CARDIOTONIC ACTIONS OF SELECTIVE PHOSPHODIESTERASE INHIBITORS IN RAT ISOLATED VENTRICULAR CARDIOMYOCYTES

1 The contractile effects of the novel cardiotonic agent HN-10200 ylsu lphinyl-2-thienyl]-1H-imidazo-[4,5-c]-pyridine hydrochloride), were ex amined and comparisons made with the responses obtained to a structura lly similar compound, sulmazole, and to a number of other compounds wh ich are known to inhibit phosphodiesterase (PDE) isoenzymes with diffe ring selectivities; namely, enoximone (PDE III inhibitor), Ro 20-1724 (PDE IV inhibitor) and 3-isobutyl-1-methylxanthine (non-selective PDE inhibitor). 2 Contractile function, as measured by mechanical shorteni ng, and biochemical systems involving cyclic AMP were investigated in ventricular cardiomyocytes isolated from adult Sprague-Dawley rats (20 0-250 g). 3 HN-10200 exerted a concentration-dependent (10(-8) M-10(-4 ) M) positive contractile effect, which was independent of alpha- or b eta-adrenoceptor, or histamine receptor stimulation. 4 The efficacies of the contractile responses to the PDE inhibitors were of the order: HN-10200 > IBMX > sulmazole > enoximone and maximum stimulations, whic h were obtained at concentrations of 10(-4) M, were 54 +/- 4%, 41 +/- 7%, 38 +/- 7% and 26 +/- 5% (mean +/- s.e.) greater than basal levels, respectively (n = 6); the basal value of contractile amplitude (dL), in the absence of PDE inhibitors was 7.39 +/- 0.18% (mean +/- s.e.). R o 20-1724 did not have any effect on contractile activity. 5 Due to lo w basal levels of cyclic nucleotides in isolated cells, accumulation o f cyclic AMP due to the presence of the PDE inhibitors was detected on ly when the levels of cyclic nucleotide were enhanced with forskolin ( 10 mum). 6 The PDE inhibitors increased levels of cyclic AMP only at c oncentrations> 10(-4) M. HN-10200 and sulmazole had similar concentrat ion-dependent profiles for the accumulation of cyclic AMP; their poten cies were lower than that of IBMX (concentrations of forskolin require d to increase cyclic AMP by 4 mumol mg-1 protein, in the presence of m aximum concentrations of the PDE inhibitors, were 13 +/- 3 mum, 14 +/- 3 mum and 3 +/- 0.6 mum [mean +/- s.e.], respectively). 7 These resul ts indicate that a similar mechanism, probably through a weak inhibiti on of the cyclic AMP-specific PDE isoenzymes, is responsible for the i ncrease in levels of cyclic AMP by HN-10200 and sulmazole. However, cy clic AMP is only partially responsible for the positive contractile ef fect of HN-10200 and, similarly, sulmazole and IBMX. The lack of appar ent increase in levels of cyclic AMP by enoximone, highlights its degr ee of selectivity for the PDE III isoenzyme, such that the PDE IV isof orm is still present in sufficient quantity to degrade cyclic AMP with in the cell. On the other hand, the potent action of Ro 20-1724 on acc umulation of cyclic AMP, in addition to the lack of effect on contract ile function, is in agreement with the selectivity of this compound fo r the PDE IV isoenzyme and compartmentalization of cyclic AMP in rat i solated ventricular cardiomyocytes.