Role of sodium in strophanthidin toxicity of Purkinje fibers.

The action of strophanthidin on the membrane potentials and contractile force of canine Purkinje fibers was studied in vitro. Purkinje fibers were exposed to strophanthidin (10(-6) M) until spontaneous fast discharge occurred. In Tyrode solution, strophanthidin increased and subsequently decreased ("mechanical toxicity") contractile force. The onset of spontaneous rhythms ("electrical toxicity") usually occurred when force was declining. In low-Na Tyrode (-71 mM NaCl), force increased: on exposure to strophanthidin, mechanical toxicity occurred sooner and electrical toxicity later. In low-Na low-Ca Tyrode, electrical toxicity developed sooner than in low-Na Tyrode. In high-Na Tyrode (+27 mM NaCl), force increased, and the time to electrical and mechanical toxicities was decreased. Increasing osmolarity (+54 mosM) with either sucrose or choline chloride increased force and shortened the time to the onset of strophanthidin toxicities. In the presence of arrhythmias, lowering [Na]o decreased transient oscillations and led to disappearance of arrhythmias. It is concluded that Na plays a role in strophanthidin-induced electrical toxicity, whereas Ca appears more important for mechanical toxicity.