Identification of Mg-transporting renal tubules and cells by ion microscopy imaging of stable isotopes.

Sites of renal Mg transport were identified in seawater killifish (Fundulus heteroclitus) using a Cameca model IMS-3f ion microscope. Killifish were given an intraperitoneal injection of the stable isotope 26Mg (99.5% enrichment) to stimulate and trace renal Mg excretion. We identified two sites of 26Mg transport in frozen freeze-dried cryosections of kidney: the proximal tubule, known to secrete Mg, and the collecting duct, heretofore not known to handle Mg. In epithelial cells of the proximal tubule, the punctate distribution of injected 26Mg suggests transcytotic excretion of Mg in bound form. In collecting ducts, a subpopulation of Mg/Ca-rich cells was identified with high accumulations of injected 26Mg. Here, the punctate distribution of 26Mg decreased from the apical to the basal region of the cells, revealing a transcytotic gradient of apparently bound Mg. Since proximal tubules of fish are implicated with Mg secretion, Mg/Ca-rich cells in the collecting duct may reabsorb Mg, thereby providing the usual two-step of renal regulation, now also for Mg.