Progesterone modulates SERCA2a expression and function in rabbit cardiomyocytes

We recently showed that progesterone treatment abolished arrhythmias and sudden cardiac death in a transgenic rabbit model of long QT syndrome type 2 (LQT2). Moreover, levels of cardiac sarco(endo) plasmic reticulum Ca2+-ATPase type 2a (SERCa2a) were upregulated in LQT2 heart extracts. We hypothesized that progesterone treatment upregulated SERCa2+a expression, thereby reducing Ca2+-dependent arrhythmias in LQT2 rabbits. We therefore investigated the effect of progesterone on SERCa2a regulation in isolated cardiomyocytes. Cardiomyocytes from neonatal (3- to 5-day-old) rabbits were isolated, cultured, and treated with progesterone and other pharmacological agents. Immunoblotting was performed on total cell lysates and sarcoplasmic reticulum-enriched membrane fractions for protein abundance, and mRNA transcripts were quantified using real-time PCR. The effect of progesterone on baseline Ca2+ transients and Ca2+ clearance was determined using digital imaging. Progesterone treatment increased the total pool of SERCa2a protein by slowing its degradation. Using various pharmacological inhibitors of degradation pathways, we showed that progesterone-associated degradation of SERCa2a involves ubiquitination, and progesterone significantly decreases the levels of ubiquitin-tagged SERCa2a polypeptides. Our digital imaging data revealed that progesterone significantly shortened the decay and duration of Ca2+ transients. Progesterone treatment increases protein levels and activity of SERCa2a. Progesterone stabilizes SERCa2a, in part, by decreasing the ubiquitination level of SERCa2a polypeptides.