Change of apparent stoichiometry of proximal-tubule Na(+)-HCO3- cotransport upon experimental reversal of its orientation.

Abstract

Electrogenic cotransport of Na+ with HCO3 has been reported in numerous tissues. It has always been shown with a net transfer of negative charge, but in some situations achieves net outward transport of both species with a stoichiometry of at least three HCO-ions per Na+ ion (3:1), and in other situations achieves net inward transport of both species and has a stoichiometry of at most two HCO-ions per Na+ ion (2:1). This suggests either that there may be more than one protein responsible for Na+-HCO-cotransport in different tissues or that if there is a single protein, its stoichiometry may differ depending on the orientation of net transport. The present study, using conventional or double-barreled ionselective microelectrodes to follow basolateral membrane potential and intracellular pH or Na+ activity in Necturus proximal convoluted tubule in vivo, shows that the orientation of the basolateral Na+-HCO-cotransporter can be reversed upon switching from a perfusate simulating normal acid-base conditions to one that imposes peritubular isohydric hypercapnia. Moreover, accompanying the reversal oforientation is a change of apparent stoichiometry from 3:1 to 2:1. Given that the observed change of orientation and accompanying change of apparent stoichiometry occur within seconds and in the same preparation, these results suggest that a single transport protein is responsible for both types of behavior.

Missense mutation T485S alters NBCe1-A electrogenicity causing proximal renal tubular acidosis

“…(28). In Necturus proximal tubules in vivo, the transporter stoichiometry was 1:3 and changed to 1:2 during acute respiratory acidosis (30). Based on these studies, it has been assumed without definitive evidence that in the human proximal tubule, NBCe1-A has a 1:3 charge transport stoichiometry.…”

Functional characterization of human NBC4 as an electrogenic Na+-HCO 3 − cotransporter (NBCe2)

“…However, when the kidney variant of NBCe1 is expressed in Xenopus oocytes, it appears to operate with a 2:1 stoichiometry (21). There is, however, evidence that the stoichiometry of NBCe1 is not fixed at a particular ratio but can change (30). Gross et al presented evidence that the HCO 3 Ϫ :Na ϩ stoichiometry might shift from 2:1 to 3:1, depending on the cell type in which it is expressed (19) and on the phosphorylation state of the protein (20).…”

Triflocin, a novel inhibitor for the Na‐HCO3 symport in the proximal tubule

“…In the present study, we were able to reverse the Triflocin effect on Vm from hyperpolarization (under physiological acid-base conditions) to depolarization (under imposed iso-hydric hypercapnia). This reversal can be understood in the light of our recent studies (Planelles et al, 1993) demonstrating reversal of the net flux of sodium and bicarbonate via the transporter under conditions of isohydric hypercapnia. A second experimental argument strengthens the hypothesis of Triflocin-associated inhibition of Na-(HCO3)n>1, namely, under physiological conditions and while the cotransport carries net efflux of bicarbonate, the addition of Triflocin leads to cytoplasmic alkalinization.…”

“…This SITS-inhibited cotransporter was initially described in the basolateral membrane of the amphibian PCT (Boron & Boulpaep, 1983), then in mammalian PCT (Yoshitomi et al, 1985;Alpern, 1985). Membrane hyperpolarization induced by SITS reflects inhibition of the outward-directed Na-(HCO3)D>l cotransport (Lopes et al, 1987;Planelles et al, 1993), while membrane depolarization induced by SITS reflects inhibition of the inward-directed Na-(HCO3)nal (Hughes et al, 1989;Planelles et al, 1993). In the present study, we were able to reverse the Triflocin effect on Vm from hyperpolarization (under physiological acid-base conditions) to depolarization (under imposed iso-hydric hypercapnia).…”

“…Table 1). We have already demonstrated that the basolateral Na-(HCO3)n>I cotransport reverses its net flux (thus promoting the influx of sodium plus HCO3 equivalents) during acute iso-hydric hypercapnia (Planelles et al, 1993). Under physiological acidbase conditions, Triflocin significantly hyperpolarizes Vm by -7.6 ± 0.3 mV (n = 10, P<0.001) (Figure 4), whereas during acute iso-hydric hypercapnia, Triflocin significantly depolarizes Vm by 4.7 ± 0.3 mV (n = 7, P<0.001) ( Figure 5).…”