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 (pHi) was 7.33 ± 0.032 (mean ± SE, n = 7) and in separate experiments, cellular Ca2+ activity (aCai) was 249.6 ± 35.54 nM (n = 28). Changing to respiratory acidosis, pHi was significantly acidified by 0.123 pH units on average and the change of aCai was +53.1 nM (n = 9 ns). In metabolic acidosis, pHi was reduced by 0.151 while aCai was reduced by 143.4. Using the 30 mM K+ solution, pHi was increased by 0.233 while aCai was reduced by 203.9, with depolarization. Both ionomycin and ouabain caused aCai to increase. In the 0.5 mM Na+ solution (replaced with BIDAC Cl), pHi was reduced by 0.177. No changes in aCai (+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 aCai (−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.