Differential identification and localization of adenylyl cyclase and glucose transporter in brain using iodinated derivatives of forskolin

Two radioiodinated derivatives of forskolin, [¹²⁵I]6-IHPP-Fsk and [¹²⁵I]7-IHPP-Fsk, were synthesized as specific ligands for adenylyl cyclase and glucose transporter, respectively. [¹²⁵I]6-IHPP-Fsk bound to bovine brain homogenates with a K_d of 9 nM and binding was inhibited by forskolin but not 1,9-dideoxyforskolin, cytochalasin B, or d-glucose. [¹²⁵I]7-IHPP-Fsk bound to bovine brain homogenates at two classes of binding sites with K_d's of 56 nM and 4.7 μM; cytochalasin B and d-glucose inhibited 75% of the high affinity binding while having no effect on the low affinity binding. [¹²⁵I]6-IHPP-Fsk and [¹²⁵I]7-IHPP-Fsk were used to localize adenylyl cyclase and glucose transporter in rat brain by receptor autoradiography. The pattern of binding obtained with [¹²⁵I]6-IHPP-Fsk was similar to that observed using [³H]forskolin to detect adenylyl cyclase. In contrast, the pattern of binding obtained with [¹²⁵I]7-IHPP-Fsk was similar to that observed by others using [³H]cytochalasin B to detect glucose transporter. These iodinated ligands are selective for adenylyl cyclase and glucose transporter and require significantly shorter exposure times to yield autoradiographs than tritiated ligands.