PDE1 Inhibition Modulates Cav1.2 Channel to Stimulate Cardiomyocyte Contraction

Rationale: cAMP activation of PKA (protein kinase A) stimulates excitation-contraction (EC) coupling, increasing cardiac contractility. This is clinically achieved by beta-ARs (beta-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 beta-ARs or PDE3i, PDE1i-stimulated inotropy is unaltered by beta-AR blockade and induces little whole-cell Ca2+ (intracellular Ca2+ concentration; [Ca2+](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 [Ca2+](i), whereas beta-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 [Ca2+](i) rise nor augmented beta-ARs. beta-ARs or PDE3i reduced myofilament Ca2+ sensitivity and increased sarcoplasmic reticulum Ca2+ 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+-Ca2+ exchanger current density. Cell shortening and [Ca2+](i) augmented by PDE1i were more sensitive to Ca(v)1.2 blockade and to premature or irregular cell contractions and [Ca2+](i) transients compared to PDE3i. Conclusions: PDE1i enhances contractility by a PKA-dependent increase in Ca(v)1.2 conductance with less total [Ca2+](i) increase, and no significant changes in sarcoplasmic reticulum [Ca2+], myofilament Ca2+-sensitivity, or phosphorylation of critical EC-coupling proteins as observed with beta-ARs and PDE3i. PDE1i could provide a novel positive inotropic therapy for heart failure without the toxicities of beta-ARs and PDE3i.

Automated summary

PDE1i enhances contractility by increasing Ca(v)1.2 conductance in a PKA-dependent manner, with lesser total [Ca2+](i) increase, and without significant changes in the phosphorylation of critical EC-coupling proteins.