Phosphatase inhibitor cantharidin blocks adenosine A(1) receptor anti-adrenergic effect in rat cardiac myocytes

Experiments were performed to examine whether the protein phosphatase inhib itor cantharidin blocks the antiadrenergic effect of adenosine A(1) recepto r stimulation. In electrically stimulated adult rat ventricular myocytes lo aded with the intracellular calcium concentration ([Ca2+](i)) indicator flu o-3, isoproterenol(10 nM) increased systolic [Ca2+](i) by 46%, increased tw itch amplitude by 56%, and increased total cellular cAMP content by 140%. T he adenosine A(1) receptor agonist 2-chloro-N-6-cyclopentlyadenosine (CCPA) reduced isoproterenol-stimulated [Ca2+](i) and contractility by 87 and 80% , respectively, but reduced cAMP content by only 18%. Cantharidin had no ef fects on myocyte [Ca2+](i), contractility, or cAMP in the absence or presen ce of isoproterenol but blocked the effects of CCPA on [Ca2+](i) and contra ctility by similar to 44%. Cantharidin had no effect on CCPA attenuation of isoproterenol-induced increases in cAMP. Pretreatment with CCPA also reduc ed the increase in contractile parameters produced by the direct cAMP-depen dent protein kinase A (PKA)activator 8-bromocAMP. These results suggest tha t activation of protein phosphatases mediate, in part, the anti-adrenergic effect of adenosine A(1) receptor activation in ventricular myocardium.