Magnesium handling in the dog kidney: a micropuncture study.

Micropuncture and clearance experiments were done on normal dogs to investigate magnesium handling by proximal and distal nephron segments. Tubular fluid electrolytes were analyzed with the electron microprobe. Tubular fluid to ultrafilterable magnesium ratio (TF/UF magnesium) was observed to rise above unity but less than the TF/P insulin ratio generated along the proximal tubule. This is in contrast to the other major cations, the ratios of which remain close to unity as water is abstracted. Tubular fluid obtained from the distal tubule contained less magnesium than the glomerular filtrate (mean TF/UF magnesium of 0.6) indicating the loop of Henle is the major nephron segment reclaiming a significant portion of the filtered load. The faction of filtered load remaining at the distal sampling site was similar to the fraction appearing in the urine (8% vs 7%) indicating very little reabsorption beyond the distal tubule in these normal states.