Intracellular pH regulation and metabolic interactions in hepatic tissues

Intracellular pH (pHi) regulation in the vertebrate liver relies heavily on ionic transport mechanisms. Liver, in common with many tissues, has plasma membrane Na(+)-H+ and Cl(-)-HCO3- electroneutral exchangers which work in opposition to tightly control pHi. Mammalian livers also possess electrogenic Na(+)-HCO3- exchangers, capable of base uptake, which, when coupled to pHi-mediated changes in membrane potential, probably confer an additional measure of pHi control, compared to fish livers, where the transporter appears to be functionally absent. It is suggested that this may be a fundamental difference between aquatic and aerial breathing. pHi regulation has barely been examined in invertebrate hepatic tissues, but already some interesting differences are apparent. Notably, an electrogenic 2Na(+)-1H+ acid-extrusion system is present in apical membranes of crustacean hepatopancreas. Despite these ionic control systems, complex acid-base disturbances (e.g., "metabolic" acidosis) have been known for some time to influence hepatic metabolism in vertebrates, but few studies have carefully examined the independent effects of the acid-base variables involved. Thus mechanistic explanations for the effects of acid-base disturbances are scarce. Ureogenesis in mammals has been well studied, and several pH-related mechanisms are evident. In contrast, the pH-insensitivity of ureogenesis in fish liver may represent a second difference between aquatic and terrestrial species. In summary, by virtue of its metabolic diversity, liver represents a potentially important organ in acid-base balance, and an interesting study tissue for interrelationships between metabolism and acid-base balance.