MODULATION OF (H-3) GLIBENCLAMIDE BINDING TO CARDIAC AND INSULINOMA MEMBRANES

The existence of a single or of multiple populations of glibenclamide binding sites is a subject of controversy. In the present study, radioligand binding techniques were employed to determine whether multiple populations of [H-3]glibenclamide binding sites exist in pancreatic tumor (insulinoma) cells. Additional studies were performed to further characterize the binding of [H-3]glibenclamide to insulinoma and cardiac membranes. [H-3]Glibenclamide bound to high (0.1 nM) and low (240 nM) affinity binding sites in insulinoma membranes. The physiological relevance of multiple populations of sites is unknown. The binding of glibenclamide to insulinoma and cardiac membranes was altered by guanine nucleotides and not adenine nucleotides. This suggests glibenclamide binding can be modulated by G-proteins. Glibenclamide binding was also modulated by divalent cations. The divalent cations, Ca2+ and Zn2+, stimulated specific glibenclamide binding to cardiac and insulinoma membranes, while Mg2+ and Mn2+ enhanced cardiac binding only. Moreover, the lowering of pH from 7.4 to 6.5 was found to enhance specific glibenclamide binding. Interestingly, the magnitude of this effect was much larger in cardiac membranes. The specific nature of the regulation of glibenclamide binding by guanine nucleotides, divalent cations and pH remains to be explored.