Stimulation of cellular autophagy by parathyroid hormone and cyclic adenosine 3',5': monophosphate in isolated tubular fragments from the rat's kidney cortex.

Tubular fragments isolated from the cortex of the rat's kidney were qualitatively and quantitatively investigated with the electron microscope. The tubules frequently burst open and became "inverted" in such a way that the rarefied brush border now formed the outer circumference. By morphometry a decrease of the average cell volume in the proximal tubular fragments was ascertained. This was mostly caused by a loss of cytoplasmic ground substance, endoplasmic reticulum and ribosomes. Cytoplasmic herniations of the basal surface, filled with free ribosomes, suggested a partial shedding of the protein synthesizing apparatus. The number of autophagic vacuoles (AV) per unit area of cytoplasm was determined in proximal tubular fragments. After isolation alone, without further incubation, the number of AV was as low as the number found in an earlier study in proximal tubular cells in situ during the diurnal minimum. After control incubation the number of AV increased to about the mean value of the AV found in cells in situ during the whole diurnal cycle. By comparison with the control incubation the number of AV increased by a factor of 1.6 to 1.7, if cyclic adenosine 3',5': monophosphate (cyclic AMP) or parathyroid hormone (PTH) were added to the incubation-medium; it now reached about the number of AV found in situ during the diurnal maximum. The increase in the number of AV paralleled that of the production of ammonia and glucose from endogenous sources under the influence of cyclic AMP and PTH. This suggests that the breakdown of cytoplasmic components by cellular autophagy could be functionally related to gluconeogenesis. A quantitative comparison between the measured production of ammonia and glucose indicates, however, that in the system of isolated tubular fragments there may exist other mechanisms of degradation, and of the provision of substrates for gluconeogenesis, than cellular autophagy only.