The Absence of an Na+/Ca(2+)Exchanger (NCX) in Bullfrog Proximal Tubules and Cellular pH is More Influential Than Cellular Ca2+ on Proximal Na Transport

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 iwas 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.

Automated summary

The functional significance of the Na+/Ca2+ exchanger (NCX) in basolateral membranes in the proximal tubule is controversial. The key factor in crosstalk between the apical and basolateral sides is not known. However, our experimental results suggest that H+ ions could play a crucial role in this crosstalk.