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 variation in the P2X receptor conductance, activated by extracellular ATP, in 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+) slope conductances were determined in OHCs of known origin from current-voltage relationships obtained from voltage ramps applied between -100 and +50 mV. Membrane conductance throughout this voltage range was greatest in OHCs originating from the basal (high frequency encoding) region of the cochlea. This gradient in OHC conductance from apex to base of the cochlea can be attributed to variation in expression of both a negatively activated K+ conductance and a TEA-sensitive outwardly rectifying K+ conductance. OHC slope conductance measured about a membrane 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 cochlea. 3. Removal of external Ca2+ reduced OHC conductance by an average of 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 produced inward currents that were progressively larger in OHCs originating from 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 for turns 4 and 1, respectively. 5. The P2X receptor conductance was determined between -100 and +50 mV by comparing voltage ramps in the presence and absence of extracellular ATP in Ca2+-free solution. The conductance was inwardly rectifying with a reversal potential close to 0 mV. Measured close to the resting membrane potential of the cells (-75 mV), 2 mM ATP elicited an average 300% increase in conductance in parallel with the systematic increase in background conductance which occurs 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 conductance 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 ATP-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).