cAMP increases K+ secretion via activation of apical IsK/KvLQT1 channels in strial marginal cells

In the cochlea, K+ is secreted by electrodiffusion across the apical membrane of strial marginal cells via the IsK/KvLQT1 ('IsK') channel. This channel complex has been reported to be activated in other systems by adenosine 3',5'-cyclic monophosphate (cAMP). Since several reports had suggested that cAMP is a second messenger in the cochlea, the effect of the cAMP pathway on transepithelial K+ secretion by strial marginal cells of the gerbil was studied. Both the transepithelial current (Isc) and K+ flux (JK) across strial marginal cell epithelium were measured; Isc in a micro-Ussing chamber and JK as the gradient of K+ concentration near the apical membrane. The apical membrane current (IIsK) and conductance (gIsK) of IsK channels were recorded with the on-cell macro-patch and the nystatin-perforated whole-cell patch clamp techniques. It has previously been shown that the apical IsK channel constitutes the primary pathway for K+ secretion. Cytoplasmic cAMP was elevated by applying dibutyryl cyclic-AMP (dbcAMP) or the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) at 37 degrees C. dbcAMP (1 mM) increased Isc by 51 +/- 4% and IIsK in on-cell and whole-cell recordings increased by 214 +/- 63% and 390 +/- 61% above the control value, respectively. IBMX (1 mM) caused transient increases of Isc by 53 +/- 3% and IIsK in on-cell recordings by 177 +/- 75% above the control value. The leak conductance due to all non-IsK channel sources did not change in the presence of dbcAMP or IBMX. dbcAMP (1 mM at 24 degrees C) increased JK by 53 +/- 16% and Isc by 18 +/- 4%. IBMX (1 mM at 24 degrees C) had no effect, suggesting reduced activity of adenylate cyclase at this temperature. Our results demonstrate that the cAMP pathway is constitutively active in strial marginal cells and that the cAMP pathway stimulates transepithelial K+ secretion by increasing IsK channel current rather than by altering another transport pathway.