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Department of Surgery, University of Kentucky College of Medicine, Lexington, Kentucky 40536
Experiments were performed to examine whether the protein phosphatase inhibitor cantharidin blocks the anti-adrenergic effect of adenosine A1 receptor stimulation. In electrically stimulated adult rat ventricular myocytes loaded with the intracellular calcium concentration ([Ca2+]i) indicator fluo-3, isoproterenol (10 nM) increased systolic [Ca2+]i by 46%, increased twitch amplitude by 56%, and increased total cellular cAMP content by 140%. The adenosine A1 receptor agonist 2-chloro-N6-cyclopentlyadenosine (CCPA) reduced isoproterenol-stimulated [Ca2+]i and contractility by 87 and 80%, respectively, but reduced cAMP content by only 18%. Cantharidin had no effects on myocyte [Ca2+]i, contractility, or cAMP in the absence or presence of isoproterenol but blocked the effects of CCPA on [Ca2+]i and contractility by ~44%. Cantharidin had no effect on CCPA attenuation of isoproterenol-induced increases in cAMP. Pretreatment with CCPA also reduced the increase in contractile parameters produced by the direct cAMP-dependent protein kinase A (PKA) activator 8-bromocAMP. These results suggest that activation of protein phosphatases mediate, in part, the anti-adrenergic effect of adenosine A1 receptor activation in ventricular myocardium.
calcium; contractility; adenosine 3',5'-cyclic monophosphate
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