Fig. 1.

Cyclic GMP-hydrolytic activities in subcellular fractions of human myocardium. Total phosphodiesterase activity was determined at 0.1 μM cGMP in the presence of 200 μMCa²⁺ and 50 nM calmodulin (CaM). Activity was also measured in the presence of 100 μM EGTA. The difference between total and Ca²⁺-independent activity was taken as Ca²⁺/calmodulin-stimulated PDE1 activity. PDE3 activity was calculated based on phosphodiesterase assays performed in the presence of EGTA using the PDE3-selective inhibitor cilostazol (0.3 μM). Each value represents the average ± S.D. of at least three determinations. Differences between normal and failing hearts with respect to Ca²⁺/calmodulin-dependent PDE1 activity and PDE3 activity were not statistically significant.