Influence of vanadate on electrophysiological and contractile parameters of atrial myocardium

Whereas vanadate increases isometric force of contraction in stimulated rat left atria (EC50: 36.6 mumol/1), it decreases force of contraction in guinea-pigs (EC50: 4.7 mumol/1). Action potential duration at 90 and 30% repolarisation is decreased in guinea-pig atria by 70 and 60%, but in rat atria by only 40 and 7% respectively. Whereas the time course of the negative inotropic action is rapid and correlates with that of the shortening of the action potential, there is no relationship between action potential changes and the slowly developing positive intropic effect. Negative and positive inotropic actions seem to be the result of two superimposed processes, whose kinetics suggest an extra- and intracellular site of action. Na+, K+-ATPase prepared from rat and guinea-pig atria and ventricle is inhibited by vanadate to a similar extent: IC50 (mumol/1) rat: atria 1.05, ventricle 0.49; guinea-pig: atria 0.75, ventricle 0.62. This indicates that the discrepancy in inotropy cannot be explained by the different sensitivity of the Na+, K+-ATPase to vanadate.