Light Transients in Intact Canine Ventricular Myocardium

The action of a novel cardiotonic agent N-hydroxy-5,6-dimethoxy-benzo[b]thiophene-2-carboximide hydrochloride (Org 30029) on intracellular aequorin light transients and isometric contractions was investigated in isolated canine ventricular trabeculae. The positive inotropic effect of Org 30029 (30 ,uM-3 mM) was consistently associated with prolongation of the duration of contraction and an increase in the amplitude of the intracellular Ca2' transients. The maximum inotropic response to Org 30029 was approximately 150% of the maximum response to isoproterenol, whereas the maximum increase in the amplitude of Ca2' transients produced by Org 30029 was only 20% of the isoproterenol-induced maximum. The duration of isometric contractions was prolonged by Org 30029, with no change in the duration of the light transients. The concentration-response curve for the positive inotropic effect of Org 30029 was shifted by carbamylcholine chloride (carbachol, 3 AM) to the right and downward, but the maximum response to Org 30029 was greater than that to isoproterenol even in the presence of carbachol. Carbachol abolished the increase in light transients and cAMP accumulation induced by Org 30029 (1 mM), whereas it only partially attenuated the positive inotropic effect of Org 30029. In the presence of carbachol (3 JM), Org 30029 increased the force of contraction in a concentration-dependent manner without augmenting the aequorin light transients. These results are compatible with the hypothesis that Org 30029 increases cardiac contractility by increasing myofilament Ca2' responsiveness. (Circulation Research 1993;72:597-606)