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Changes in contractility and the levels of adenosine 3': 5'-cyclic monophosphoric acid (cAMP) were assessed in isolated cat cardiac muscle in response to soluble isoproterenol and isoproterenol immobilized on glass beads. Drug-induced positive inotropic responses were compared to the maximum isometric force achieved with paired electrical stimulation, a potent physiological inotropic stimulus.. Isoproterenol (1 uM) in solution increased the force of contraction 0.832 4 0.165 g at 60 sec in eight muscles tested, which at 60 and 120 sec averaged 65.5 4 6.5% and 82.9 d 8.8%, respectively, of the force with paired electrical stimulation. Isoproterenol immobilized on glass beads gave positive inotropic responses similar to those for the soluble form of the drug. Placement of only three isoproterenol-glass beads on the muscles increased the force of contraction 0.742 d 0.166 g at 60 sec (n = 11), which at 60 and 120 sec averaged 45.1 7.0% and 58.6 4 6.4%, respectively, of the force with paired electrical stimulation. The magnitude of this response indicates that the increased force was developed by at least 60% of the cells in each muscle. Control levels of cAMP were 0.527 4 0.049 pmol/mg of tissue wet weight, n = 11. cAMP levels 60 sec after 1 MM soluble isoproterenol was added were 1.212 4 0.085 pmol/mg; in contrast, the levels of cAMP in response to isoproterenol immobilized on glass beads at 60 sec were 0.490 4 0.060 pmol/mg, not significantly different from control levels. These data indicate that cAMP may not be involved in the propagation of the inotropic response that must have occurred in these cardiac muscles and raise questions as to the physiological significance of the large cAMP increases that occur in response to soluble drugs and hormones.

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