AT1 receptor antagonist restores cardiac ryanodine receptor function, rendering isoproterenol-induced failing heart less susceptible to Ca2+-leak induced by oxidative stress

Background The Ca2+ regulatory proteins in the sarcoplasmic reticulum (SR) play a key role in the pathogen-1 esis of heart failure. In the present study the effect of chronic beta-receptor-stimulation on cardiac and SR functions was assessed, with or without angiotensin-II receptor antagonist treatment recently reported to have anti-beta-adrenergic activity. Methods and Results Rats were treated with isoproterenol with (+) or without (-) candesartan (CAN) and then SR vesicles were isolated from the left ventricular muscle. Both Ca2+-uptake and the amount of SR Ca2+- ATPase were significantly lower in the CAN(-) group than in the shams, but those were almost normally restored in the CAN(+). Although the level of the protein kinase A (PKA)-phosphorylation of the SR Ca2+ release channel, known as the ryanodine receptor (RyR2), was elevated in the CAN(-), no Ca2+-leak was detected. However, SIN-1 (O-2(-) donor) induced Ca2+-leak in the CAN(-) at a 10-fold lower dose than in the sham and CAN(+). In cardiomyocytes, SIN-1 decreased cell shortening and the peak Ca2+ transient and prolonged time from peak to 70% decline in CAN (-), again at 10-fold lower dose than in the sham and CAN(+). Conclusion Chronic beta-receptor-stimulation did not induce any Ca2+-leak from the SR, whereas Ca2+-leak was easily induced when oxidative stress was applied to the PKA-phosphorylated RyR2. Candesartan not only improved Ca2+-uptake, but also prevented PKA-phosphorylation, rendering the SR less susceptible to Ca2+-leak.