I-125-Apamin and photolabile derivatives of the toxin have been used to investigate the binding properties and subunit composition of small conductance Ca2+-activated potassium channels (SKCa, channels) expressed on plasma membranes from rat brain, rabbit liver, or rat pheochromocytoma (PC12) cells, On all preparations, I-125-apamin recognized single classes of acceptor binding sites with similar high affinity (Kd similar to 3-6 pM), Gallamine, however, was found to readily discriminate between I-125-apamin accepters present in these preparations, showing a maximal approx ninefold difference in affinity for accepters expressed by rabbit liver or PC 12 cells. Affinity-labeling patterns revealed the expression of different hetero-oligomeric combinations of high (86 or 59 kDa) and low (33 or 30 kDa) molecular mass I-125-apamin-binding polypeptides, consistent with pharmacological differences. Alternative expression of either 86- or 59-kDa polypeptides appeared to be the most important factor influencing gallamine's affinity for SKCa channel subtypes, Both high- and low-molecular-mass polypeptides are integral membrane proteins, the latter being glycosylated in a tissue-specific manner. (C) 1997 Academic Press.