Electron-microscopic demonstration of alkaline-phosphatase activity in the juxtaglomerular apparatus

In the rat nephron, alkaline phosphatase (ALP) was observed, with the light microscope, to be present on the brush border of the proximal tubule and in a small band of cells of the juxtaglomerular apparatus. With the electron microscope, ALP activity could not be demonstrated in the macula densa cells proper, but was seen for the first time in a narrow zone of cells interposed between the macula densa and the vascular pole of the renal corpuscle. The lead phosphate precipitates were precisely localized on the plasma membranes which form an intricate network of cytoplasmic interdigitations. Since ALP is known to be involved in some steps of phosphate transport, the present morphological data might be considered as an indication for a role of phosphate as a signal ion for the autoregulation of glomerular filtration.