Biochimica et Biophysica Acta, 942 (1988) 353-356 353 Elsevier BBA 70565 BBA Report Hypotonicity increases apical membrane CI- conductance in Necturus enterocytes F. Giraldez a M.A. Valverde a and F.V. Sepfilveda b a Departamento de Bioquimica y Biologia Molecular y Fisiologia, Universidad de Valladolia~ Valladolid (Spain) and b AFRC Institute of Animal Physiology and Genetics Research, Babraham, Cambridge (U.K.) (Received29 January 1988) Key words: Apical membrane; Membranepotential; Chloride permeability; (Necturus enterocyte) The effect of hypotonicity on epithelial cells of Necturus small intestine has been studied using conventional and CI--selective microelectrodes. Exposure to a mucosal solution made hypoosmotic by 70 mosmolal provokes a rapid dilution of intracellular C!-, consistent with a perfect osmometer behaviour of the cell. The swollen cells showed an increased apical membrane C!- conductance. The increased CI- conductance might be involved in regulatory volume decrease. Epithelial cells in the small intestine of Necturus maculosus have recently been shown to have a conductance to C1- which can dominate the ionic selectivity of the apical membrane [1]. This con- ductance is modulated by cyclic AMP and has been proposed to be the permeability pathway activated during sodium-coupled transport of so- lutes and during secretagogue-induced fluid secre- tion [1,2]. An increase in electrodiffusional mem- brane permeability to C1- has also been shown to take place in other cell types as a consequence of cell volume expansion, during what has been termed regulatory volume decrease [3]. In the pre- sent report we have used intracellular microelec- trodes to investigate whether Necturus apical membrane C1- conductance can be regulated by cell swelling. Our data show that apical C1- con- ductance is effectively activated when enterocytes are swollen in hypotonic medium. Correspondence: F.V. Sepfllveda, AFRC Institute of Animal Physiologyand Genetics Research, Babraham, Cambridge CB2 4AT, U.K. A continuous recording of the apical membrane potential, E m, of a Necturus enterocyte is shown in Fig. 1. Replacement of all mucosal C1- by gluconate produced a fast depolarisation which, as it has been shown elsewhere [1], reflects the pres- ence of a C1- conductance in the apical mem- brane. Decreasing the osmolality of the mucosal solution by 70 mosM evoked a hyperpolarisation. Under hyposmotic conditions, the replacement of C1- produced a depolarisation which was larger than in normal, isotonic Ringer (50 compared with 33 mV). This change in the size of depolarisa- tion corresponds to an increase in the relative C1- permeability of the mucosal membrane, r, from 2.7 to 6.3 (see legend to Table I for calculation of fl). In these experiments the tissue was short-cir- cuited except for short periods in which transepi- thelial potential, E t, was clamped at 10 mV. The deflections produced on E m are proportional to the apical membrane resistance, and the ratio AEm/AE t can be taken as a measure of the frac- tional resistance of the mucosal membrane, fR m. In hypotonic Ringer the size of the deflections in the E TM trace was reduced, suggesting that in this 0005-2736/88/$03.50 © 1988 ElsevierSciencePublishers B.V. (BiomedicalDivision)