Effects of 3,6-dimethamidodibenzopyriodonium citrate (I-65) on action potentials and contractions in guinea pig papillary muscles

Synchronous recordings of action potentials and contractile force in isolated guinea pig papillary muscles administered 3,6-dimethamidodibenzopyriodonium citrate (I-65) 30 mumol/L showed excitation-contraction uncoupling, ie, the contractile force was markedly depressed although the Vmax and amplitudes of the action potentials were unaffected. At the same concentration, I-65 shortened the duration of the action potentials at 50% and 90% repolarization. However, I-65 at lower concentrations (1-10 mumol/L) decreased Vmax and the amplitude of the slow action potentials which were elicited by isoprenaline in papillary muscles were rendered partially depolarized by KCl 25 mmol/L Tyrode solution. These effects were partially reversed by elevation of the concentration of Ca2+ from 2.7 to 5.4 mmol/L. I-65 also suppressed the abnormal automaticity elicited by BaCl2 and DAD and triggered activity induced by ouabain. The results suggest that I-65 may be a calcium channel blocker.