Renal tubular calcium transport: effects of changes in filtered calcium load.

Micropuncture experiments were performed in thyroparathyroidectomized dogs to examine the influence of changes in filtered calcium load on segmental tubular calcium reabsorption. Filtered calcium load was changed either by reducing glomerular filtration rate (GFR) by aortic clamping (group I) or by progressive calcium infusion (group II) to increase plasma ultrafilterable calcium concentration (UFCa). The results suggest that fractional proximal calcium reabsorption responds similarly to altered filtered load, whether produced by changes in GFR or UFCa. In contrast, fractional reabsorption by the loop segment is progressively reduced as UFCa is increased but is relatively unchanged by alterations in filtered load secondary to altered GFR. These data indicate a specific parathyroid hormone-independent reciprocal effect of UFCa on calcium reabsorption in the loop segment, which may be an important determinant of urinary calcium excretion.