Lack of detectable change in cyclic AMP during the cardiac inotropic response to isoproterenol immobilized on glass beads.

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 muM) in solution increased the force of contraction 0.832 plus or minus 0.165 g at 60 sec in eight muscles tested, which at 60 and 120 sec averaged 65.5 plus or minus 6.5% and 82.9 plus or minus 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 plus or minus 0.166 g at 60 sec (n equals 11), which at 60 and 120 sec averaged 45.1 plus or minus 7.0% and 58.6 plus or minus 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 plus or minus 0.049 pmol/mg of tissue wet weight, n equals 11. cAMP levels 60 sec after 1 muM soluble isoproterenol was added were 1.212 plus or minus 0.085 pmol/mg; in contrast, the levels of cAMP in response to isoproterenol immobilized on glass beads at 60 sec were 0.490 plus or minus 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.