title: Effects of papaverine on Ca++-dependent action potentials in guinea-pig myocardium depolarized by potassium.
abstract: Ventricular and atrial tissues isolated from guinea-pig hearts were depolarized by exposure to 22 mM K+ to study the effects of papaverine on the Ca++-mediated slow inward current (Isi). Papaverine at 5 x 10(-6) to 5 x 10(-5) M caused a concentration-dependent decrease in the threshold current required to excite the field-stimulated tissues and an enhancement of action potential amplitude, action potential duration and maximum rate of depolarization of the Ca++-dependent action potentials. In ventricular preparations, 5 x 10(-5) and 10(-4) M papaverine invariably induced rhythmic spontaneous activity that was abolished by verapamil. It is proposed that the membranal electrical effects of papaverine in the K+-depolarized myocardium are caused by an enhancement of Isi and a decrease in K+ conductance. However, at the highest concentration studied (10(-4) M), papaverine inhibited Isi in atrial tissue, demonstrating that papaverine exerts another action on Isi opposite to that observed at lower concentrations.
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001464:Barium / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D005260:Female; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects*; D006325:Heart Atria / Q000187:drug effects; D006352:Heart Ventricles / Q000187:drug effects; D066298:In Vitro Techniques; D008297:Male; D010208:Papaverine / Q000494:pharmacology*; D011188:Potassium / Q000494:pharmacology*; D014700:Verapamil / Q000494:pharmacology
chemical_list: D001464:Barium; D014700:Verapamil; D010208:Papaverine; D011188:Potassium; D002118:Calcium