INFLUENCE OF FORSKOLIN AND CARBACHOL ON INTESTINAL-ABSORPTION OF HORSERADISH-PEROXIDASE IN THE GOLDFISH (CARASSIUS-AURATUS)

The transepithelial route for mucosa-to-serosa transport of the tracer macromolecule horseradish peroxidase (HRP; MW 40 kDa) and modulation of this transport by forskolin and carbachol have been studied in vitr o in stripped goldfish intestinal epithelium mounted in Ussing-type ch ambers. Uptake and transport have been investigated by measuring the H RP flux from the mucosal to serosal sides by an enzymatic method and b y visualising HRP reaction products in the mucosa with electron-micros copical techniques. Both the cholinergic agonist carbachol (which is t hought to increase intracellular Ca2+ and activate protein kinase C ac tivity) and forskolin (a direct activator of adenylylcyclase) affect t he amount of enzymatically active HRP in the tissue. In control tissue , HRP product is found only within the epithelial cells, the transepit helial flux reaching a constant value of about 1.5 pmoles/cm(2) per h. Carbachol increases the amount of HRP product in the cells, but has n o significant effect on the HRP flux compared with control values. For skolin decreases the amount of HRP product in the cells; however, in t he presence of forskolin, the lateral intercellular spaces become fill ed with HRP product. HRP is found in the lamina propria and the transe pithelial protein flux increases more than 2.5-fold. In the presence o f forskolin plus carbachol, the results are no different from the cont rol. It is concluded that carbachol increases the endocytotic uptake o f HRP, whereas forskolin inhibits the uptake but increases the paracel lular permeability for HRP in goldfish intestine.