PDE1 inhibition modulates Ca v 1.2 channel to stimulate cardiomyocyte contraction

Rationale Cyclic adenosine monophosphate (cAMP) activation of protein kinase A (PKA) stimulates excitation-contraction coupling, increasing cardiac contractility. This is clinically leveraged by beta-adrenergic stimulation (β-ARs) or phosphodiesterase-3 inhibition (PDE3i), though both approaches are limited by arrhythmia and chronic myocardial toxicity. Phosphodiesterase-1 inhibition (PDE1i) also augments cAMP and was recently shown in rabbit cardiomyocytes to augment contraction independent of β-AR stimulation or blockade, and with less intracellular calcium rise than β-ARs or PDE3i. Early testing of PDE1 inhibition in humans with neuro-degenerative disease and dilated heart failure has commenced. Yet, the molecular mechanisms for PDE1i inotropic effects remain largely unknown. Objective Define the mechanism(s) whereby PDE1i increases contractility. Methods and Results Primary guinea pig myocytes which express the cAMP-hydrolyzing PDE1C isoform found in larger mammals and humans were studied. The potent, selective PDE1i (ITI-214) did not alter cell shortening or Ca 2+ transients under resting conditions whereas both increased with β-ARs or PDE3i. However, PDE1i enhanced shortening with less Ca 2+ rise in a PKA-dependent manner when combined with low-dose adenylate cyclase stimulation (Forskolin). Unlike PDE3i, PDE1i did not augment β-AR responses. Whereas β-ARs reduced myofilament Ca 2+ sensitivity and increased sarcoplasmic reticular Ca 2+ content in conjunction with greater phosphorylation of troponin I, myosin binding protein C, and phospholamban, PDE1i did none of this. However, PDE1i increased Ca v 1.2 channel conductance similar to PDE3i in a PKA-dependent manner. Myocyte shortening and peak Ca 2+ transients were more sensitive to Ca v 1.2 blockade with nitrendipine combined with PDE1i versus PDE3i. Lastly, PDE1i was found to be far less arrythmogenic than PDE3i. Conclusions PDE1i enhances contractility by a PKA-dependent increase in Ca v 1.2 conductance without concomitant myofilament desensitization. The result is less rise in intracellular Ca 2+ and arrhythmia compared to β-ARs and/or PDE3i. PDE1i could be a novel positive inotrope for failing hearts without the toxicities of β-ARs and PDE3i.