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Inhibition of β-adrenergic receptor (β-AR) signaling is one of the most common therapeutic approaches for patients with arrhythmias. Adenylyl cyclase (AC) is the key enzyme responsible for transducing β-AR stimulation to increases in cAMP. The two major AC isoforms in the heart are types 5 and 6. Therefore, it is surprising that prior studies on overexpreβion of AC5 and AC6 in transgenic (Tg) mice have not examined mediation of arrhythmogenesis. Our goal was to examine the proarrhythmic substrate in AC5Tg hearts. Intracellular calcium ion (Ca2+i) was imaged in fluo-4 AM-loaded ventricular myocytes. The sarcoplasmic reticulum (SR) Ca2+ content, fractional Ca2+ release, and twitch Ca2+ transient were significantly higher in the AC5Tg vs. wild-type (WT) myocytes, indicating Ca2+ overload in AC5Tg myocytes. Action potential (AP) duration was significantly longer in AC5Tg than in WT myocytes. Additionally, AC5Tg myocytes developed spontaneous Ca2+ waves in a larger fraction compared with WT myocytes, particularly when cells were exposed to isoproterenol. The Ca2+ waves further induced afterdepolarizations and triggered APs. AC5Tg hearts had increased level of SERCA2a, oxidized Ca2+/calmodulin-dependent protein kinase II (CaMKII), and phosphorylation of ryanodine receptors (RyR) at the CaMKII site, especially after isoproterenol treatment. This was consistent with higher reactive oxygen species production in AC5Tg myocytes after isoproterenol treatment. Isoproterenol induced more severe arrhythmias in AC5Tg than in WT mice. We conclude that AC5 overexpreβion promotes arrhythmogenesis, by inducing SR Ca2+ overload and hyperactivation of RyR (phosphorylation by CaMKII), which in turn induces spontaneous Ca2+ waves and afterdepolarizations. [ABSTRACT FROM AUTHOR]

Abnormal intracellular Ca 2+ handling is an important factor in the progressive functional decline of dystrophic muscle. In the present study, we investigated the function of sarco(endo)plasmic reticulum (SR) Ca 2+ ATPase (SERCA) in various dystrophic muscles of mouse models of Duchenne muscular dystrophy. Our studies show that the protein expression of sarcolipin, a key regulator of the SERCA pump is abnormally high and correlates with decreased maximum velocity of SR Ca 2+ uptake in the soleus, diaphragm and quadriceps of mild ( mdx) and severe ( mdx:utr−/−) dystrophic mice. These changes are more pronounced in the muscles of mdx:utr−/− mice. We also found increased expression of SERCA2a and calsequestrin specifically in the dystrophic quadriceps. Immunostaining analysis further showed that SERCA2a expression is associated both with fibers expressing slow-type myosin and regenerating fibers expressing embryonic myosin. Together, our data suggest that sarcolipin upregulation is a common secondary alteration in all dystrophic muscles and contributes to the abnormal elevation of intracellular Ca 2+ concentration via SERCA inhibition. [ABSTRACT FROM AUTHOR]

Inhibition of β-adrenergic receptor (β-AR) signaling is one of the most common therapeutic approaches for patients with arrhythmias. Adenylyl cyclase (AC) is the key enzyme responsible for transducing β-AR stimulation to increases in cAMP. The two major AC isoforms in the heart are types 5 and 6. Therefore, it is surprising that prior studies on overexpression of AC5 and AC6 in transgenic (Tg) mice have not examined mediation of arrhythmogenesis. Our goal was to examine the proarrhythmic substrate in AC5Tg hearts. Intracellular calcium ion (Ca(2+) i) was imaged in fluo-4 AM-loaded ventricular myocytes. The sarcoplasmic reticulum (SR) Ca(2+) content, fractional Ca(2+) release, and twitch Ca(2+) transient were significantly higher in the AC5Tg vs. wild-type (WT) myocytes, indicating Ca(2+) overload in AC5Tg myocytes. Action potential (AP) duration was significantly longer in AC5Tg than in WT myocytes. Additionally, AC5Tg myocytes developed spontaneous Ca(2+) waves in a larger fraction compared with WT myocytes, particularly when cells were exposed to isoproterenol. The Ca(2+) waves further induced afterdepolarizations and triggered APs. AC5Tg hearts had increased level of SERCA2a, oxidized Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), and phosphorylation of ryanodine receptors (RyR) at the CaMKII site, especially after isoproterenol treatment. This was consistent with higher reactive oxygen species production in AC5Tg myocytes after isoproterenol treatment. Isoproterenol induced more severe arrhythmias in AC5Tg than in WT mice. We conclude that AC5 overexpression promotes arrhythmogenesis, by inducing SR Ca(2+) overload and hyperactivation of RyR (phosphorylation by CaMKII), which in turn induces spontaneous Ca(2+) waves and afterdepolarizations., (Copyright © 2015 the American Physiological Society.)