title: Acetylcholine response in guinea pig outer hair cells. II. Activation of a small conductance Ca(2+)-activated K+ channel.
abstract: The type of K+ channel involved in the acetylcholine (ACh) evoked response (Ksub; sub stands for suberyldicholine) in guinea pig outer hair cells (OHCs) is still uncertain. The present study tests the hypotheses that Ksub is one of the following: a big conductance Ca(2+)-dependent K+ channel (BK), a small conductance Ca(2+)-dependent K+ channel (SK), a KA type of K+ channel, or a Kn type of K+ channel. Patch-clamp technique in the whole-cell mode was used to record from single guinea pig OHCs. ACh (100 microM) was applied to voltage-clamped OHCs and the ACh-induced currents (IACh) were measured. Charybdotoxin (100 nM) had no effect on IACh, while apamin (1 microM) blocked more than 90% of IACh. Lowering the external Ca2+ concentration caused a hyperpolarizing shift of the IACh monitored as a function of the prepulse voltage. Increasing internal Mg2+ (Mgi2+) concentration caused a reduction in the outward IACh without affecting the inward IACh. The Ksub channel was found to be permeable to Cs+. In Cs+ solutions, IACh was 45% of the IACh in K+ solutions. The block of IACh by apamin, the dependence on extracellular Ca2+, the incomplete block of IACh by Cs+, and the ACh-induced Cs+ currents favor the hypothesis that Ksub belongs to the SK type of channels. An ionotropic/nicotinic nature of the ACh mechanism of action is favored. It is suggested that, in vivo, the amplitude of the ACh-induced hyperpolarization may depend on the Ca2+/Mg2+ ratio inside and outside the cell.
authors.affiliation: Kresge Hearing Research Laboratory of the South, Department of Otorhinolaryngology and Biocommunication, Louisiana State University Medical Center, New Orleans 70112-2234, USA.
authors.forename: A P
authors.initials: AP
authors.lastname: Nenov
authors.forename: C
authors.initials: C
authors.lastname: Norris
authors.forename: R P
authors.initials: RP
authors.lastname: Bobbin
mesh_terms: D000109:Acetylcholine / Q000633:toxicity*; D000818:Animals; D001030:Apamin / Q000633:toxicity; D002118:Calcium / Q000378:metabolism*; D002469:Cell Separation; D002586:Cesium / Q000633:toxicity; D018999:Charybdotoxin / Q000633:toxicity; D004553:Electric Conductivity; D006168:Guinea Pigs; D018072:Hair Cells, Auditory, Outer / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism; D008564:Membrane Potentials / Q000187:drug effects; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000378:metabolism; D026902:Potassium Channel Blockers
chemical_list: D026902:Potassium Channel Blockers; D018999:Charybdotoxin; D002586:Cesium; D001030:Apamin; D008274:Magnesium; D000109:Acetylcholine; D011188:Potassium; D002118:Calcium