REGULATED CL TRANSPORT, K AND CL PERMEABILITY, AND EXOCYTOSIS IN T84 CELLS

We measured stimulant-induced changes of exocytosis that are associated with increases in Cl secretion (i.e., short circuit current, I-SC, in mu A/cm(2)) and apical (ap) Cl permeability (P-Cl) and basolateral (bl) K permeability (P-K) (both in cm/s) in T84 monolayers. P-Cl and P-K were measured by permeabilizing the bl or ap membrane with nystatin. P-Cl was also measured with a fluorescent dye 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ). A noninvasive and sensitive method (release of (SO4)-S-35-labeled glycosaminoglycan [GAG], a fluid-phase marker of Golgi-derived vesicles) was used to measure exocytosis at both ap and bl membranes. At rest, I-SC = 3.6, P-K = 0.8 X 10(-6), P-Cl = 2.1 X 10(-6) with SPQ and 2.4 X 10(-6) electrically, and there was constitutive GAG secretion (i.e., exocytosis) to both ap and bl sides (bl >2 X ap). Carbachol (C) increased: I-SC (Delta = 18.6), P-K (6.5X), P-Cl (1.8-2.9X), and exocytosis to both ap (2.2-3.5X) and bl (2.0-3.0X) membranes. Forskolin (F) increased I-SC (Delta = 29), P-Cl (5.5-11X) and an exocytosis (1.5-2X), but had no effect on P-K or bl exocytosis. Synergistic effects on I-SC occurred when C was added to F-treated cells but not vice versa, even though the characteristic effects of F + C on P-Cl, P-K, and/or GAG secretion were identical to those exhibited when stimulants were added individually. Cl secretion results from coordinated activation of channels at ap and bl membranes, and exocytosis may play a role in these events.