1. The relaxing action of the beta‐adrenergic agent, isoprenaline, on the porcine coronary artery was investigated in relation to the cyclic AMP level, the endogenous binding of cyclic AMP to the regulatory unit of cyclic AMP dependent protein kinase or the phosphorylation as a result of activation of protein kinase of the muscle homogenate in Krebs solution and excess [K]o solution. These relations were also compared with those of the rat cardiac muscle, in which isoprenaline showed a positive inotropic action. 2. Excess [K]o decreased the cyclic AMP level in proportion to the amplitude of K‐induced contracture in the porcine coronary artery. Isoprenaline increased the cyclic AMP level in Krebs solution, while it had no effect in excess [K]o. 3. In the porcine coronary artery, the particulate fraction possessed only 5% of the total cyclic AMP dependent protein kinase, while in the rat cardiac muscle, the particulate fraction was 25% of the total protein kinase. 4. The cyclic AMP dependent protein kinase in the particulate fraction of the porcine coronary artery was already saturated with the endogenous cyclic AMP. However, the binding of cyclic AMP to the protein kinase in the particulate fraction in the cardiac muscle and in the cytosol fraction of both tissues were increased in accordance with the cyclic AMP level. In the coronary artery, the protein kinase in the cytosol fraction was bound to a greater extent with cyclic AMP than was measured in the rat cardiac muscle. 5. In the rat cardiac muscle, isoprenaline enhanced the phosphorylation, detected by autoradiography of SDS gel electrophoresis in individual fractions of phosphorylated protein, while little enhancement was observed in the porcine coronary artery. 6. These observations led to the conclusion that in the porcine coronary artery, beta‐adrenergic agent increases the levels of cyclic AMP but does not increase the phosphorylation. If the phosphorylation catalysed by cyclic AMP dependent protein kinase was utilized for Ca mobilization in the cell, the change in the cyclic AMP level would probably not have a causal relation to the muscle tone. This conclusion, however, may not be applicable in the case of the cardiac muscle.
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