TRANSCYTOTIC VESICLE FUSION WITH CANALICULAR MEMBRANES IS MODULATED BY PHOSPHOLIPID SPECIES - IMPLICATIONS FOR BILIARY LIPID SECRETION

Phospholipid species modulate bile metastability and the subselection of such species for biliary secretion occurs at the canalicular membra ne. In this study, the role of phospholipid head groups and hydrophobi c indices in transcytotic vesicle fusion with the canalicular membrane inner leaflet was investigated using rat canalicular membrane vesicle s (CMV) and liposomes. The CMV were purified from Sprague-Dawley rat l iver, and small unilamellar vesicles (SUV) of phosphatidylserine (PS), phosphatidylcholine (PC) and mixtures of PS/PC (1:1, 2:1 and 4:1) wer e labelled with 8 mol% of octadecyl rhodamine 13 chloride (R18). The P C species used in this study were egg yolk PC (EYPC), soybean PC (SBPC ), dipalmitoyl PC (DPPC) and dilinoleoyl PC (DLPC). Fusion of SW with CMV was initiated by the addition of a millimolar concentration of Ca2 + and the degree of fusion was estimated by the increase of R18 fluore scence. Ca2+-dependent fusion of SW consisting of PS, and PS/PC (4:1) with CMV was observed (PS > PS/PC; 4:1), whereas no detectable fusion was evident between CMV and SW of PC alone or PS/PC (1:1 or 2:1). The rank order of fusibility between CMV and SUV of PS/PC (4:1) containing various PC species was PS/DLPC>PS/SBPC>PS/EYPC>PS/DPPC. The hydrophob ic index of PC as determined by high performance Liquid chromatography (HPLC) was related closely to liposome fusibility (r=-0.88). These re sults suggest that transcytotic vesicle fusion with the canalicular me mbrane inner leaflet is regulated by the phospholipid hydrophobicity o f the vesicles.