Abstract
The stomachs of most vertebrates operate at an acidic pH of 2 generated by the gastric H+/K+-ATPase located in parietal cells. The acidic pH in stomachs of vertebrates is believed to aid digestion and to protect against environmental pathogens. Little attention has been placed on whether acidic gastric pH regulation is a vertebrate character or a deuterostome ancestral trait. Here, we report alkaline conditions up to pH 10.5 in the larval digestive systems of ambulacraria (echinoderm + hemichordate), the closest relative of the chordate. Microelectrode measurements in combination with specific inhibitors for acid-base transporters and ion pumps demonstrated that the gastric alkalization machinery in sea urchin larvae is mainly based on direct H+ secretion from the stomach lumen and involves a conserved set of ion pumps and transporters. Hemichordate larvae additionally utilized HCO3− transport pathways to generate even more alkaline digestive conditions. Molecular analyses in combination with acidification experiments supported these findings and identified genes coding for ion pumps energizing gastric alkalization. Given that insect larval guts were also reported to be alkaline, our discovery raises the hypothesis that the bilaterian ancestor utilized alkaline digestive system while the vertebrate lineage has evolved a strategy to strongly acidify their stomachs.
Highlights
The highly acidic pH of most vertebrate stomachs is believed to protect against environmental pathogens and the mechanisms of gastric acidification are well described[1,2,3]
In contrast to acidic digestive systems found in most vertebrates, the gastric pH of echinoid pluteus and hemichordate tornaria larvae was highly alkaline and the pH was 9.10 ± 0.02 (n = 1 1) and 10.35 ± 0.01 (n = 5) in S. purpuratus and P. flava, respectively
Our results showed that gastric alkalization in sea urchin larvae is inhibited by ouabain and bafilomycin, specific inhibitors for the Na+/K+-ATPase (NKA) and the V-type H+-ATPase (VHA), respectively (Fig. 1B+C)
Summary
The highly acidic pH of most vertebrate stomachs is believed to protect against environmental pathogens and the mechanisms of gastric acidification are well described[1,2,3]. The H+/K+-ATPase (HKA) in luminal membranes of parietal cells drives H+ ions into the stomach lumen leading to an accumulation of hydrochloric acid[4] This enables vertebrates to generate highly acidic conditions in their digestive systems. Midgut alkalization in lepidopteran and dipteran insect larvae, is energized by an V-H+-ATPase (VHA) coupled to processes that result in net export of H+ from the luminal space[5,6,7]. These observations demonstrate that different gastric pH regulatory mechanisms have evolved in the animal kingdom. Sea urchins are prime organisms to study regulatory mechanisms controlling early development[11], many functions of the larval gut remain unexplored
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