Target size analysis of skeletal muscle Ca2+ channels

[3H]PN 200-110, a potent chiral benzoxadiazol 1,4-dihydropyridine Ca2+ antagonist was used to label guinea pig skeletal muscle Ca2+ channels. [3H]PN 200-110 binds with a Ksub(d) of approximately 1 nM to a homogeneous population of non-interacting binding sites; d-cis-diltiazem, but not 1-cis-diltiazem increases the Bsub(max) of [3H]PN 200-110 by 25% and slows the dissociation rate 3-fold at 370C. Target size analysis of the [3H]PN 200-110-labelled Ca2+ channels with 10 MeV electrons gave an Msub(r) of 136000 which was reduced to 75000 by d-cis-diltiazem treatment of membranes. It is concluded that positive heterotropic allosteric regulation by d-cis-diltiazem is accompanied by channel oligomer dissociation. (Auth.)