Regulation of ionic currents by protein kinase A and intracellular calcium in outer hair cells isolated from the guinea-pig cochlea

Two prominent potassium currents, termed I-K and I-K,I-n, and a cation current are found in outer hair cells (OHCs) of the guinea-pig cochlea. We report here whole-cell recordings which indicate that the currents are regulated by intracellular factors. 8-bromo-cAMP (500 mu M), a membrane-permeable cAMP analogue, activated potassium currents in OHCs in both apical and basal turns of the cochlea. In OHCs from the cochlear apex, the drug effect was largest at potentials positive to -40 mV, indicating I-K as the target. In short cells from the cochlear base, both I-K and I-K,I-n were affected. The effects of 8-bromo-cAMP could be blocked by the presence of 1 mu M H-89 (a protein kinase A inhibitor) in the patch pipette solution. Extracellular application of 10 nM okadaic acid, a protein phosphatase inhibitor, also activated both potassium currents. Currents were also modulated by intracellular calcium. I-K was activated in long cells by photorelease of calcium from the caged compound nitr5. Cation current activation required calcium release by photolysis of DM-nitrophen, a compound releasing more calcium. The results show that OHC potassium channels are regulated by background phosphorylation through protein kinase A and dephosphorylation by protein phosphatase. Cellular calcium also activates I-K and the cation channel, but with different sensitivities.