Epinephrine binding to rat adipocytes and their subcellular fractions.

Abstract

The binding of epinephrine to adipocytes and adipocyte plasma membrane, mitochondrial and microsomal fractions was characterized. Scatchard analysis of epinephrine binding to adipocytes revealed two orders of binding sites with association constants of 5.3 × l06M–1 and 1.1 × lO5M–1. The total binding capacity for each site was 0.8 and 17 pmoles epinephrine per mg protein, respectively. Adipocyte plasma membranes had a high and low affinity binding site with association constants of 2.4 × l06M–1 and 3.1 × lO4M–1 and maximal binding of 8 and 170 pmoles per mg protein. The dissociation constants derived from the high affinity association constant for adipocytes and plasma membranes were consistent with the level of hormone necessary to cause a half-maximal increase of adenylate cyclase, cyclic AMP and lipolysis. The binding sites on the adipocyte could be accounted for by the sites on the plasma membrane since the plasma membrane protein constitute about 10% of the adipocyte protein. The high affinity association constants for mitochondria and microsomes were slightly less than the plasma membranes and the maximal binding about double. No significant difference was found between the specific binding (picomoles/mg protein) of 6.7 × 10–7M epinephrine to plasma membranes, mitochondria or microsomes. Liver plasma membranes bound more than twice that of liver mitochondria. Time course studies revealed two phases of epinephrine binding associated with the high affinity site of all three adipocyte fractions. The first phase was rapid and larger with a sharp inflection point by 10 min with a second smaller and slower phase of binding occurring over the next 30 min. Analog studies revealed that isoproterenol was more effective than epinephrine in preventing [3H] epinephrine binding. L-dopa, DL-alpha methyl dopa and DL-3, 4 dihydroxy mandelic acid decreased binding; metanephrine stimulated binding while L-tyrosine partiallyblocked epinephrine binding to mitochondria and microsomes but not at all to plasma membranes. Propranolol at levels known to block lipolysis or adenylate cyclase activation by epinephrine decreased epinephrine binding to the plasma membranes by 30% while phentolaminc had no effecton binding. The present data show binding sites exist on the plasma membranes that can be classified as receptors by classic criteria. These findings in conjunction with previous studies raised a question as to whether all of these high affinity binding sites are truly receptors or only a small proportion of them at a time are coupled to the adenylate cyclase and thereby responsible forintiating a response in this enzyme. (Endocrinology94: 719, 1974)