cAMP and forskolin inhibit potassium currents in rat taste receptor cells by different mechanisms

In gustatory transduction, adenosine 3',5'-cyclic monophosphate (cAMP) has been suggested to close potassium channels when elevated by sweet stimuli or to open cAMP-gated cation channels when depressed by bitter stimuli. These experiments examine the effect of cAMP on whole cell currents from posterior taste receptor cells with standard patch-clamp techniques. Elevating cytosolic cAMP by pipette administration, membrane-permeant analogs [8-(4-chlorophenylthio)-cAMP (CPT-cAMP) and dibutyryl-cAMP], or by phosphodiesterase inhibition [3-isobutyl-1-methylxanthine (IBMX)] produced poorly reversible inhibitions of outward potassium currents by up to 33%. Unexpectedly, middle to high concentrations of forskolin (> 5 microM) profoundly and reversibly inhibited these currents (95%) with greatly accelerated inactivation kinetics. 1,9-Dideoxyforskolin, an ineffective activator of adenylate cyclase, was similarly potent. Kinase inhibitors effectively blocked the effects of cAMP elevations produced by IBMX or CPT-cAMP but did not block these forskolin actions. However, at low concentrations (5 microM), forskolin reduced potassium currents in a phosphorylation-dependent manner. Collectively, these data suggest that cAMP produces a phosphorylation-dependent inhibition of outward potassium currents but that forskolin's actions are independent of cAMP or phosphorylation except at low concentration. cAMP was also effective in altering the waveform of the gustatory action potential, implying it may modify transmission of gustatory information to the brain.