Inhibition of forskolin-activated adenylate cyclase by ethanol and other solvents

Ethanol and a variety of solvents are known to activate basal and Gpp(NH)p- and hormone-stimulated adenylate cyclase. We report here that ethanol and other solvents inhibit the activation of adenylate cyclase by forskolin. In the presence of 10 microM forskolin, 2% ethanol gives about 20% inhibition and 5% ethanol gives 40% inhibition of enzyme activity. Analysis of ethanol inhibition at several forskolin concentrations suggests that inhibition is competitive versus forskolin. Thus the effect of ethanol is greater at low forskolin concentrations and minimal at high concentrations. In addition to ethanol, inhibition of forskolin activation was observed with acetone, n-butanol, t-butanol, dimethyl formamide, dioxane, methanol and n-propanol. Dimethyl sulfoxide was inhibitory only at high concentrations (10%). Since some solvent is needed to prepare forskolin solutions and to maintain solubility at higher concentrations, the inhibitory effects reported here are an important consideration in studies employing forskolin activation. To minimize solvent inhibition we recommend that dimethyl sulfoxide be used to prepare forskolin solutions. At concentrations of 5% and less, dimethyl sulfoxide gives little if any inhibition of forskolin activation and causes only small increases in basal activity.