High affinity binding of a calcium channel antagonist to smooth and cardiac muscle

Specific binding of the Ca²⁺-channel antagonist nitrendipine, a close structural analog of nifedipine, has been measured in microsomal membrane fractions from guinea pig ileal longitudinal smooth muscle. The dissociation constant was 0.18 nanomole per liter and maximum binding was 1.14 picomoles per milligram of protein. Binding with very similar characteristics was found in a rat ventricle preparation. This high affinity binding was sensitive to displacement by a series of 1,4-dihydropyridine analogs of nifedipine with an activity sequence correlating well with that determined for inhibition of mechanical responses in the intact smooth muscle.