specific binding sites for KATP channel openers with vascular characteristics. The sensitivity of binding to mild collagenase treatment suggests that these sites are located on a membrane protein; in addition, the data suggest that these sites are localized on smooth muscle and/or renin secreting cells of the afferent vascular endings attached to some of the glomeruli. Their estimated density (1,500 microns-2) is much higher than that of KATP channels in smooth muscle.