Statistical models for renal micropuncture studies

Distal tubule bicarbonate accumulation in vivo. Effect of flow and transtubular bicarbonate gradients

We have performed microperfusion studies on distal tubules of normal and alkalotic rats in an attempt to demonstrate in vivo bicarbonate secretion. All perfusion solutions were free of phosphate and other nonbicarbonate buffers. In both normal and alkalotic rats, distal perfusions elicited significant tCO2 entry only at high flow (24 nl/min). Even when perfusate tCO2 concentration closely matched plasma tCO2 concentration (30 mM tCO2), significant tCO2 entry again occurred at high flow. This was associated with a rise of the perfusate tCO2 concentration, which indicated net entry of tCO2 against a concentration gradient. In this "symmetrical" perfusion situation, acetazolamide blockade prevented tCO2 entry. Accordingly: distal tubule tCO2 entry is demonstrable in both alkalotic and normal rats at high flow rates; increasing perfusate tCO2 concentration can suppress tCO2 entry; and entry can occur in the absence of a gradient and this effect can be blocked by acetazolamide.

An in vivo microperfusion study of distal tubule bicarbonate reabsorption in normal and ammonium chloride rats

For many years it has been thought that distal nephron hydrogen ion secretion can be importantly modulated by factors such as sodium delivery, sodium avidity, and potassium stores. Free flow micropuncture studies have also indicated that the rate of bicarbonate delivery may also alter the rate of bicarbonate reabsorption. The present studies were undertaken to examine possible luminal influences on total CO2 reabsorption in microperfused distal tubules in the rat in vivo. Tubules from normal and acidotic rats were perfused with five solutions in a manner that induced changes in bicarbonate load, sodium and potassium fluxes (JNa, JK), and luminal sulfate concentration. in each collected perfusate, simultaneous analyses were undertaken to determine water reabsorption, Na, and K concentrations using graphite furnace atomic absorption spectroscopy and total CO2 by microcalorimetry. Using factorial analysis of covariance to account for confounding effects on total CO2 flux (JtCO2) such as water reabsorption, distal tubules of acidotic rats reabsorbed CO2 in the range of 50-112 pmol X min-1 X mm-1 X These JtCO2 values were not significantly correlated with HCO3 load, JNa, or JK despite changes in the latter from net reabsorption to net secretion. Distal tubules of rats with normal acid-base status had JtCO2 values which were neither significantly different from zero nor correlated with changes in JK and JNa. Further, doubling the load from 250-500 pmol/min (by doubling the perfusion rate of 25-mM HCO3 solutions) did not stimulate JtCO2 in these normal animals. Accordingly, these acute in vivo microperfusion studies indicate for the first time that neither load nor potassium or sodium fluxes are important modulators of distal tubule bicarbonate reabsorption.

Loop of Henle bicarbonate accumulation in vivo in the rat

We have carried out perfusion studies on hydropenic and bicarbonate-loaded rats to provide direct in vivo observations on bicarbonate accumulation in the short loops of Henle. Analysis of early distal tubular fluid was made during bicarbonate-free saline perfusion from the end proximal to the early distal site, documenting accumulation of "new" bicarbonate. During perfusion in hydropenic rats, steady-state bicarbonate concentrations were suggested by early distal values of approximately equal to mM, which were independent of perfusion rate and virtually indistinguishable from bicarbonate concentration measured during free flow when filtered bicarbonate was allowed to enter the loop. Thus, loop bicarconate accumulation was apparently sufficient to allow new bicarbonate to enter at a rate comparable to that delivered to the early distal site during free flow, recognizing of course that free-flow delivery rates are the result of complex components of filtration and bidirectional fluxes. In bicarbonate-loaded rats, however, bicarbonate accumulation rates although higher than in hydropenia, were much lower than free-flow delivery rates. Furthermore, early distal bicarbonate concentrations during bicarbonate loading fell as perfusion rate increased, presumably because of a limitation to increasing ionic bicarbonate entry.