CALCIUM CHANNELS - THE BETA-SUBUNIT INCREASES THE AFFINITY OF DIHYDROPYRIDINE AND CA2+ BINDING-SITES OF THE ALPHA(1)-SUBUNIT

A Ca2+ channel alpha(1)-subunit derived from rabbit heart was transiently expressed in COS-7 cells. The dihydropyridine (+)-isradipine had low affinity (K-i = 34.3 nM) for the alpha(1)-subunit in the absence of the beta-subunit due to rapid dissociation (k(-1) = 0.11 min(-1)). Co-expression of the beta-subunit resulted in a > 35-fold increase in (+)-isradipine binding affinity (K-i = 0.9 nM) due to decreased dissociation (k(-1) of 0.007 min(-1)). Higher DHP binding affinity was associated with an increase of the apparent affinity of Ca2+ ions for the channel. Our data suggest that the beta-subunit affects the coordination of Ca2+ ions with sites that are coupled to the dihydropyridine binding domain and by this mechanism increases the affinity for these ligands.