A NEW PROTOCOL FOR THE MEASUREMENT OF PICOMOLE QUANTITIES OF MAGNESIUM IN RAT RENAL TUBULAR FLUID

The analysis of picomolar quantities of magnesium by electrothermal at omic absorption spectrophotometry (EAAS) was studied using a Perkin-El mer-Zeeman 3030 spectrophotometer. The absorbance signal was not heavi ly dependent on the atomization temperature, but was greatly reduced w hen ashing temperatures in excess of 1200 degrees C were applied. The magnesium signal was significantly depressed in the presence of excess chloride in the sample matrix. However, use of NH4NO3 as a matrix mod ifier was sufficient to overcome this artefact. The analytical sensiti vity was 0.15 absorbance units pmol(-1) and the detection limit was 0. 04 pmol. Using nanolitre constriction pipettes to dispense standards, the mean coefficient of variation was 5%. Measurement of magnesium han dling in the rat proximal convoluted tubule revealed a significant cor relation between the tubular fluid-to-plasma ultrafiltrate (TF/UF) con centration ratio for magnesium and the tubular fluid-to-plasma (TF/P) concentration ratio for [H-3]inulin (r(2) = 0.56, n = 17). This indica ted that magnesium is concentrated during its passage along the proxim al tubule. In contrast, this was not the case for sodium (r(2) = 0.11, n = 16). Mean (TF/UF)(Mg) (1.16 +/- 0.07, n = 17) for random puncture s was significantly greater than that for sodium ((TF/UF)(Na) = 1.02 /- 0.02, n = 16). Despite concentration of magnesium in the lumen, sig nificant net reabsorption of magnesium was observed along the length o f the tubule (fractional reabsorption, FRMg = 19.4 +/- 3.0%, n = 17). In conclusion, EAAS provides a highly sensitive, reproducible and tech nically simple method for measuring picomolar quantities of magnesium in renal tubular fluid.