The Activation of Na+-K+-2Cl- Cotransporter in Response to the Hypertonic Stress and Muscarinic Stimulation in Rat Parotid Acini

Abstract

Na^(+)-K^(+)-2Cl^(-) cotransporter has been shown to be activated in response to volume shrinkage in a number of cell types. In this study, we investigated the role of cell shrinkage in the activation of Na^(+)-K^(+)-2Cl^(-) cotransporter in rat parotid acinar cells during hypertonic stress and muscarinic stimulation. We measured the recovery rate of the intracellular pH(pH_1) from NH_4^(+)-induced alkaline load in 2＇, 7＇bis(2-carboxy-ethyl)-5(6＇)-carboxyfluorescein (BCECF)-loaded cells as an index of the activity of Na^(+)-K^(+)-2Cl^(-) contransporter. The rate of pH_1 recovery was enhanced by cell shrinkage caused by a 20％ increase in perfusate osmolarity, This recovery was markedly inhibited by pretreatment with bumetanide, a blocker of Na^(+)-K^(+)-2Cl^(-) cotransporter. The rate of pHi recovery was also enhanced by 10μM carbachol stimulation which caused comparable shrinkage. Muscarinic stimulation induced an increase in [Ca^2+]_1, whereas hypertonic shrinkage failed to elevate [Ca2+]_1, Therefore, Ca^2+ does not seem to be the universal messenger for the activation of Na^(+)-K^(+)-2Cl^(-) cotransporter. In addition, the prevention of cell shrinkage during carbachol stimulation by reducing the perfusate osmolarity did not inhibit the activation of the Na^(+)-K^(+)-2Cl^(-) cotransporter. These results suggest that volume shrinkage alone is not sufficient to account for the marked activation of the Na^(+)-K^(+)-2Cl^(-) cotransporter observed during muscarinic stimulation. Other components of the intracellular signalling pathway are likely to be of great significance.