Hearts from AC8TG mice develop a higher contractility (LVSP) and larger Ca 2؉ transients than NTG mice, with (surprisingly) no modification in L-type Ca 2؉ channel current (I Ca,L) (1). In this study, we examined the cardiac response of AC8TG mice to ␤-adrenergic and muscarinic agonists and IBMX, a cyclic nucleotide phosphodiesterase (PDE) inhibitor. Stimulation of LVSP and I Ca,L by isoprenaline (ISO, 100 nM) was twofold smaller in AC8TG vs. NTG mice. In contrast, IBMX (100 M) produced a twofold higher stimulation of I Ca,L in AC8TG vs. NTG mice. IBMX (10 M) increased LVSP by 40% in both types of mice, but contraction and relaxation were hastened in AC8TG mice only. Carbachol (10 M) had no effect on basal contractility in NTG hearts but decreased LVSP by 50% in AC8TG mice. PDE assays demonstrated an increase in cAMP-PDE activity in AC8TG hearts, mainly due to an increase in the hydrolytic activity of PDE4 and PDE1 toward cAMP and a decrease in the activity of PDE1 and PDE2 toward cGMP. We conclude that cardiac expression of AC8 is accompanied by a rearrangement of PDE isoforms, leading to a strong compartmentation of the cAMP signal that shields L-type Ca 2؉ channels and protects the cardiomyocytes from Ca 2؉ overload.