title: Forms of adenylate cyclase, activation and/or potentiation by forskolin.
abstract: Activation of different forms of adenylate cyclases (AC) by forskolin and displacement of [14,15-3H]dihydroforskolin binding from membranes by forskolin in the absence or presence of specific stimulatory hormone and beta, gamma-imidoguanosine 5'-triphosphate (Gpp(NH)p) have been studied. These conditions have been used to generate forskolin dose-response curves of AC activation. A plot of enzyme activation versus apparent forskolin-binding showed a linear and a nonlinear relationship, respectively, in the absence or presence of the other two stimulators. The latter relationship can be fitted by two linear regression lines with a defined intercept, the slopes of which represent two distinct binding-activation (B-A) effects. The B-A effects of forskolin for rat adipocyte and liver membranes in the absence of stimulatory hormone and Gpp(NH)p were 10 and 8 (pmol X min-1) X (pmol)-1, respectively. The B-A effects for the same membranes in the presence of the other two stimulators were 69 (high) and 13 (low) (pmol X min-1) X (pmol)-1 for adipocyte membrane, and 83 (high) and 9 (low) (pmol X min-1) X (pmol)-1 for liver membrane. The ratio of potentiation of forskolin-activated enzyme activity to the unmodified forskolin-stimulated activity (P-A ratio) was determined without the binding data. At 3 microM forskolin, with and without 230 epinephrine and 10 microM Gpp(NH)p, the P-A ratio was 3.7, decreasing to 1.1 with the addition 100 microM forskolin. The line representing a high B-A effect and a resulting high P-A ratio appears to describe the interactions between forskolin and the AC stimulated by epinephrine and Gpp(NH)p. The line of low B-A effect may represent the interaction between forskolin and the basal AC. Two peaks of AC activity were eluted from forskolin-Sepharose column. They have apparent differences in sensitivity to Gpp(NH)p and affinity for forskolin. Based on the results available thus far, with consideration for known limitations of the methodology, a working model has been proposed for forskolin activation of AC.
mesh_terms: D000262:Adenylyl Cyclases / Q000302:isolation & purification / Q000378:metabolism*; D000273:Adipose Tissue / Q000201:enzymology; D000818:Animals; D001665:Binding Sites; D002462:Cell Membrane / Q000201:enzymology; D002846:Chromatography, Affinity; D005576:Colforsin / Q000494:pharmacology*; D004357:Drug Synergism; D004789:Enzyme Activation / Q000187:drug effects; D004837:Epinephrine / Q000494:pharmacology; D005934:Glucagon / Q000494:pharmacology; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D008099:Liver / Q000201:enzymology; D051381:Rats
chemical_list: D005576:Colforsin; D006165:Guanylyl Imidodiphosphate; D005934:Glucagon; D000262:Adenylyl Cyclases; D004837:Epinephrine