Background and purpose: β-Adrenoceptors play a major role in regulating myocardial function through cAMP-dependent pathways. Different phosphodiesterases (PDEs) regulate intracellular cAMP-pools and thereby contribute to the compartmentalization of cAMP-dependent effects. We explored the involvement of PDEs in limiting the β(2) adrenoceptor-mediated positive inotropic (PIR) and lusitropic (LR) responses in sham-operated (Sham) and failing rat hearts.
Experimental approach: Extensive myocardial infarctions were induced by coronary artery ligation in Wistar rats. Rats developing heart failure were studied 6 weeks after surgery. Contractility was measured in left ventricular strips from failing and Sham hearts. cAMP was quantified by RIA.
Key results: In ventricular strips, stimulation of β(2) -adrenoceptors with (-)-adrenaline (300 nM CGP20712A present) exerted a small PIR and LR. In Sham hearts, β(2) -adrenoceptor-mediated as well as β(1) -adrenoceptor-mediated PIR and LR were increased by selective inhibition of either PDE3 (1 µM cilostamide) or PDE4 (10 µM rolipram). In failing rat hearts, PDE3 inhibition enhanced PIR and LR to both β(1) - and β(2) -adrenoceptor stimulation while PDE4 inhibition had no effect on these responses despite a significant increase in cAMP levels. Combined PDE3/4 inhibition further enhanced the PIR and LR of β(2) - and β(1) -adrenoceptor activation both in Sham and failing hearts, compared with PDE3 inhibition alone. PDE4 enzyme activity was reduced in failing hearts.
Conclusions and implications: Both PDE3 and PDE4 attenuated β(2) - and β(1) -adrenoceptor-mediated contractile responses in Sham hearts. In failing hearts, these responses are attenuated solely by PDE3 and thus even selective PDE3 inhibitors may provide a profound enhancement of β-adrenoceptor-mediated responses in heart failure.
© 2010 The Authors. Journal compilation © 2010 The British Pharmacological Society.