1 The responses of the electrically-driven right ventricle strip of the guinea-pig heart to diazepam were recorded in the absence and in the presence of di erent selective cyclic nucleotide phosphodiesterase (PDE) inhibitors. 2 Diazepam, at concentrations ranging from 1 mM to 100 mM, was devoid of e ect on the contractile force in this preparation. 3 Conversely, diazepam (5 mM ± 100 mM) produced a consistent positive inotropic response in the presence of a concentration (1 mM), that was without e ect in the absence of diazepam, of either of the selective PDE 3 inhibitors milrinone or SK&F 94120, but not in the presence of the selective PDE 4 inhibitor rolipram. 4 This e ect of diazepam was not g-aminobutyric acid (GABA)-dependent, since it was neither mimicked nor potentiated by GABA, and was not a ected by either a high concentration (5 mM) of the antagonists of the benzodiazepine/GABA/channel chloride receptor complex, picrotoxin,¯umazenil and b-carboline-3-carboxylic acid methyl ester (bCCMe), or by the inverse agonists, b-carboline-3-carboxylic acid N-methylamide (bCCMa) and methyl 6,7-dimethoxy-4-ethyl-b-carboline-3-carboxylate (DMCM, 0.1 mM). Furthermore, a speci®c antagonist of the peripheral benzodiazepine receptors, PK 11195 (5 mM), did not in¯uence the e ect of diazepam. 5 Biochemical studies with isolated PDEs, con®rmed that diazepam selectively inhibits type 4 PDE from guinea-pig right ventricle rather than the other PDEs present in that tissue. The compound inhibited this enzyme in a non-competitive manner. Diazepam was also able to inhibit PDE 5, the cyclic GMP speci®c PDE absent from cardiac muscle, with a potency close to that shown for PDE 4. 6 Diazepam displaced the selective type 4 PDE inhibitor, rolipram from its high a nity binding site in rat brain cortex membranes, and also potentiated the rise in cyclic AMP levels induced by isoprenaline in guinea-pig eosinophils, where only type 4 PDE is present. 7 The PDE inhibitory properties of diazepam were shared, although with lower potency, by other structurally-related benzodiazepines, that also displaced [ 3 H]-rolipram from its high a nity binding site. The order of potency found for these compounds in these assays was not related to their potencies as modulators of the GABA receptor through its benzodiazepine binding site. 8 The pharmacological and biochemical data presented in this study indicate that diazepam behaves as a selective type 4 PDE inhibitor in cardiac tissue and this e ect seems neither to be mediated by the benzodiazepine/GABA/channel chloride receptor complex nor by peripheral type benzodiazepine receptors.
