THE ROLE OF INOSITOL TRISPHOSPHATE ON ACH-INDUCED OUTWARD CURRENTS INBULLFROG SACCULAR HAIR-CELLS

Acetylcholine (ACh) is considered as the most likely candidate for a n eurotransmitter of the efferent synapse onto hair cell. In this paper, the nature of this cholinergic receptor mechanism on dissociated bull frog saccular hair cell was examined by using whole cell recording and Ca2+ sensitive fluorophotometric technique. Both the ACh-induced curr ent and the increase of [Ca2+ ](i) were observed in an oscillatory man ner, and were the largest around the basal part of the cell where the efferent synapse is thought to make a contact with the membrane. The r eversal potential of ACh-induced current indicated that ACh activated a K+ conductance. The ACh-induced current was reversibly blocked by at ropine, d-tubocurarine (dTC), apamin, tetraethylammonium (TEA) and qui nine. Neither muscarine nor nicotine mimicked the ACh-induced current. When GTP gamma S was injected into a hair cell, the first ACh applica tion induced an outward current of transient kinetics, but in subseque nt trials ACh-induced current lost its decay phase. Intracellularly in jected D-myo-inositol 1,4,5-trisphosphate (InsP(3)) generated outward currents. Intracellularly injected heparin suppressed ACh-induced curr ents, and lithium (Li+) increased ACh-induced currents. These results indicate that ACh activates a receptor coupled with a guanine nucleoti de binding protein (G-protein) which triggers metabolic cascades of In sP(3) and Ca2+ leading to the activation of the Ca2+-activated K+ chan nel.