PROPERTIES OF 2 MULTISUBSTRATE CL- CHANNELS FROM HUMAN SYNCYTIOTROPHOBLAST RECONSTITUTED ON PLANAR LIPID BILAYERS

We describe the first successful reconstitution of placental ionic cha nnels on planar lipid bilayers. An apical plasma membrane-enriched ves icle fraction from human syncytiotrophoblast at term was prepared by f ollowing isotonic agitation, differential centrifugation, and Mg2+-ind uced selective precipitation of nonapical membranes, and its purity wa s assessed by biochemical and morphological marker analysis. We have a lready reported that, unlike previous patch-clamp studies, nonselectiv e cation channels were incorporated in most cases, a result consistent with the higher permeability for cations as compared with C1(-) and w ith the low apical membrane potential difference at term revealed by f luorescent probe partition studies, and microelectrode techniques. In this paper, we report that C1(-)-selective channels were incorporated in 4% of successful reconstitutions (14 out of 353) and that their ana lysis revealed two types of activity. One of them was consistent with a voltage-dependent, 100-pS channel while the other was consistent wit h the lateral association of 47-pS conductive units, giving rise to mu ltibarrelled, DIDS-sensitive channels of variable conductance (300 to 650 pS). The latter displayed a very complex behavior which included c ooperative gating of conductive units, long-lived substates, voltage-d ependent entry into an apparent inactivated state, and flickering acti vity. The role of the reported C1(-) channels in transplacental ion tr ansport and/or syncytium homeostasis remains to be determined.