Ultracytochemical Study of the Pathway of Ca2+ Transportation across the Epithelial Cell Layer of a Nephron.

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Abstract

The pathway of Ca2+ absorption across the renal epithelial layer was ultracytochemically investigated in nephrons using an oxalate-pyroantimonate technique. Reaction products were strongly detected in the inter-cellular space in proximal tubules, and in intracellular organelles in distal tubules in the cortex and the outer layer of the outer medulla. These findings are suggestive of paracellular transport of Ca2+ in the proximal tubule, and transcellular transport in the distal tubule. The transcellular pathway of Ca2+ is of interest, as intracellular Ca2+ concentrations must be maintained. Ca-precipitates, in the distal tubule cells, were detected en masse in mitochondria, and to a lesser extent, in smooth endoplasmic reticulum, Golgi apparatus and vesicles. Precipitates were rare in lysosomes and rough endoplasmic reticulum. Calcium containing organelles distributed throughout the cytoplasm. Free precipitates in the cytoplasmic matrix were only rarely seen in the apical portion of the cells. It seems that Ca2+immediately after entering a cell, binds to calcium binding proteins, or is sequestered to organelles like mitochondria, Golgi apparatus, smooth endoplasmic reticulum or vesicles. In this way, fluctuations in intra-cellular Ca2+ concentrations are avoided. These organelles may be involved in Ca-transport in Ca-absorbing cells.

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