Divalent cations on action potentials of dog heart

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Abstract
The effects of Mn, Ni, Hg, Cd, Cu, Zn, Sr, and Mg on transmembrane action potentials of Purldnje and ventricular muscle fibers on dog heart were studied. The concentrations of cations administered varied between 10-3 and 10-5 M. All except Mg altered the action potential of both fibers. Only Hg produced a marked alteration of action potential of Purkinje fiber with little change in ventricular fiber action potential. The most consistent alteration produced by the divalent cations was either a shortening or lengthening of plateau phase of action potentials of both fibers. Ni and Mg lengthened the plateau phase, whereas Cu, Hg, Zn, Cd and Sr shortened it. Cu, Hg, Zn, and Cd produced a greater degree of shortening of duration of action potential in Purldnje than in ventricular fiber. These four cations have an affinity for binding sulfhydryl groups, and the greater modifying effect on the Purldnje fiber produced by these four cations suggests that this fiber may be more sulfhydryl dependent than ventricular fiber. The lengthening of plateau phase can be ascribed to a decrease in K conductance, and the shortening of the plateau phase can be attributed to an increase in K conductance. Changes in Ca conductance may also be implicated in plateau phase[long dash]a decrease in Ca conductance being associated with a shortening and an increase in calcium conductance with a lengthening of plateau phase.