MEMBRANE CROSSTALK IN SECRETORY EPITHELIAL-CELLS MEDIATED BY INTRACELLULAR CHLORIDE CONCENTRATION

Fluid secretion by epithelial cells is modulated by agents which activ ate Cl- channels in the apical membrane. To sustain secretion, Cl- inf lux across the basolateral membrane must also be accelerated. To exami ne cellular mechanisms which couple Cl- efflux across the apical membr ane to Na+-coupled Cl- entry across the basolateral membrane, we emplo yed optical imaging techniques utilizing single rat salivary acinar ce lls. Na+ influx was negligible in resting cells, but was rapidly incre ased by carbachol due to activation of a Na+-H+ exchanger, Na+-K+-2Cl( -) cotransporter and likely a non-selective cation channel. Receptor s timulation was not necessary since elevation of [Ca2(+)](i) by thapsig argin activated the Naf transporters at equivalent rates. Cell acidifi cation, activation of protein kinase C, and cell shrinkage, other even ts associated with the rise of [Ca2+](i), had little effect on Na+ tra nsport in resting cells. Nevertheless, stimulation of cells in a mediu m which prevented normal Ca2+-induced cell shrinkage prevented activat ion of all three pathways. The block of the activation was not overcom e by osmotic shrinkage, but was relieved when intracellular Cl- concen tration ([Cl-](i)) was allowed to fall, including conditions in which [Cl-](i) fell in the absence of cell shrinkage. Activation of a Na+-H exchanger, Na+-K+-2Cl(-) cotransporter, and non-selective cation chan nel therefore exhibit a requirement for agonist-induced fall in [Cl-]( i). Low [Cl-](i) may create a permissive environment for Ca2+-dependen t activation of multiple Na+ transport pathways, providing for crossta lk which coordinates transport activities of apical and basolateral me mbranes in secretory epithelial cells.