Urinary acidification in turtle bladder is due to a reversible proton-translocating ATPase

Urinary acidification is a critical process in maintaining pH homeostasis, but the underlying mechanisms remain unclear in certain species. Prior research has shown that epithelial tissues can transport protons across membranes to acidify urine. However, the specific molecular mechanism responsible for this transport in turtle bladders had not been resolved. Some studies suggest that proton transport could be driven by either a proton-translocating ATPase or a redox pump. This uncertainty left a gap in understanding how epithelial cells manage proton gradients under adverse conditions. The question of whether ATP synthesis occurs under high proton gradients remained unanswered. No prior work had directly tested ATP levels in epithelial cells under varying proton gradients. That uncertainty drove the need for an experiment that could distinguish between these two possible mechanisms. This gap motivated a study to measure ATP synthesis in turtle bladder epithelial cells under controlled proton gradients. The goal was to determine whether a proton-translocating ATPase or a redox pump is responsible for urinary acidification.

The aim of this study was to determine the mechanism responsible for urinary acidification in turtle bladders. Specifically, the researchers sought to distinguish whether a proton-translocating ATPase or a redox pump drives proton transport under adverse electrochemical gradients. The study focused on measuring ATP synthesis in epithelial cells under controlled proton gradients. The motivation was to test whether high proton gradients could reverse the direction of proton transport and lead to ATP synthesis. The researchers hypothesized that ATP synthesis would occur if a proton-translocating ATPase was involved. They also wanted to assess whether inhibitors like dinitrophenol or dicyclohexylcarbodiimide could block ATP synthesis. The study aimed to provide direct evidence of ATP synthesis under varying proton gradients. This would clarify the role of a proton-translocating ATPase in urinary acidification.

The researchers applied adverse proton electrochemical gradients across the turtle urinary bladder epithelium. They measured ATP levels in epithelial cells after poisoning them to stop ATP synthesis. The cells were exposed to proton gradients ranging from 120 to 310 mV. At each gradient, ATP levels were recorded to detect synthesis. Dinitrophenol was used to test if ATP synthesis could be blocked by a proton carrier. Dicyclohexylcarbodiimide was applied to assess inhibition at the cell surface. The study compared ATP synthesis at different gradient levels. The presence of ATP synthesis at high gradients suggested a reversible proton-translocating ATPase.

At proton gradients of 120 mV or less, no ATP synthesis was observed in epithelial cells. At gradients above 120 mV, ATP synthesis increased linearly with the gradient. At 310 mV, ATP synthesis was detectable and proportional to the gradient. Dinitrophenol prevented ATP synthesis at 310 mV by acting as a proton carrier. Dicyclohexylcarbodiimide also blocked ATP synthesis at 310 mV by inhibiting proton transport at the cell surface. These findings indicate that ATP synthesis occurs only when proton gradients exceed a threshold. The inhibition by both compounds suggests that proton transport is tightly coupled to ATP synthesis. The results support the presence of a reversible proton-translocating ATPase in the mucosal border.

The study concludes that a reversible proton-translocating ATPase in the mucosal border of the turtle bladder is responsible for urinary acidification. The authors propose that ATP synthesis occurs only when proton gradients exceed a threshold of 120 mV. The linear increase in ATP synthesis at higher gradients supports this mechanism. Inhibition by dinitrophenol and dicyclohexylcarbodiimide confirms the role of proton transport in ATP synthesis. The findings suggest that proton transport and ATP synthesis are tightly coupled. The authors suggest that the ATPase reverses direction under high proton gradients. This reversal leads to ATP synthesis rather than consumption. The study provides direct evidence for a proton-translocating ATPase in urinary acidification.