Urocortin 2 Regulates Sarcoplasmic Reticulum Calcium via Phosphorylation of Phospholamban and SERCA Activation and Protects against Pro-Arrhythmic Alternans in Cardiac Myocytes from Normal and Failing Hearts

We tested the effect of the cardioactive peptide Urocortin 2 (Ucn2; 100 nM) on action potential (AP)-induced Ca transients (CaT), sarcoplasmic reticulum (SR) Ca load and cardiac Ca alternans. Experiments were performed on single rabbit atrial and ventricular myocytes from normal and failing hearts. Chronic heart failure (HF) was induced by combined pressure- and volume-overload. Ca and mechanical alternans was induced by electrical pacing. Changes in cytosolic [Ca]i and [Ca]SR were monitored with fluorescent indicators, in conjunction with sarcomere length measurements. The average Ca alternans ratio (AR = 1-S/L; S: small amplitude CaT; L: large-amplitude CaT) was 0.51 in normal atrial myocytes, and 0.43 and 0.53 in normal and HF ventricular myocytes, respectively. Ucn2 increased SR Ca load in normal, and to a larger degree in HF ventricular cells, and enhanced AP-induced SR Ca depletion transients. Baseline phosphorylation levels of phospholamban (PLN) at Ser16 were increased in HF myocytes, and Ucn2 stimulation further augmented PLN phosphorylation in normal myocytes (∼10-fold) and to a lesser extent (∼5-fold) in HF ventricular myocytes. Ucn2 completely abolished Ca and mechanical alternans (within 2-3 min) in normal atrial as well as in normal and HF ventricular myocytes. In the presence of Astressin-2B (1 µM; inhibition of corticotropin-releasing factor receptor; CRFR2) and H89 (1 µM; protein kinase A inhibition) Ucn2 failed to rescue alternans. In conclusion, we propose that Ucn2 rescues cardiac alternans via CRFR2-mediated stimulation of PKA, phosphorylation of PLN and enhanced cytosolic Ca sequestration by SERCA.