title: A new protocol for the measurement of picomole quantities of magnesium in rat renal tubular fluid.
abstract: The analysis of picomolar quantities of magnesium by electrothermal atomic absorption spectrophotometry (EAAS) was studied using a Perkin-Elmer-Zeeman 3030 spectrophotometer. The absorbance signal was not heavily dependent on the atomization temperature, but was greatly reduced when 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 modifier was sufficient to overcome this artefact. The analytical sensitivity 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 handling in the rat proximal convoluted tubule revealed a significant correlation between the tubular fluid-to-plasma ultrafiltrate (TF/UF) concentration ratio for magnesium and the tubular fluid-to-plasma (TF/P) concentration ratio for [3H]inulin (r2 = 0.56, n = 17). This indicated that magnesium is concentrated during its passage along the proximal tubule. In contrast, this was not the case for sodium (r2 = 0.11, n = 16). Mean (TF/UF)Mg (1.16 +/- 0.07, n = 17) for random punctures was significantly greater than that for sodium ((TF/UF)Na = 1.02 +/- 0.02, n = 16). Despite concentration of magnesium in the lumen, significant net reabsorption of magnesium was observed along the length of the tubule (fractional reabsorption, FRMg = 19.4 +/- 3.0%, n = 17). In conclusion, EAAS provides a highly sensitive, reproducible and technically simple method for measuring picomolar quantities of magnesium in renal tubular fluid.
authors.affiliation: School of Biological Sciences, University of Manchester, UK. jkibble@sheffield.ac.uk
authors.forename: J D
authors.initials: JD
authors.lastname: Kibble
authors.forename: N
authors.initials: N
authors.lastname: Audsley
authors.forename: J P
authors.initials: JP
authors.lastname: Day
authors.forename: R
authors.initials: R
authors.lastname: Green
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000502:physiology; D007687:Kidney Tubules, Proximal / Q000737:chemistry*; D008274:Magnesium / Q000032:analysis* / Q000378:metabolism; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000378:metabolism; D013053:Spectrophotometry / Q000379:methods; D013696:Temperature; D014462:Ultrafiltration
chemical_list: D012964:Sodium; D008274:Magnesium