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

Two prominent potassium currents, termed I-K and I-K,I-n, and a cation curr ent are found in outer hair cells (OHCs) of the guinea-pig cochlea. We repo rt here whole-cell recordings which indicate that the currents are regulate d by intracellular factors. 8-bromo-cAMP (500 mu M), a membrane-permeable c AMP 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 a pplication of 10 nM okadaic acid, a protein phosphatase inhibitor, also act ivated both potassium currents. Currents were also modulated by intracellul ar calcium. I-K was activated in long cells by photorelease of calcium from the caged compound nitr5. Cation current activation required calcium relea se by photolysis of DM-nitrophen, a compound releasing more calcium. The re sults show that OHC potassium channels are regulated by background phosphor ylation through protein kinase A and dephosphorylation by protein phosphata se. Cellular calcium also activates I-K and the cation channel, but with di fferent sensitivities.