Cholinergic-induced HCO3- loss from rat parotid acini.

The effect of cholinergic stimulation on the magnitude of anion loss from collagenase-isolated rat parotid acini suspended in a Krebs-Ringer bicarbonate buffer was characterized by means of 36Cl. It was found that on stimulation, the acinar Cl- concentration decreases from 62.3 mM to 22.6 mM in about 10 seconds after addition of carbachol. Concomitant with the dramatic net loss of Cl- a transient fall in the intracellular pH (pHi) of 0.1 pH-units occurred and that was interpreted as being due to acinar loss of HCO3-. The magnitude of the HCO3- loss was estimated in two different ways. From calculation of the intracellular buffer capacity at the unstimulated pHi of 7.28 it was found that the stimulation-induced fall of 0.1 pH-units is due to an efflux of about 5.5 mM from the acini. Another estimate of the HCO3- loss was based on the assumption that the ratio of HCO3- distribution on the internal and external sites of the plasma membranes is distributed according to the ratio for Cl-. By this calculation a HCO3- loss of 5.6 mM was found, which is in good accordance with the value obtained from the change in pHi. It was concluded that on cholinergic stimulation the acinar cells within seconds produce an isotonic fluid containing Cl- as the predominant anion and a small amount of HCO3-.