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A Ca2+ channel alpha 1-subunit derived from rabbit heart was transiently expressed in COS-7 cells. The dihydropyridine (+)-isradipine had low affinity (Ki = 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 (Ki = 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.
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