Cardiotonic actions of selective phosphodiesterase inhibitors in rat isolated ventricular cardiomyocytes

Abstract

1 The contractile effects of the novel cardiotonic agent HN-10200 (2-[3-methoxy-5-methylsulphinyl-2-thienyl]-lH-imidazo-[4,5-c]-pyridine hydrochloride), were examined and comparisons made with the responses obtained to a structurally similar compound, sulmazole, and to a number of other compounds which are known to inhibit phosphodiesterase (PDE) isoenzymes with differing selectivities; namely, enoximone (PDE III inhibitor), Ro 20-1724 (PDE IV inhibitor) and 3-isobutyl-1-methylxanthine (nonselective PDE inhibitor). 2 Contractile function, as measured by mechanical shortening, and biochemical systems involving cyclic AMP were investigated in ventricular cardiomyocytes isolated from adult Sprague-Dawley rats (200-250 g). 3 HN-10200 exerted a concentration-dependent (10-8 M-10-4 M) positive contractile effect, which was independent of a-or 0-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, which were obtained at concentrations of 10-4M, 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.). Ro 20-1724 did not have any effect on contractile activity. 5 Due to low basal levels of cyclic nucleotides in isolated cells, accumulation of cyclic AMP due to the presence of the PDE inhibitors was detected only when the levels of cyclic nucleotide were enhanced with forskolin (10 fM). 6 The PDE inhibitors increased levels of cyclic AMP only at concentrations> 10-' M. HN-10200 and sulmazole had similar concentration-dependent profiles for the accumulation of cyclic AMP; their potencies were lower than that of IBMX (concentrations of forskolin required to increase cyclic AMP by 4 pmol mg-' protein, in the presence of maximum concentrations of the PDE inhibitors, were 13 ± 3 11M, 14 ± 3 JAM and 3 ± 0.6 JAM [mean ± s.e.], respectively). 7 These results indicate that a similar mechanism, probably through a weak inhibition of the cyclic AMP-specific PDE isoenzymes, is responsible for the increase in levels of cyclic AMP by HN-10200 and sulmazole. However, cyclic AMP is only partially responsible for the positive contractile effect of HN-10200 and, similarly, sulmazole and IBMX. The lack of apparent increase in levels of cyclic AMP by enoximone, highlights its degree of selectivity for the PDE III isoenzyme, such that the PDE IV isoform is still present in sufficient quantity to degrade cyclic AMP within the cell. On the other hand, the potent action of Ro 20-1724 on accumulation of cyclic AMP, in addition to the lack of effect on contractile function, is in agreement with the selectivity of this compound for the PDE IV isoenzyme and compartmentalization of cyclic AMP in rat isolated ventricular cardiomyocytes.

Effects of type‐selective phosphodiesterase inhibitors on glucose‐induced insulin secretion and islet phosphodiesterase activity

“…The 30% inhibition produced by zaprinast in the supernatant fraction, but not consistently in the pellet, also supported the previous demonstration of an islet cytosolic Ca2+-calmodulin sensitive (type I) PDE (Capito et al, 1986 (Kelso et al, 1993). In contrast, the type IV selective PDE inhibitor Ro 20-1724 failed to increase contractile activity of cardiomyocytes, despite producing marked increase in cyclic AMP (Kelso et al, 1993). The failure of type III inhibitors alone to increase islet cyclic AMP levels could be due to cyclic AMP hydrolysis by other, non-inhibited PDE isoenzymes.…”

“…It remains possible that other cyclic AMP-independent actions may be involved, as suggested for the effect of PDE inhibitors on ventricular cardiomyocytes (Kelso et al, 1993). Blockade of adenosine receptors, referred to above, is unlikely to be implicated in the actions of IBMX or of the other PDE inhibitors since, in the present work, adenosine either did not significantly modify insulin secretion or tended to increase it.…”

“…Similar discrepancies between functional effects of selective PDE inhibitors and their effects on cyclic AMP accumulation have been reported in other systems. For example, in ventricular cardiomyocytes, compounds with PDE inhibitory activity increased cyclic AMP accumulation only at concentrations that produced maximal response on contractile function and enoxamine, the selective PDE III inhibitor, failed to produce any increase in cyclic AMP (Kelso et al, 1993). In contrast, the type IV selective PDE inhibitor Ro 20-1724 failed to increase contractile activity of cardiomyocytes, despite producing marked increase in cyclic AMP (Kelso et al, 1993).…”

“…For example, in ventricular cardiomyocytes, compounds with PDE inhibitory activity increased cyclic AMP accumulation only at concentrations that produced maximal response on contractile function and enoxamine, the selective PDE III inhibitor, failed to produce any increase in cyclic AMP (Kelso et al, 1993). In contrast, the type IV selective PDE inhibitor Ro 20-1724 failed to increase contractile activity of cardiomyocytes, despite producing marked increase in cyclic AMP (Kelso et al, 1993). The failure of type III inhibitors alone to increase islet cyclic AMP levels could be due to cyclic AMP hydrolysis by other, non-inhibited PDE isoenzymes.…”

Myocardial Phosphodiesterases and Regulation of Cardiac Contractility in Health and Cardiac Disease

“…A similar finding was reported with regards to papaverine-induced effects in isolated guinea-pig heart [226]. In isolated rat cardiac myocytes, a significant increase in intracellular cAMP induced by Ro 20-1724, a selective PDE4 inhibitor, was not associated with an effect on contractile function [227]. In isolated, perfused guineapig heart preparations, rolipram, a selective PDE4 inhibitor, produced more pronounced cAMP accumulation and myocardial PKA activation as compared to SK&F 94120, a selective PDE3 inhibitor [72].…”