Examples of cell shortening during 1-Hz field-stimulation confirm comparable fractional shortening normalized to resting cell length, L/L0, of paced cardiomyocytes (mean 7.6% in WT versus 7.3% in PDE5-TG, >60 cells per group, A). Contraction, indicated as percent cell shortening, significantly increased following isoproterenol stimulation in both genotypes (n=13 and 16 for WT and PDE5-TG) (B). Following PDE5 inhibition, contraction is modestly reduced compared to baseline and enhanced again after β-adrenergic stimulation; relative changes are similar in WT (n=5) and PDE5-TG cardiomyocytes (n=5) (C). Amplitude and voltage dependence of L-type calcium channel current ICaL were similar in both genotypes (n=11 WT, n=11 PDE5-TG) (D). *P<0.05 for the effect of the intervention compared to baseline.