Papillary MusclesGuinea PigsGermine AcetatesCevanesFendilineAction PotentialsMyocardial ContractionMicroelectrodesQuinidineOuabainMembrane PotentialsHeartCalcium Channel BlockersElectric StimulationMusclesMuscle, SmoothMuscle ProteinsMuscle ContractionChordae TendineaeMuscle, SkeletalMuscle Fibers, SkeletalMuscle, Smooth, VascularMitral Valve InsufficiencyMuscle DevelopmentMitral ValveFerretsMyocardiumHeart VentriclesGuinea