Maria Luisa Vargas

Ontogenic changes of the control by phosphodiesterase‐3 and ‐4 of 5‐HT responses in porcine heart and relevance to human atrial 5‐HT4 receptors

Background and purpose:  Atrial inotropic responses to 5‐HT mediated through 5‐HT4 receptors fade, presumably through phosphodiesterase (PDE) activity. We investigated the influence of a selective inhibitor of PDE3 (cilostamide) or of PDE4 (rolipram) on the fade of 5‐HT responses in atrial muscle.

Phosphodiesterase PDE3 blunts the positive inotropic and cyclic AMP enhancing effects of CGP12177 but not of noradrenaline in rat ventricle

The cardiostimulant effects of CGP12177, mediated through a β1‐adrenoceptor site with low affinity for (−)‐propranolol, are potentiated by the nonselective PDE inhibitor IBMX but the role of PDE isoenzymes is unknown. We studied the effects of the PDE3‐selective inhibitor cilostamide (300 nM) and PDE4‐selective inhibitor rolipram (1 μM) on the positive inotropic and cyclic AMP‐enhancing effects of CGP12177 and noradrenaline in right ventricular strips of rat. CGP12177 (under (−)‐propranolol 200 nM) only increased contractile force in the presence of either cilostamide or rolipram with −logEC50M 6.7 (Emax=23% over basal) and 7.1 (Emax=50%) respectively. The combination of cilostamide and rolipram caused CGP12177 to enhance contractile force with −logEC50M=7.7 and Emax=178%. The positive inotropic effects of noradrenaline (−logEC50M=6.9) were potentiated by rolipram (−logEC50M=7.4) but not by cilostamide (−logEC50M=7.0). In the presence of rolipram and (−)‐propranolol, noradrenaline (2 μM) and CGP12177 (10 μM) produced matching inotropic effects but failed to increase cyclic AMP levels. 20 μM (−)‐noradrenaline increased cyclic AMP levels, a response further enhanced by rolipram. Both PDE3 and PDE4 of rat ventricle appear to hydrolyse cyclic AMP generated through the low‐affinity β1‐adrenoceptor site, thereby preventing inotropic responses of CGP12177. When (−)‐noradrenaline interacts with the β1‐adrenoceptor, the generated cyclic AMP is hydrolysed only by PDE4, thereby reducing cardiostimulation.

Diazepam enhances inotropic responses to dopamine in rat ventricular myocardium.

Diazepam inhibits phosphodiesterase type 4 and enhances the effect of some 3′,5′-cyclic adenosine monophosphate (cAMP)-dependent positive inotropic drugs. We sought to determine whether diazepam and the selective phosphodiesterase type 4 inhibitor rolipram enhances the contractile response and cAMP levels induced by dopamine in rat myocardium. Dopamine (3–100 &mgr;M) produced concentration-dependent positive inotropic effects (−log EC50 = 5.21 ± 0.2, n = 5), which were augmented in the presence of 10 &mgr;M diazepam (-log EC50 = 5.40 ± 0.08, n = 6, P < 0.05) or 1 &mgr;M rolipram (-log EC50 = 5.41 ± 0.1, n = 6, P < 0.05). The effect of diazepam was not mimicked by 100 &mgr;M &ggr;-aminobutyric acid nor it was antagonized by a 5 &mgr;M concentration of the blockers of central and peripheral type benzodiazepine receptors, flumazenil and PK 11195. cAMP levels (pmol/g) produced by dopamine (744.4 ± 111.8, n = 5) in this tissue were enhanced by the presence of diazepam (1073 ± 97.7, n = 6, P < 0.05) or rolipram (1034.0 ± 245.2, n = 5, P < 0.05). Therefore, diazepam, like rolipram, augments the inotropic and biochemical effects of dopamine in rat myocardium. This effect is not mediated by benzodiazepine receptors but is probably the consequence of the phosphodiesterase type 4 inhibitory activity of diazepam.

Function of cardiac β1- and β2-adrenoceptors of newborn piglets: Role of phosphodiesterases PDE3 and PDE4

The structures of porcine and human beta(2)-adrenoceptors differ but the repercussions for porcine cardiac function are unknown. We investigated the function of porcine beta(2)-adrenoceptors in 3 cardiac regions, sinoatrial node, left atrium and right ventricle of newborn piglets. Both (-)-noradrenaline and (-)-adrenaline caused sinoatrial tachycardia: 60+/-10% and 62+/-7% of the maximum response (E(max)) to (-)-noradrenaline (-logEC(50)=9.0) and (-)-adrenaline (-logEC(50)=7.5) respectively, were resistant to antagonism by the beta(1)-selective CGP20712A (2-hydroxy-5-[2-[[2-hydroxy-3-[4-[1-methyl-4-(trifluoromethyl)-1H-imidazol-2-yl]phenoxy]propyl]amino]ethoxy]-benzamide) (300 nM) but antagonized by beta(2)-selective ICI118551 (erythro(+/-)-[1-(2,3-dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol) (50 nM), consistent with mediation through beta(2)-adrenoceptors. The phosphodiesterase3-selective inhibitor cilostamide and phosphodiesterase4-selective inhibitor rolipram did not affect catecholamine chronotropic potencies. Only small CGP20712A-resistant positive inotropic effects of (-)-adrenaline were detected in the left atria (13+/-2% of E(max)) and ventricular trabeculae (14+/-5% of E(max)). The atrial inotropic responses to (-)-noradrenaline and (-)-adrenaline faded; fades were prevented by rolipram but not cilostamide or concurrent cilostamide+rolipram respectively. (-)-Noradrenaline (ICI118551 present) increased left atrial cAMP levels through beta(1)-adrenoceptors that were markedly enhanced by rolipram but unaffected by cilostamide. Concurrent cilostamide+rolipram uncovered inotropic and cAMP responses to (-)-adrenaline (CGP20712A present). We conclude that sinoatrial beta(2)-adrenoceptors are more important than beta(1)-adrenoceptors in the mediation of tachycardia caused by both (-)-noradrenaline and (-)-adrenaline in the newborn piglet. beta(2)-adrenoceptors have only a minor role in the mediation of left atrial and ventricular inotropic effects of (-)-adrenaline. Catecholamine-evoked tachycardia is not controlled by PDE3 or PDE4. PDE4, but not PDE3, controls the atrial inotropic and cAMP beta(1)-adrenoceptor-mediated responses to (-)-noradrenaline. Both PDE3 and PDE4 blunt left atrial inotropic and cAMP responses to (-)-adrenaline through beta(2)-adrenoceptors.

Mechanism of action of B-HT 933 (azepexole) in rat vas deferens and guinea-pig ileum.

The mechanism of action of B-HT 933 (azepexole) was studied on the rat vas deferens and myenteric plexus-longitudinal muscle (MP-LM) of the guinea-pig ileum. The drug caused a concentration-dependent inhibition of the twitch response in both preparations. The maximal inhibition in both preparations was 80-90%. B-HT 933 did not affect the cumulative dose-response curves of the vas deferens and of MP-LM to noradrenaline (NA) and acetylcholine (Ach) respectively. Yohimbine antagonized in a competitive way the twitch inhibitory effect of B-HT 933 on vas deferens and on MP-LM; the pA2 values were 8.62 and 8.5, respectively. The twitch inhibitory effects of B-HT 933 were not antagonized by propranolol. The results suggest that the action of B-HT 933 is mediated by stimulation of presynaptic alpha 2-receptors.