pH regulation and bicarbonate transport of isolated porcine submucosal glands.

We have previously demonstrated that the airway serous cell line Calu-3 employs a number of pH regulatory mechanisms required for bicarbonate secretion by these cells. The aim of the present study was to investigate the pH regulatory mechanisms of serous cells of freshly isolated submucosal glands (SMG). Porcine SMG were dissected out of pig tracheas obtained from a local slaughterhouse. Single glands were transferred into the chamber of an inverted microscope, immobilized by two holding pipettes and the serous cells loaded with the fluorescent pH probe 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF). Fluorescence was monitored from small areas consisting of up to 20 cells. The fluorescence ratio of the emission after excitation at 488 nm and 436 nm respectively was used to estimate cytosolic pH (pH(i)). Resting pH(i) of SMG cells in the absence of HCO(3)(-)/CO(2) was 7.1 +/- 0.16 (n=24). Addition of a solution buffered with HCO(3)(-)/CO(2) to the bath transiently acidified the cells by 0.18 +/- 0.03 (n=18). pH(i) rapidly recovered to a slightly more alkaline value than baseline pH(i). Removal of the HCO(3)(-)/CO(2) buffer strongly alkalinized SMG cells by 0.2 +/- 0.03 (n=18). To challenge pH regulatory mechanisms we exposed the cells to 20 mmol/L NH4(+) in the absence and presence of HCO(3)(-)/CO(2). In both cases we observed a rapid increase in pH(i) followed by a slight recovery. Washout of NH4(+) strongly acidified the cells. Realkalinization of pH(i) could only be observed in the presence of Na(+). This effect was inhibited by the addition of the specific Na(+)/H(+) exchanger isoform 1 (NHE1) blocker 3-methylsulfonyl-4-piperidinobenzoyl guanidine hydrochloride (HOE 694, 10-100 micromol/L) with an half maximal inhibitory concentration (IC(50)) of approximately 20 micromol/L. Full recovery of pH(i) in the presence of HOE 694 was observed when the cells were bathed in HCO(3)(-)/CO(2) solution. Addition of forskolin (5 micromol/L) in the presence of HCO(3)(-)/CO(2) did not significantly alter pH(i) or change pH(i) recovery after acid loading. We conclude that SMG cells possess both HCO(3)(-) dependent and HCO(3)(-) independent pH(i); regulatory mechanisms that require the presence of extracellular Na(+). Further studies are required to understand whether bicarbonate is only transported to regulate pH(i) or whether this transport determines the overall secretory capacity of SMG serous cells.