Excitation Contraction CouplingMuscle ContractionMyocytes, CardiacMyocardial ContractionIsometric ContractionMyocardiumHeartHeart VentriclesCalciumUterine ContractionElectric StimulationAction PotentialsCardiac OutputGuinea PigsArrhythmias, CardiacMuscle, SmoothRats, Sprague-DawleyCells, CulturedVentricular FunctionTime FactorsMembrane Potentials