The importance of the Golgi complex for epithelial ion transport in Drosophila Malpighian tubules, studied by electron microscopy, cytochemistry and X-ray microanalysis.

The distribution of potassium in the cells of Drosophila Malpighian tubules is not homogeneous. In the microvilli of the apical part of the cell the cytoplasmic potassium content was found to be 2 to 3 times higher than in the neighboring intermediate cytoplasm. Data obtained by electron microscopy, histochemistry and electron probe X-ray microanalysis indicate that glucosaminoglycans (GAGs), synthesized by the Golgi-ER complex, are responsible for potassium accumulation in the apical microvilli. Vesicles bud from the Golgi complex and then move to the apical cell region, where they discharge their contents into the cytoplasm or into the lumen. Budded vesicles also discharge their contents into the hemolymph space between the folds of the basal plasma membrane. GAGs, transformed to proteoglycans (PGs), were identified on the folds of the basal cell surface including basal lamina by reaction with alcian blue. Brefeldin A (BFA) was found to disintegrate Golgi-ER structures to vesicles, whereas budded vesicles vanished. Within the microvilli the K+-content decreased to 32%, the water content to 77%. These data provide evidence that the ER-Golgi complex is involved in the delivery of GAGs (and PGs) into the luminal space and the hemolymph. After disintegration of the Golgi complex, GAGs are missing as temporary ion stores from the vicinity of the membrane transporters.