Forskolin: unique diterpene activator of adenylate cyclase in membranes and in intact cells.

TLDR

Forskolin is a diterpene that activates adenylate cyclase in rat cerebral cortical membranes with an EC50 of 5–10 µM in a rapid, reversible manner. It activates the enzyme through a unique mechanism that combines direct stimulation of adenylate cyclase with facilitation of receptor or guanyl‑nucleotide‑binding subunit modulation. Forskolin activates adenylate cyclase independently of exogenous guanyl nucleotides, is potentiated by GTP/GDP, shows non‑additive interactions with p[NH]ppG but additive with fluoride, is not inhibited by manganese, elicits a rapid 35‑fold rise in cAMP in cortical slices, and low doses amplify responses to multiple neurotransmitters, with tissue‑specific potency relative to fluoride.

Abstract

The diterpene, forskolin [half-maximal effective concentration (EC50), 5-10 microM] activates adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] in rat cerebral cortical membranes in a rapid and reversible manner. Activation is not dependent on exogenous guanyl nucleotides and is not inhibited by guanosine 5'-O-(2-thiodiphosphate) when assayed with adenosine 5'-[beta, gamma-imido]triphosphate as substrate. GTP and GDP potentiate responses to forskolin. The activations of adenylate cyclase by forskolin and guanosine 5'-[beta, gamma-imido]triphosphate p[NH]ppG are not additive, whereas activations by forskolin and fluoride are additive or partially additive. The responses of adenylate cyclase to forskolin or fluoride are not inhibited by manganese ions, whereas the response to p[NH]ppG is completely blocked. Activation of adenylate cyclase by forskolin is considerably greater than the activation by fluoride in membranes from rat cerebellum, striatum, heart, and liver, while being about equal or less than the activation by fluoride in other tissues. Forskolin (EC50, 25 microM) causes a rapid and readily reversible 35-fold elevation of cyclic AMP in rat cerebral cortical slices that is not blocked by a variety of neurotransmitter antagonists. Low concentrations of forskolin (1 microM) augment the response of cyclic AMP-generating systems in brain slices to norepinephrine, isoproterenol, histamine, adenosine, prostaglandin E2, and vasoactive intestinal peptide. Forskolin would appear to activate adenylate cyclase through a unique mechanism involving both direct activation of the enzyme and facilitation or potentiation of the modulation of enzyme activity by receptors or the guanyl nucleotide-binding subunit, or both.