Evidence for HCO₃⁻ and NH₃/NH₄⁺-dependent pH regulatory mechanisms in the alkaline midgut of the sea urchin larva

Alkaline digestive systems are well described for some insect species and their larval stages. More recently, larvae of the members of ambulacraria superphylum consisting of echinoderms and hemichordates were also discovered to have highly alkaline midguts (pH 9.5-10.5) with the underlying acid-base regulatory mechanisms largely unknown. Using pharmacological inhibition of acid-base transporters in conjunction with ion-selective microelectrode measurements and pH-sensitive dyes, we investigated intracellular and extracellular pH regulatory mechanisms of midgut epithelial cells of a sea urchin (<i>Strongylocentrotus purpuratus</i>) larva. Our findings suggest that vacuolar-type H⁺-ATPase (inhibited by bafilomycin a1), carbonic anhydrase (inhibited by acetazolamide), anion-exchangers (inhibited by 4,4'-diisothiocyano-2,2'-disulfonic acid or DIDS), and soluble adenylyl cyclase (inhibited by KH7) play crucial roles in cellular acid-base regulation as well as midgut alkalization. Ammonia excretion rates were decreased in the presence of bafilomycin and colchicine, pointing toward vesicular [Formula: see text] trapping and exocytosis mechanism in eliminating nitrogenous proton equivalents from midgut cells. Finally, midgut perfusion studies revealed ouabain-sensitive luminal [Formula: see text] uptake, suggesting a role for Na⁺/K⁺-ATPase-mediated ammonia transport in midgut alkalization. This comprehensive pharmacological analysis provides a new working model relying on the CO₂/[Formula: see text] and NH₃/[Formula: see text] buffer systems for midgut alkalization in the sea urchin larva. These findings are discussed in the context of other alkalizing systems with strong implications for the conserved role of [Formula: see text] and NH₃-driven mechanism of midgut alkalization across the animal kingdom.<b>NEW & NOTEWORTHY</b> Sea urchin larvae evolved highly alkaline conditions in their digestive tracts, and the underlying acid-base regulatory mechanisms are little understood. Here we present evidence that the process of luminal alkalization is cAMP-dependent. Furthermore, our data point toward the involvement of bicarbonate and ammonia in regulating midgut fluid pH. These results identified a novel mechanism for luminal alkalization in the digestive tract of a marine animal with strong implications for other alkalizing systems in animals.