The Absence of an Na+/Ca2+Exchanger (NCX) in Bullfrog Proximal Tubules and Cellular pH is More Influential Than Cellular Ca2+ on Proximal Na Transport

Abstract

Background/aims: The functional significance of the Na⁺/Ca²⁺ exchanger (NCX) in basolateral membranes in the proximal tubule remains controversial. The key factor in crosstalk between the apical and basolateral sides is not known.

Methods: We investigated the basolateral membranes, using double-barreled Ca²⁺ or pH ion-selective microelectrodes. We used doubly perfused bullfrog kidneys in vivo, and switched the basolateral solution (renal portal vein) to experimental solutions.

Results: In the control, cellular pH (pH_i) was 7.33 ± 0.032 (mean ± SE, n = 7) and in separate experiments, cellular Ca²⁺ activity (aCa_i) was 249.6 ± 35.54 nM (n = 28). Changing to respiratory acidosis, pH_i was significantly acidified by 0.123 pH units on average and the change of aCa_i was +53.1 nM (n = 9 ns). In metabolic acidosis, pH_i was reduced by 0.151 while aCa_i was reduced by 143.4. Using the 30 mM K⁺ solution, pH_i was increased by 0.233 while aCa_i was reduced by 203.9, with depolarization. Both ionomycin and ouabain caused aCa_i to increase. In the 0.5 mM Na⁺ solution (replaced with BIDAC Cl), pH_i was reduced by 0.177. No changes in aCa_i (+49.8 ns) were observed although we recorded depolarization of 15.2 mV. In the 0.5 mM Na⁺ solution, replaced with raffinose, no changes in aCa_i (-126.4 ns) were observed with depolarization (6.5 ns).

Conclusion: Our results suggest that thermodynamic calculations of cellular Na⁺ concentration led to the conclusion that either a Na⁺/HCO₃^- exchanger (NBC) or NCX could be present in the same basolateral membrane. H⁺ ions are the most plausible key factor in the crosstalk.

Keywords: Na+/HCO3− cotransporter (NBC); Na+/Ca2+ exchanger (NCX); Respiratory acidosis; Metabolic acidosis; Depolarization with high K+; Low Na+ replacement with or without Cl−; Proximal tubule.