Relation of dark cells to the transport of H+/HCO3- and K+ ions: a microperfusion study in the rat submaxillary duct.

Relation of dark cells to the transport of H/HCO3 and K ions: A microperfusion study in the rat submaxillary duct. Transport of Na', and H1HC01 by the epithelium of the main excretory duct of the rat submaxillary gland was determined in rats subjected to metabolic acidosis and metabolic alkalosis. In addition, the morphology of the epithelium, which is composed of basal, light, and dark cells, was studied to find a structural functional correlationship. The following results were obtained: In metabolic acidosis, HC03 secretion decreased from 2.4 1.4 to 0.1 0.9 nmoles/min duct (P < 0.05) and K' secretion decreased from 28 3 to 22 3 nmoles/min 'duct (P < 0.05). Na reabsorption was reduced from 38 3 to 31 4 nmoles/min duct (P < 0.2). In metabolic alkalosis, HC01 secretion increased from 2.4 1.4 to 9.9 3.1 nmoles/min duct (P < 0.05) and K secretion rose from 28 3 to 35 4 nmoles/min duct (P < 0.05). Na reabsorption was not changed significantly. The incidence of the three cell types, identified by their response to PAS, varied with respect to light and dark cells only. A diminution or augmentation of light cells (Li, PAS) occurred to the benefit or to the expense, respectively, of dark cells (III, PAS). In metabolic acidosis, 17 1% dark cells were visualized by PAS staining as compared with 10 IT in the controls (P < 0.01). The number of light cells decreased at the same time from 61 3% to 54 3% (P < 0.01), whereas the undifferentiated basal cells remained constant at 29 1% of the total duct cells. In metabolic alkalosis the number of visible dark cells diminished from 10 1% to 7 1% (P < 0.01) and the number of light cells rose from 61 3% to 64 3% (P < 0.01), whereas the number of basal cells again remained constant at 29 1%. In terms of their morphology, light cells have to be divided into two groups: lIa and IIb. lIb light cells closely resemble dark cells (Ill, PAS) with respect to their shape and nuclear characteristics. ha type light cells always remain constant, whereas llb cells totally disappear in metabolic acidosis but are augmented in metabolic alkalosis. Among the dark cells (Iii) there are PAS and HALMI cells (lila) and PAS and HALMI (IlIb) cells. These two subtypes are subject to alterations occurring in the same manner: increase in acidosis and decrease in alkalosis. That is, the direction of their evolution runs opposite to that of the hib cells, it can therefore be concluded that the lib, lila, and ilib cells constitute a constant percentage of the duct cell population, the socalled dark cell system. Within this group, the morphological transformations occur in response to changes in acid-base balance. A structural evaluation of the findings yields a positive correlation between the number of PAS cells (lila and ilIb) and H' secretion (or HC03 reabsorption) and a negative correlation with K secretion into the lumen. In the cortical collecting duct of the kidney and in the toad urinary bladder, increased H transport is also associated with morpho-