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The effects of chronic exposure to the adenosine antagonist theophylline (Theo) and dibutyryl cyclic-AMP, a membrane-permeant derivative of the second messenger 3', 5'-cyclic-AMP (cAMP), on contractions and adenosine receptor levels in cultured cardiocytes were studied. Binding of the A1-adenosine receptor antagonist [3H]8-cyclopentyl-1,3-dipropylxanthine ([3H]CPX) was used to monitor the level of the receptors in intact cardiocytes. Both Theo and cAMP stimulated the rate of contraction and also increased the density of adenosine receptors. The Bmax value for [3H]CPX binding to intact cardiocytes was increased by 45-47% following 4 days of exposure to either 50 microM Theo or 100 microM cAMP. Scatchard analysis indicated that the affinity of the A1 receptors for [3H]CPX remained unchanged (Kd 0.1-0.2 nM). No significant differences were observed in protein content or in cell number. A linear correlation was achieved between the level of A1-adenosine receptors and heart rate at various Theo and dibutyryl-cAMP concentrations, although Theo was more efficient in elevation of the receptor density. Increases of 82, 78, 138 and 235% in A1 receptor density and increases of 63, 59, 66 and 150% in heart rate were obtained following 5 days of treatment with 1, 10, and 1000 microM of Theo, respectively. It is concluded that there is a linkage between the rate of cardiac contractions and the level of adenosine receptors. Thus, changes in the density of adenosine receptors may compensate for chronic drug-induced changes in cardiac contractile activity so as to restore conditions to the normal state.

Effects of Theophylline and Dibutyryl-cAMP on Adenosine Receptors and Heart Rate in Cultured Cardiocytes