The protective action of amlodipine on cardiac negative inotropism caused by prolonged incubation in vitro

The mechanism of the antihypertensive action of the 1,4-dihydropyridine Ca2+ antagonist amlodipine was studied in isolated ventricular strips and aortic rings from Wistar rats after oral treatment with amlodipine 15 mg/kg/day for one week. The contractions evoked by electrical stimulation of isolated strips from right ventricles pretreated with amlodipine (5 nM) were unaffected during the first hour after mounting, but they decreased in magnitude after prolonged incubation (4 hr). However, the decrease in response of these preparations after prolonged incubation was less than that observed in strips prepared from untreated rats. A negative inotropic effect of amlodipine was observed at concentrations higher than 300 nM. In the presence of lower concentrations of amlodipine (5 nM - 30 nM) after prolonged incubation, the contractions of ventricular strips were significantly more sustained than in the absence of amlodipine. Likewise, the decrease in contractility evoked by increasing the stimulation frequency from 1 to 3 Hz was reduced in amlodipine treated rats. The recovery of contractility was improved when stimulation frequency was returned to 1 Hz. On the other hand, when rat ventricular strips pretreated with amlodipine (5 nM) were exposed to isoprenaline (3 mu M), the contractions evoked by isoprenaline were enhanced. The isoprenaline effect was not altered with 300 nM amlodipine, but with 3 mu M became weak and was significantly lower than in strips treated with isoprenaline alone. In addition, treatment with amlodipine produced a marked decrease in the contractions evoked by 100 mM KCl solution in isolated aortic rings when compared to untreated rats. This inhibition was produced in a time-dependent manner with an IC50 equal to 30 and 3 nM after 2 and 45 min of contraction, respectively. Ex vivo results show that amlodipine treatment decreased aortic contractility without producing a negative inotropic effect although there was an occupation of cardiac Ca2+ channels. These results suggest that a protective effect of amlodipine on cardiac negative inotropism is produced by prolonged incubation in vitro.