Evidence for forskolin binding sites in rat myocardium

Forskolin binding sites have been identified in a preparation of rat myocardium using [3H]forskolin and [3H]14,15-dihydroforskolin ([3H]DHF) as radioligands. It could be shown that both ligands bind to the same site with only a slight difference in their affinities. Using the filtration method the binding sites were characterized by an affinity of about 250 nM for forskolin and about 650 nM for dihydroforskolin (DHF). The binding capacity was about 5 pmol/mg protein with both compounds. When using the centrifugation technique similar binding affinities were obtained; the binding capacity, however, was around 3 fold higher than with the filtration method. Under all conditions only one single group of independent binding sites was found. In contrast, stimulation of adenylate cyclase by forskolin was found to be negatively cooperative. These results indicate that stimulation of adenylate cyclase by forskolin is a very complicated mechanism and the binding procedure of forskolin to its binding sites can represent only a small part of this process.