HCO3(-)-dependent ACh-activated Na+ influx in sheep parotid secretory endpieces

In the present study we used the Na)+)-sensitive fluorescent dye SBFI and optical measurement of endpiece volume to investigate the transport of Na+ in sheep parotid secretory cells. Sheep parotid endpiece cells bathed in a HCO3(-)-free Cl(-)-rich solution had a resting intracellular Na+ concentration ([Na+]i) of 17 +/- 2 mmol/l (n = 39). Exposure of the cells to a 2-min pulse of acetylcholine (ACh) (3 x 10(-7) mol/l) in a HCO3(-)-free bathing solution produced no change in [Na+]i or in cell volume. Changing from a Cl(-)-containing HCO(3-)-free bath solution to a Cl- solution containing 25 mmol/l HCO3- caused the endpieces to swell by 8 +/- 2% (n = 11) and the [Na+]i to increase by 10 +/- 2 mmol/l (n = 14). Subsequent exposure of the cells to ACh led to shrinkage of the cells by 12 +/- 2% from the volume in the HCO3(-)-containing solution prior to ACh exposure, with the maximum decrease occurring after 29 +/- 7 s (n = 9). This shrinkage was accompanied by a rapid and transient increase in [Na+]i, the [Na+]i reaching a peak at 70 +/- 5 mmol/l above the unstimulated level (n = 9). Substitution of gluconate for Cl- did not significantly alter the effects of HCO3- on unstimulated [Na+]i or endpiece volume, nor did it significantly inhibit the effects of ACh on these two parameters when HCO3- was present.(ABSTRACT TRUNCATED AT 250 WORDS)