INTESTINAL PARACELLULAR PEPTIDE-TRANSPORT - MOBILIZATION OF INTRACELLULAR CALCIUM AS A MECHANISM OF TIGHT JUNCTIONAL OPENING BY 4-PHENYLAZOBENZOXYCARBONYL-PRO-LEU-GLY-PRO-D-ARG (PZ-PEPTIDE) IN THE RABBIT DESCENDING COLON AND CACO-2 CELL MONOLAYERS

The objective of this study was to test the hypothesis that 4-phenylaz obenzoxycarbonyl-Pro-Leu-Gly-Pro-D-Arg (Pz-peptide) mobilized intracel lular Ca2+ in Caco-2 cell monolayers and the descending colon of the a lbino rabbit, thereby triggering tight junctional opening. Experiments in cell suspensions revealed that Pz-peptide indeed caused an increas e in [Ca2+](i) in a concentration-dependent manner over the 1-10 mM ra nge. Such an increase appeared to correlate with the transient rise in inositol-1,4,5-triphosphate (IP3), a product of phosphoinositol-4,5-b iphosphate (PIP2) hydrolysis mediated by phospholipase C (PLC). The in crease in [Ca2+](i) was inhibited by an intracellular Ca2+ release blo cker, TMB-8, thus supporting the notion of mobilization of Ca2+ from i ntracellular stores. A possible link between the Na+ channel and PLC a ctivation by Pz-peptide was suggested by the lack of enhancement in IP 3 and [Ca2+](i) in the presence of either neomycin (a PLC inhibitor) o r amiloride (a Na+ channel blocker). Finally, blunting the rise in Ca2 + and IP3 concentrations by GDP beta S (a non-hydrolyzable analog of G TP) raises the interesting possibility that a G-protein may be involve d in the tight junctional opening triggered by Pz-peptide.