AJP - Renal Physiology, Vol 252, Issue 6 1158-F1166, Copyright © 1987 by American Physiological Society
In vivo measurement of tubular fluid ferrocyanide with carbon-fiber microelectrodes
L. C. Moore, C. Clausen, E. F. Bowden and A. Birzgalis
Techniques to construct carbon-fiber microelectrodes and to measure
ferrocyanide ion concentration in single nephrons are described. The
measurement involves polarizing an inert carbon-fiber microelectrode 500 mV
positive with respect to a Ag-AgCl reference, while measuring the faradic
current produced by the oxidation of ferrocyanide. A carbon fiber (5-7
micron diam) is heat sealed into a glass micropipette that is then
sharpened, silanized, and electrochemically pretreated to minimize
electrode degradation by protein. Circuit diagrams for an inexpensive
voltage clamp-current monitor and a data sampling device are presented. The
electrodes show a linear response to changes in ferrocyanide concentration
in large and very small (20 nl) volumes in vitro. The electrodes were used
in an electrochemical microassay to determine tubular fluid-to-plasma
ferrocyanide concentration ratios and nephron filtration rates with
proximal micropuncture samples. The results show excellent agreement with
paired determinations using [3H]inulin. In vivo proximal tubule perfusion
experiments show a rapid linear response to changes in tubular fluid
ferrocyanide concentration. These electrodes permit rapid quantitative
measurements of ferrocyanide concentration and water transport in the
proximal tubule and may be useful in other biological systems.
