Rationale:
cAMP activation of PKA (protein kinase A) stimulates excitation-contraction (EC) coupling, increasing cardiac contractility. This is clinically achieved by β-ARs (β-adrenergic receptor) stimulation or PDE3i (inhibition of phosphodiesterase type-3), although both approaches are limited by arrhythmia and chronic myocardial toxicity. PDE1i (Phosphodiesterase type-1 inhibition) also augments cAMP and enhances contractility in intact dogs and rabbits. Unlike β-ARs or PDE3i, PDE1i-stimulated inotropy is unaltered by β-AR blockade and induces little whole-cell Ca
2+
(intracellular Ca
2+
concentration; [Ca
2+
]
i
) increase. Positive inotropy from PDE1i was recently reported in human heart failure. However, mechanisms for this effect remain unknown.
Objective:
Define the mechanism(s) whereby PDE1i increases myocyte contractility.
Methods and Results:
We studied primary guinea pig myocytes that express the PDE1C isoform found in larger mammals and humans. In quiescent cells, the potent, selective PDE1i (ITI-214) did not alter cell shortening or [Ca
2+
]
i
, whereas β-ARs or PDE3i increased both. When combined with low-dose adenylate cyclase stimulation, PDE1i enhanced shortening in a PKA-dependent manner but unlike PDE3i, induced little [Ca
2+
]
i
rise nor augmented β-ARs. β-ARs or PDE3i reduced myofilament Ca
2+
sensitivity and increased sarcoplasmic reticulum Ca
2+
content and phosphorylation of PKA-targeted serines on TnI (troponin I), MYBP-C (myosin binding protein C), and PLN (phospholamban). PDE1i did not significantly alter any of these. However, PDE1i increased Ca
v
1.2 channel conductance similarly as PDE3i (both PKA dependent), without altering Na
+
-Ca
2+
exchanger current density. Cell shortening and [Ca
2+
]
i
augmented by PDE1i were more sensitive to Ca
v
1.2 blockade and to premature or irregular cell contractions and [Ca
2+
]
i
transients compared to PDE3i.
Conclusions:
PDE1i enhances contractility by a PKA-dependent increase in Ca
v
1.2 conductance with less total [Ca
2+
]
i
increase, and no significant changes in sarcoplasmic reticulum [Ca
2+
], myofilament Ca
2+
-sensitivity, or phosphorylation of critical EC-coupling proteins as observed with β-ARs and PDE3i. PDE1i could provide a novel positive inotropic therapy for heart failure without the toxicities of β-ARs and PDE3i.