VARIATION IN EXPRESSION OF THE OUTER HAIR CELL P2X RECEPTOR CONDUCTANCE ALONG THE GUINEA-PIG COCHLEA

1. Whole-cell patch-clamp recordings were used to determine the variat ion in the P2X receptor conductance, activated by extracellular ATP, i n outer hair cells (OHCs) isolated from each of the four turns of the guinea-pig cochlea. 2. In standard solution (containing 1.5 mM Ca2+) s lope conductances were determined in OHCs of known origin from current -voltage relationships obtained from voltage ramps applied between -10 0 and +50 mV. Membrane conductance throughout this voltage range was g reatest in OHCs originating from the basal (high frequency encoding) r egion of the cochlea. This gradient in OHC conductance from apex to ba se of the cochlea can be attributed to variation in expression of both a negatively activated K+ conductance and a TEA-sensitive outwardly r ectifying K+ conductance. OHC slope conductance measured about a membr ane potential of -75 mV increased from a mean of 33.5 nS in the apical region (turn 4) to 96.8 nS in the basal region (turn 1) of the cochle a. 3. Removal of external Ca2+ reduced OHC conductance by an average o f 25%, reflecting a Ca2+ dependence of the background conductances in these cells. In zero external Ca2+ the mean slope conductance measured at -75 mV in the apical turn was 25.0 nS compared with 73.8 nS in the basal turn. 4. In Ca2+-free solution both 2 mM and 4 mu M ATP produce d inward currents that were progressively larger in OHCs originating f rom more basal regions of the cochlea. The steady-state inward current elicited by 2 mM extracellular ATP increased from -1.44 to -3.26 nA f or turns 4 and 1, respectively. 5. The P2X receptor conductance was de termined between -100 and +50 mV by comparing voltage ramps in the pre sence and absence of extracellular ATP in Ca2+-free solution. The cond uctance was inwardly rectifying with a reversal potential close to 0 m V. Measured close to the resting membrane potential of the cells (-75 mV), 2 mM ATP elicited an average 300% increase in conductance in para llel with the systematic increase in background conductance which occu rs in OHCs originating from the more basal regions of the cochlea. The conductance at -75 mV activated by 2 mM ATP increased from a mean of 59.6 nS in turn 4 OHCs to a mean of 166.2 nS in turn 1 OHCs. The condu ctance activated by 4 mu M ATP was also greater in the basal turn OHCs (45.3 nS) than in the apical region OHCs (5.9 nS). 6. The number of A TP-gated ion channels on individual OHCs, presumed to be localized to the stereocilia, increases from approximately 6000 in turn 4 cells to 16 500 in turn 1 cells, based on estimates of unitary conductance and average maximum ATP-activated OHC conductance (2 mM ATP).