LONG-LASTING ANTIADRENERGIC EFFECT OF 7-OXO-PROSTACYCLIN IN THE HEART - A CYCLOHEXIMIDE SENSITIVE INCREASE OF PHOSPHODIESTERASE ISOFORM-I AND ISOFORM-IV ACTIVITIES

Evidence is accumulating that 7-oxo-prostacyclin (7-oxo-PGI(2)) induces a delayed indirect anti-adrenergic and cytoprotective effect on the myocardium, the mechanism of which is still unclear. To demonstrate that a single application of 7-oxo-PGI(2) (50 mu g/kg i.m.) 48 h prior to starting experiments attenuates the isoprenaline inducible inotropic response and accumulation of cAMP, isolated hearts of pretreated animals were perfused in the Langendorff mode with and without isoprenaline (1 to 100 nM). The late anti-adrenergic effect of the drug was manifested by a significant attenuation in the elevation of cAMP levels as well as in contractile force development. This effect was not due to changes in cAMP generation as there were identical beta(1)-adrenoceptor densities and affinities (as calculated from [H-3]-CGP binding studies), G(i) and G(alpha s) protein patterns (as taken from Western blots) as well as adenylyl cyclase activity measurements in the hearts studied. The anti-adrenergic potency of 7-oxo-PGI(2), however, was found to be related to a significant rise in cyclic nucleotide hydrolysis by phosphodiesterase (PDE). Using the fast-performance liquid chromatographic separation for PDE isoforms, a significant increase in the activity of PDE isoforms I and IV (260 +/- 28 vs 110 +/- 12 pmol cGMP/min x enzyme fraction and 77 +/- 11 vs 34 +/- 3 pmol cAMP/min x enzyme fraction, respectively) was found in the solubilized fraction of cardiac membranes in comparison to untreated controls; PDE IV activity was also increased in the cytosolic fraction (106 +/- 14 vs 65 +/- 6 pmol cAMP/min x enzyme fraction). The hypothesis that the delayed anti-adrenergic effect of 7-oxo-PGI(2) is initiated by an induction and accelerated synthesis of PDE I and IV in the heart is underlined by the fact that cycloheximide suppresses completely both the rise in PDE activities and the anti-adrenergic effects studied. It is suggested that an inducible predominance of cAMP degradation over its generation may be of relevance in processes related to heart protection.