Effect of NaCl infusion on urinary Ca++ and Mg++ during reduction in their filtered loads

Abstract

Hypotonic saline was infused to 7 anesthetized dogs while glomerular filtration rate (GFR) was reduced by inflation of an intra-aortic balloon located above the renal arteries. Despite a substantial reduction in filtered load, Ca excretion (UCaV) increased when Na excretion (UnaV) was enhanced. The clearances (C) of Ca and Na were linearly related during normal and reduced GFR without significant difference between their regression slopes. During reduced GFR the change in urinary Mg (UMgV) was dependent on the magnitude of Na diuresis. When UNaV was greatly augmented, UMgV increased but fell when the increment in UNaV was only moderate. The relations between CMg and CNa were linear at normal and reduced GFR, but the regression slopes differed significantly. The increment in CMg with increased CNa was less when GFR was reduced. Expansion of extracellular volume during saline infusion may decrease not only Na but Ca and Mg reab-sorption as well. Although these studies demonstrate the close association in the renal handling of these ions, they indicate that UMgV may be more critically influenced by a fall in filtered load than UCaV.