A1-adenosine receptor-mediated inhibition of isoproterenol-stimulated protein phosphorylation in ventricular myocytes. Evidence against a cAMP-dependent effect.

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Abstract
CAMP content and protein phosphorylation were determined in unlabeled and 32P-labeled guinea pig ventricular myocytes. Isoproterenol (10 nM, 37 degrees C, 10 seconds) increased cAMP content (236%) and phospholamban (265%) and troponin I (135%) phosphorylation in ventricular myocytes. When isoproterenol (0-300 nM) and the A1-adenosine receptor agonist (-)-N6-phenylisopropyladenosine (PIA, 1 microM) or the A1- and A2-adenosine receptor agonist 5'-(N-ethylcarboxamido)-adenosine (NECA, 1 microM) were administered simultaneously, both adenosine receptor agonists attenuated phospholamban phosphorylation to approximately the same extent (40%). The EC50 value for isoproterenol to phosphorylate phospholamban was 8 +/- 1 nM (n = 3), which increased to 31 +/- 4 nM (n = 3) in the presence of PIA or NECA. IC50 values for PIA or NECA to decrease the phosphorylation of phospholamban were 30 or 32 nM in 10 nM isoproterenol-stimulated cells and 80 or 85 nM in 30 nM isoproterenol-stimulated cells. Both adenosine receptor agonists failed to inhibit the phosphorylation of troponin I. However, acetylcholine (2 microM) in the presence of 10 nM isoproterenol inhibited phosphorylation of phospholamban as well as troponin I in ventricular cells. These effects were antagonized by 10 microM atropine. The effects of PIA and NECA on phosphorylation were antagonized by the A1-selective adenosine receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (1 microM) but not by the A2-selective adenosine receptor antagonist 9-chloro-2-(2-furanyl)-5,6-dihydro-1,2,4,triazolo(1,5-c)quinazolin -5-imine (1 microM). PIA and NECA did not reduce cAMP levels in isoproterenol-stimulated cells. We conclude that phospholamban phosphorylation was inhibited by A1-adenosine receptor activation and that these effects on phospholamban phosphorylation cannot be explained by a reduction in cAMP levels.

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