Activation of adenylate cyclase and inhibition of glucose transport in rat adipocytes by forskolin analogues: Structural determinants for distinct sites of action

Forskolin and four analogues of forskolin, 7-β-[γ-(N'-methylpiperazino)-butyryloxy]-7-desacetylforskolin, 7-desacetylforskolin 7-toxyl-7-desacetylforskolin, and 1,9-dideoxyforskolin, were tested for their ability to activate adenylate cyclase, inhibit glucose transport, and inhibit cytochalasin B binding in rat adipocyte membranes. Forskolin was the most potent analogue in activating adenylate cyclase with an EC₅₀ of 2 μM, whereas 7-β-[γ-(N'-methylpiperazino)butyryloxy]-7-desacetylforskolin and 7-desacetylforskolin were less potent, with EC₅₀ values of 3 μM and 20 μM, respectively. The 7-tosyl-7-desacetylforskolin and 1,9-dideoxyforskolin did not stimulate adenylate cyclase even at the highest concentrations tested (100 μM). In contrast, forskolin and all of the analogues were able to fully inhibit glucose transport in adipocyte plasma membranes. The order of potency for the inhibition was forskolin > 7-β-[γ-(N'-methylpiperazino)butyryloxy]-7-desacetylforskolin > 7-desacetylforskolin > 7-tosyl-7-desacetylforskolin > 1,9-dideoxyforskolin, and the EC₅₀ values were 0.24 μM, 1.8 μM, 7.1 μM 8.8 μM, and 12.8 μM, respectively. Cytochalasin B binding to rat adipocyte membranes was inhibited by forskolin and the four analogues with the same order of potency as observed for the inhibition of glucose transport. Thus, the site of action of forskolin which is responsible for the inhibition of glucose transport and cytochasin B binding exhibits structural requirements for forskolin and its analogues that are different from those of the site responsible for the activation of adenylate cyclase.