Titratable Acid, Pco₂, Bicarbonate and Ammonium Ions along the Rat Proximal Tubule

Abstract

KARLMARK, B. and B. G. DANIELSON. Titratable acid, Pcoz, bicarbonate and ammonium ions along the rat proximal tubule. Acta physiol. scand. 1974. 91. 243 -258.

Direct determination of PCO2 in the rat renal cortex.

“…The finding of an elevated Pco2 throughout the renal cortex, although somewhat surprising initially, is quite compatible with previously reported values for in situ pH (2,3,9) and bicarbonate concentration (9,12,18). In fact, if one assumes reaction equilibrium in the proximal tubule, a pH of 6.71, and a bicarbonate concentration of 8 mM, the calculated Pco2 for these values would be 65 mm Hg.…”

“…In contrast, our electrode has the following characteristics: sensitivity of 57-61 mV/log10 Pco2 (near theoretic or Nernstian response), stability ofintercept, a tip diameter sufficiently small to allow direct measurement in all segments of the nephron, and does not exhibit sensitivity to 02 or N2. The present study, therefore, circumvents the disadvantages inherent in the indirect and direct methodology employed in the studies cited above (12,13). Repeated determinations (68 proximal tubules) in 21 control rats clearly establish that proximal tubular Pco2 exceeds arterial Pco2 under normal acid-base conditions (Table II).…”

“…Karlmark and Danielson (12) were the first to suggest that tubular fluid might not be in equilibrium with arterial blood. Despite the wide range of CO2 tensions reported by these investigators, the inference was made that a declining profile for Pco2 existed along the length of the proximal tubule (12). In this study, however, the Pco2 oftubular fluid was not determined directly, but was calculated by indirect techniques from measurements ofin situ pH.…”

“…Most studies have necessarily assumed identical values for arterial blood and tubular fluid Pco2 (2, 3, 9). In the few studies in which an attempt has been made to estimate Pco2 either indirectly or directly, the calculated (12,19), or determined (13) CO2 tension in proximal tubular fluid exceeded that ofarterial blood. Moreover, a profile was described in which the Pco2 fell along the length of the proximal tubule, reaching equilibrium with arterial blood in the later segments (12).…”

Ammonium Handling by Superficial and Juxtamedullary Nephrons in the Rat

“…While the biochemical events involved in the renal production of ammonia seem clear (1,2), the secretory site(s) in the nephron are less well defined. Micropuncture studies in rats (3)(4)(5)(6)(7) reported that the concentration of ammonium in fluid obtained along the proximal segments of surface nephrons was higher than could be accounted for by water extraction suggesting that ammonium entry occurs in this segment of the renal tubule. Further, these studies reported a higher concentration of this buffer in tubular fluid when rats were J. Clin.…”