In vitro and in vivo characterization of 2,6-dimethyl-3,5-dicarbomethoxy-4-(2-isothiocyano)phenyl-1,4-dihydrop ridine as a Ca²⁺ channel antagonist

The actions of 2,6-dimethyl-3,5-dicarbomethoxy-4-(2-isothiocyano)phenyl-1,4-dihydrop ridine (o-NCS-DHP), a nifedipine analog bearing a reactive group, have been characterized in vitro by pharmacological and radioligand binding techniques in a number of smooth muscles and in vivo by blood pressure and radioligand binding. o-NCS-DHP exhibits persistent, but slowly reversible, antagonism in guinea pig ileal longitudinal smooth muscle, guinea pig bladder, taenia coli, rat portal vein, and rat tail artery to receptor responses (muscarinic and α-adrenoceptor) and K⁺ depolarization initiated responses. Duration of response was significantly longer than that of equivalent concentrations of nifedipine. In many tissues a component of antagonism produced by o-NCS-DHP was not reversed by repeated washing over the duration of the experiment (up to 2 or 7 h). A comparison of the actions of o-NCS-DHP and its isomers m-NCS-DHP and p-NCS-DHP revealed the former to be significantly longer lasting in rat tail artery against K⁺ depolarization induced responses. A similar profile was exhibited when the Ca²⁺ channel activator Bay K 8644 was employed as the stimulant, but the antagonism produced by all three compounds was fully reversed with sufficiently prolonged washing. In vivo administration of o-NCS-DHP (5-25 mg/kg) produced a persistent reduction of [³H]nitrendipine binding in rat brain, gut, and heart characterized as B(max), but not K(D), changes. No effects on [³H]dihydroalprenolol or [³H]quinuclidinyl benzilate binding were detected. Binding site recoveries were characterized by t 1/2 values of 35-50 h, and these were significantly prolonged to 91-107 h in animals treated with cycloheximide. Recovery of [³H]nitrendipine binding sites correlated with blood pressure restoration in spontaneously hypertensive rats. These data suggest that o-NCS-DHP possesses both reversible and irreversible actions. The reversible actions are unusually persistent compared with nifedipine and other 1,4-dihydropyridine analogs. This persistent, but reversible component, may be accompanied by an irreversible action particularly at the higher concentrations employed in the in vivo experiments.