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.
D Z Levine, M K Byers, R A McLeod, J A Luisello, S Raman