Lipid-loading and unloading of lipophorin in the midgut epithelium of dragonfly larvae ( Aeshna cyanea) : A biochemical and immunocytochemical study

Abstract

Radioactive oleic acid orally infused into dragonfly larvae ( Aeshna cyanea) was recovered in the haemolymph in the esterified form as diacylglycerol which was transported by lipophorin. Radioactive diacylglycerol was released in vitro into the surrounding medium from the isolated, ligated midgut which had been preloaded in vivo with radiolabelled oleic acid. The release was largely dependent on the temperature and the presence of haemolymph in the incubation medium. On the other hand, the ligated midgut isolated from fasting larvae, was also able to absorb radioactive diacylglycerol from the haemolymph and to further esterify it to triacylglycerol. These results demonstrate that the midgut epithelium cannot only release but also absorb diacylglycerol at the basal side, which strongly suggests that both lipid-loading and unloading of lipophorin occur in the midgut probably in dependence of physiological conditions. Lipophorin penetrated across the basal lamina into the basolateral intercellular clefts of the midgut epithelium as evidenced by immunocytochemistry on the light and electron microscopic level, using affinity-purified polyclonal antibodies against lipophorin. The apicolateral intercellular clefts, i.e. the sites of the smooth septate junctions, and the intercellular clefts between the undifferentiated cells of the regeneration nidi remained unlabelled by antilipophorin. Incubation experiments with fluorescently labelled lipophorin suggested that lipophorin of the intercellular clefts was exchanged with lipophorin of the circulating haemolymph. Although the enterocytes showed clear endocytotic activity at the basolateral plasma membranes when incubated with horse radish peroxidase, lipophorin was not detected inside the cells by immunocytochemistry. Consequently, indications of lipophorin secretion and internalization were lacking. These results support the conclusion that lipid-loading and unloading of lipophorin occur extracellularly along the basolateral plasma membranes of the differentiated absorptive enterocytes.