Spontaneous water secretion in T84 cells: effects of STa enterotoxin, bumetanide, VIP, forskolin, and A-23187

Abstract

The regulated Cl− secretory apparatus of T84 cells responds to several pharmacological agents via different second messengers (Ca2+, cAMP, cGMP). However, information about water movements in T84 cells has not been available. In the absence of osmotic or chemical gradient, we observed a net secretory transepithelial volume flux ( J w = −0.16 ± 0.02 μl · min−1 · cm−2) in parallel with moderate short-circuit current values ( I sc = 1.55 ± 0.23 μA/cm2). The secretory J wreversibly reverted to an absorptive value when A-23187 was added to the serosal bath. Vasoactive intestinal polypeptide increased I sc, but, unexpectedly, J w was not affected. Bumetanide, an inhibitor of basolateral Na+-K+-2Cl−cotransporter, completely blocked secretory J wwith no change in I sc. Conversely, serosal forskolin increased I sc, but J w switched from secretory to absorptive values. Escherichia coli heat-stable enterotoxin increased secretory J w and I sc. No difference between the absorptive and secretory unidirectional Cl−fluxes was observed in basal conditions, but after STa stimulation, a significant net secretory Cl− flux developed. We conclude that, under these conditions, the presence of secretory or absorptive J w values cannot be shown by I sc and ion flux studies. Furthermore, RT-PCR experiments indicate that aquaporins were not expressed in T84 cells. The molecular pathway for water secretion appears to be transcellular, moving through the lipid bilayer or, as recently proposed, through water-solute cotransporters.