Characterization of [3H]forskolin binding sites in the iris-ciliary body of the albino rabbit.

[3H]Forskolin binding sites were identified using membranes prepared from the iris-ciliary body of adult, albino rabbits. Scatchard analysis of saturation binding experiments demonstrated that [3H]forskolin bound to a single population of high affinity sites. The Kd and Bmax values were 8.7 +/- 0.9 nM and 119.0 +/- 30.9 fmol/mg prot. using membranes prepared from frozen tissue and 17.0 +/- 6.2 nM and 184.4 +/- 47.2 fmol/mg prot. using fresh tissue. The binding of [3H]forskolin was magnesium-dependent. The Bmax was enhanced by sodium fluoride and Gpp(NH)p, a nonhydrolyzable guanine nucleotide analog. Forskolin was the most potent inhibitor of [3H]forskolin binding; two commercially-available analogs were weaker inhibitors. In an adenylate cyclase assay, there was the same rank order of potency to enhance enzyme activity. Based upon binding affinities, magnesium-dependence, sensitivity to sodium fluoride and Gpp(NH)p, rank order of potencies of analogs and correlation of binding with adenylate cyclase activity, these studies suggest that the [3H]forskolin binding site in the iris-ciliary body is similar to the binding site in other tissues.