Cyclic AMP modulates electrical excitability of insect oöcytes (Rhodnius prolixus)

The effects of cyclic AMP on resting potentials and calcium action potentials in Rhodnius oöcytes have been investigated using intracellular recording techniques. Extracellular application of cyclic AMP or either of two phosphodiesterase-resistant derivatives, 8-bromo-cAMP and 8-(4-chlorophenylthio)-cAMP, was associated with a progressive decline in action potential overshoot and duration and the appearance of a hyperpolarizing afterpotential. Similar effects were observed when the intracellular concentration of cAMP, as measured by a competitive protein binding assay, was elevated by application of 3-isobutyl-1-methylxanthine or forskolin. Reduced excitability was not correlated with changes in the resting potential. Extracellular adenosine and 8-bromo-cGMP did not alter excitability, and responses to cAMP were reduced by tetraethylammonium or barium. When action potentials were blocked by cobalt, a hyperpolarizing after potential appeared after termination of a depolarizing current pulse during application of 8-(4-chlorophenylthio)-cAMP. It is suggested that reduced excitability and the hyperpolarizing after potential in the presence of cAMP are mediated by an increase in a voltage-dependent potassium conductance. Possible developmental consequences of the effects of cAMP on excitability are discussed.