Phosphodiesterase-4 blunts inotropism and arrhythmias but not sinoatrial tachycardia of (-)-adrenaline mediated through mouse cardiac beta(1)-adrenoceptors

Abstract

Background and purpose: beta(1) and beta(2)-adrenoceptors coexist in murine heart but beta(2)-adrenoceptor-mediated effects have not been detected in atrial and ventricular tissues, possibly due to marked phosphodiesterase (PDE) activity. We investigated the influence of the PDE3 inhibitor cilostamide and PDE4 inhibitor rolipram on the effects of (-)-adrenaline in three regions of murine heart.

Experimental approach: (-)-Adrenaline-evoked cardiostimulation was compared on sinoatrial beating rate, left atrial and right ventricular contractile force in isolated tissues from 129SvxC57B1/6 cross mice. Ventricular arrhythmic contractions were also assessed.

Key results: Both rolipram (1 microM) and cilostamide (300 nM) caused transient sinoatrial tachycardia but neither enhanced the chronotropic potency of (-)-adrenaline. Rolipram potentiated 19-fold (left atrium) and 7-fold (right ventricle) the inotropic effects of (-)-adrenaline. (-)-Adrenaline elicited concentration-dependent ventricular arrhythmias that were potentiated by rolipram. All effects of (-)-adrenaline were antagonized by the beta(1)-adrenoceptor-selective antagonist CGP20712A (300 nM). Cilostamide (300 nM) did not increase the chronotropic and inotropic potencies of (-)-adrenaline, but administered jointly with rolipram in the presence of CGP20712A, uncovered left atrial inotropic effects of (-)-adrenaline that were prevented by the beta(2)-adrenoceptor-selective antagonist ICI118551.

Conclusions and implications: PDE4 blunts the beta(1)-adrenoceptor-mediated effects of (-)-adrenaline in left atrium and right ventricle but not in sinoatrial node. Both PDE3 and PDE4 reduce basal sinoatrial rate in a compartment distinct from the beta(1)-adrenoceptor compartment. PDE3 and PDE4, acting in concert, prevent left atrial beta(2)-adrenoceptor-mediated inotropy. PDE4 partially protects the right ventricle against (-)-adrenaline-evoked arrhythmias.

Figure 1

Potentiation of the effects of (−)-adrenaline by rolipram but not by cilostamide on left atrium. Representative experiments, depicting cumulative concentration–effect curves for (−)-adrenaline in the absence of PDE inhibitors (a), in the presence of rolipram (b) and in the presence of cilostamide (c). Black spots indicate −log(−)-adrenaline concentrations, achieved by cumulative administration. Ca, CaCl₂ (9 mM); Iso, (−)-isoprenaline (200 μM).

Figure 2

Positive inotropic and arrhythmic effects of (−)-adrenaline on right ventricular walls; influence of PDE inhibitors. Representative experiments, depicting cumulative concentration–effect curves for (−)-adrenaline in the absence of PDE inhibitors (a), presence of rolipram (b) and presence of cilostamide (c). Black spots indicate −log(−)-adrenaline concentrations, achieved by cumulative administration. Ca, CaCl₂ (9 mM); Iso, (−)-isoprenaline (200 μM).

Figure 3

Rolipram potentiates the positive inotropic and arrhythmic effects of (−)-adrenaline on murine right ventricular wall. Comparison of the effects of increasing (−)-adrenaline concentrations on a right ventricular wall in the absence (left hand panels) and another right ventricular wall in the presence of rolipram (right hand panels). Please note extrasystoles at 10 and 100 μM (−)-adrenaline in the absence of rolipram and with rolipram and all (−)-adrenaline concentrations in the presence of rolipram. (−)-Adrenaline 10 μM in the absence of rolipram and (−)-adrenaline 0.01 and 100 μM in the presence of rolipram caused episodes of tachycardia.

Figure 4

Potentiation of the effects of (−)-adrenaline by rolipram, mediated through β₁-adrenoceptors, on left atria (b) and right ventricular walls (c), but not on sinoatrial pacemaker (a) in the absence and presence of CGP20712A. Lack of effect of cilostamide on (−)-adrenaline potency. A single concentration–effect curve for (−)-adrenaline was determined in the absence or presence of CGP20712A. IPDE, PDE inhibitor. ^*Increase of the E_max of (−)-adrenaline by rolipram (^*P<0.03).

Figure 5

Increased incidence of (−)-adrenaline-evoked arrhythmias by rolipram (a) and blockade by CGP20712A (b). %Arrythmias is the % of ventricles that showed extrasystoles and/or ventricular tachycardia at each (−)-adrenaline concentration. Lines depict the dependence of arrhythmias on −log(−)-adrenaline concentration. The slopes of the lines were 0.126 and 0.096 in the absence and presence of rolipram, respectively. B, basal; ISO, (−)-isoprenaline 200 μM;R, rolipram.

Figure 6

Cilostamide (Cil) and rolipram (Rol), administered together, potentiate the chronotropic effects of (−)-adrenaline (a) and uncover β₂AR-mediated inotropic effects in the presence of CGP20712A, sensitive to blockade by ICI118551, in left atrium (b) but not in right ventricle (c). Data from left atria in the presence of CGP20712A were fitted for two β-adrenoceptor populations with −log EC₅₀ M of 7.93±0.30 (β₂-adrenoceptor) and 5.53±0.16 (β₁-adrenoceptor) and fractional E_max of 0.74±0.07 (β₁-adrenoceptor) and 0.26±0.07 (β₂-adrenoceptor). The left atrial −log EC₅₀ M of the curve for (−)-adrenaline in the presence of both CGP20712A and ICI118551 was 5.44±0.08.