Calcium antagonistic action of corynanthine in isolated guinea pig left atria

It was found that the negative inotropic action of corynanthine (Cor) was unaffected by pretreatment of prazosin. Cor and verapamil (Ver) non-competitively shifted isoprenaline (Iso) dose-response relationships to the right (pD: = 3.40, 5.43, respectively) and calcium dose-response curves to the right (pD : = 3.83, 6.11 , respectively), although propranolol competitively shifted Iso dose-response curve ( pA2 = 6.90). Cor 0.3 mmol/L and Ver 30 ymol/L reversed the frequency-dependent positive staircase phenomenon to negative one. The results suggest that Cor. like Ver, is able to block the Ca2+ inward flow through slow channel.