The antimony microelectrode in kidney micropuncture.

The use of pH-sensitive microelectrodes for the determination of ttubular pH in amplhibian neplirons dates from the early days of kidney micropuncture. Soon after this metlhodology was introduced by Riclhards and hiis coworkers, MIontgomery and Pierce(1,2) publislhed their papers on the (letermination of tubular pH in Necturus and in frogs by means of a quinlhydrone microelectrode. In this microelectrode, the pH-sensitive element is a tlhin platinum wire covered by quinlhydrone crystals, located inside a collection micropipete. Tubular fluid is aspirated into this pipete until it contacts the wire, and the voltage between this electrode and a reference electrode is measured and is proportional to tlle sample's pH. Originally the pipete was filled with mercury. Later, other autlhors filled it witlh mineral oil equiilibrated witlh CO,, witlh wlhiclh the tubular fltuid had to equilibrate(3-5). Using this metlhod, Gottschalk et al.(5) as well as Giebisclh et al.(6) were able to slhow tlhat, in the mammalian neplhron, acidification starts in the proximal tutbule, a different finding from that of Montgoinery and Pierce, wlho lhad slhown that, in the amphlibian, acidification was a function only of the distal tubule. Two basic meclhanisms can be considered the cause of tubular acidification or bicarbonate reabsorption. One is the secretion of H+ ions into the tubular lumen by the tubular cell, and the otlher is the reabsorption of alkaline salts like sodium bicarbonate as such. In the first situation, bicarbonate would be decomposed into CO., and water through the formation and subsequent dehydration of carbonic acid in the tubular lumen. The observation by Walser and MIudge(7) that the hydrogen ion secretory rate in the mammalian neplhron calculated upon assumption of the first mechanism would exceed the rate of uncatalyzed dehydration of carbonic acid at a concentration equal to that in plasma, in the tubular lumen, led to the expectation of a disequilibrium pH at this site unless this dehydratior