Effects of vanadate ions on action potentials and tension development in papillary muscles and cultured heart muscle cells.

In isolated and electrically driven (1/s) papillary muscles peak tension and, to a slighter degree, resting tension increased through treatment with Na3VO4 in concentrations higher than 10(-4)M. The positive inotropic effect was found to be transient. With vanadate, the resting membrane potential became more negative and rate of rise and overshoot of the action potential increased. Positivity and duration of the action potential plateau was markedly lessened by vanadate. These vanadate-induced changes of the electrical activity were not transient and hardly recovered during washout. Single cardiac cells in culture showed increased beat frequency when treated with vanadate ions in excess of 10(-4)M. This increase was fast but transient and led to cessation of spontaneous discharge. Simultaneously with the initial increase of beat frequency, maximum of rate of rise, diastolic potential and overshoot of the action potential also increased. Cells inactivated by vanadate (5 X 10(-4)M and higher concentrations) were relaxed in diastole and could be excited by intracellular stimulation.