Molecular Pharmacology, Vol 10, 582-596, Copyright © 1974 by the American Society for Pharmacology and Experimental Therapeutics

Binding of dl-[³H]Epinephrine to Proteins of Rat Ventricular Muscle: Nonidentity with Beta Adrenergic Receptors

¹ Department of Pharmacology, University of Colorado Medical Center, Denver, Colorado 80220

The binding of dl-[³H]epinephrine to proteins in a supernatant fraction obtained from rat ventricular muscle has been studied and compared with the properties which would be expected of beta adrenergic receptors studied in vitro. The binding to heart proteins took place over a time scale of hours and was correlated with the time course of destruction of epinephrine as determined by alumina chromatography. The binding of dl-[³H]epinephrine was markedly temperature-dependent. The rate of binding was enhanced if tissues were stored at -22°, homogenates were stored at 4° or -22°, or homogenates were heated to 95°. Binding was not reversed by excess propranolol, nonradioactive epinephrine, or strong acid. Binding activity was distributed uniformly with the distribution of protein on differential centrifugation, while adenylate cyclase activity was associated with rapidly sedimenting membrane fragments. The stimulation of adenylate cyclase activity by l-epinephrine was prevented by dl-propranolol but was not affected by d-epinephrine or catechol. The binding reaction was blocked, on the other hand, by both the d and l stereoisomers of norepinephrine and epinephrine, by ascorbic acid, and by catechol. It was not affected by propranolol and was only slightly reduced by non-catechol inhibitors of catechol O-methyltransferase. Binding that appeared to be quantitatively similar to that seen with heart supernatant proteins was observed when bovine serum albumin was incubated with [³H]epinephrine. The results suggest that the binding of catecholamines to proteins is a nonspecific interaction which is dependent upon the oxidative destruction of the amines. The binding does not appear to reflect interaction with beta adrenergic receptors or with any other specifiable constituent of cardiac tissue.

Note:
ACKNOWLEDGMENT We thank Ms. Jane Andrews for excellent technical assistance.