Electromechanical Effects of Protamine and Verapamil in Rat Papillary Muscle

Abstract

The electromechanical effects of protamine sulfate and the calcium channel blocker verapamil on rat cardiac and skeletal muscles were studied using isolated left ventricular papillary muscle and phrenic nerve-hemidiaphragm preparations. Protamine produced significant decreases in isometric force in the cardiac tissue and contracture developed at concentrations of 40 and 80 mg·L⁻¹. Isometric force also decreased significantly with verapamil at concentrations of 0.757 and 7.57 mg·L⁻¹. Both drugs caused significant decreases in the contractile force of hemidiaphragm muscle when the tissue was stimulated indirectly. Protamine and verapamil caused the resting membrane potential and the amplitude of the action potential to decrease in cardiac tissue and overshoot failed to develop with 80 mg·L⁻¹ of protamine or 7.57 mg·L⁻¹ of verapamil. These bioelectrical changes developed in a dose-dependent manner. It was concluded that protamine had a similar effect to that of calcium channel blockers and it may act through a reduction of cellular calcium. This effect on cardiac tissue may be mediated through the sarcolemmal ion pumps or channels, leading to changes in calcium homeostasis.