Light and electron microscopic studies of rat kidney after administration of inhibitors of the citric acid cycle in vivo. A morphological and histochemical study of the pars recta during malonate poisoning.

Abstract

Extensive conglomerations of smooth endoplasmic reticulum (SER) formed in pars recta proximal tubules of rat kidney 15 to 30 minutes after single subcutaneous injections of malonic acid (0.5 M given at 1.0 ml per 100g body weight). Simultaneously rough endoplasmic reticulum (RER) disappeared but within 4 hours the normal ER pattern was restored. During this period numerous large multivesicular bodies (MVB) accumulated. In addition to vesicles many MVB contained mitochondria, cytoplasm and even smaller MVB. Apparent fusion of thinner SER membrane with thicker membrane limiting MVB was observed rarely in both perfusion and immersion fixed tissues. Histochemical studies on altered ER, labelled with thiamine pyrophosphatase and unbuffered osmium staining, failed to demonstrate that MVB vesicles were ER derived or that ER was incorporated into MVB by fusion with the limiting membrane. This must be considered a fixation artifact. Horseradish peroxidase, a pinocytotic tracer, showed MVB to have pinocytotic origin. The vesicles apparently formed by budding of cytoplasm into MVB matrices. Acid phosphatase activity was acquired by MVB as they moved towards mid-cell and the enclosed organelles underwent digestion. It is clear that MVB of pars recta are ambilysosomes digesting both autologous and heterologous materials but there was no evidence that altered ER underwent digestion within them and it is likely that rapid alterations in ER resulted from rearrangements of existing membrane moieties. MVB have been implicated in autophagy in other cell types but their autophagic role in sublethally injured kidney tubules has received little attention.