Prostacyclin Synthesis, and Ultrastructure in Isolated Rat Hearts

An extensive investigation of the cardiac actions of phorbol esters and the potential role of the Na+-H+ exchanger in those actions was carried out using isolated rat hearts. Sixty minutes of perfusion with 10-` M phorbol 12-myristate 13-acetate (PMA) or 10-8 M phorbol 12,13-dibutyrate (PDBu) produced marked cardiac dysfunction associated with depressed contrac-tility, coronary constriction, and elevated resting tension, the latter being particularly evident with PMA. These effects were also associated with disturbances in tissue levels of energy metabolites manifested primarily by a reduction in ATP and an elevation in lactate. Furthermore , both phorbols produced a sustained stimulation of the release of 6-ketoprostaglandin F1, (6-keto PGF1), the hydrolysis product of prostacyclin (prostaglandin 12). Amiloride, an inhibitor of the Na+-H+ exchanger, significantly attenuated the loss in contractility and elevation in coronary pressure as well as the stimulated release of 6-keto PGF1l but was without effect on elevations in resting tension or on changes in energy metabolites. Increasing concentrations of PMA or PDBu 10-fold resulted in a much more rapid and severe (>80%Yo loss in contractile function after 30 minutes) effect that was nonetheless qualitatively identical to that seen with the lower concentrations of phorbol. However, the effects were not prevented by amiloride. Surprisingly, 4a-phorbol 12,13-didecanoate (a.PDD, 10-6 M), which does not activate protein kinase C, was found to be a potent inhibitor of cardiac function (>80%o loss in contractility and 50%o increase in resting tension) after 30 minutes of perfusion, although these effects were not associated with changes in levels of energy metabolites or with elevations in coronary pressure. Similarly, none of the actions of this compound were attenuated by amiloride. In contrast to the sustained effects of other phorbols on 6-keto PGF1, release, the effect of a.PDD was transient (<10 minutes). In all hearts studied, the marked depression in contractile function caused by all phorbol esters occurred in the absence of any ultrastructural changes. 4ac-Phorbol (10-' M), which does not activate protein kinase C, was without effect on any parameter studied. Our results demonstrate very complex effects of phorbol esters on numerous parameters of cardiac function, including an amiloride-sensitive component that occurs at low concentrations. The latter observation suggests the involvement of Na+-H+ exchange activation, possibly occurring as a consequence of protein kinase C stimulation, in mediation of the effects of phorbol esters at low concentrations. In addition, the inability of amiloride to attenuate the cardiac actions of high …