Marking the tip location of PO2 microelectrodes or glass micropipettes

The necessity of localizing the PO2 recording sites in the carotid body of cats has led us to modify our O2 microelectrode to be able to mark the position of the tip. Gold is sputtered on the electrode, and then iron is plated on the gold. The carotid body of cats was exposed, and PO2 was recorded at different depths in the body after which iron was deposited. At the end of the experiment, the excised body was placed in a 1:1 mixture of 4% potassium ferrocyanide and 4% acetic acid for 1 h or more. After that, the body was fixed in a 1:1 mixture of 40% formaldehyde and 95% ethyl alcohol for 1-2 days. Paraffin sections cut at 10 micrometer revealed the recording sites as bright blue spots. Since the cathode size can be made as small as 1 micrometer, discrete spots (a few microns or larger depending on current and time) can be produced at the PO2 recording sites. Several locations can be marked in a few minutes without damage to the tissue or the electrode. In 21 cats all of the 62 marked spots were found. In 16 peripheral (depth 50-150 micrometers) locations the PO2 averaged 101 Torr and in 46 deep locations 73 Torr. This technique is applicable to glass micropipettes and is a unique tool in correlating the structure and function of various tissues.