H⁺, Na⁺, K⁺, and Amino Acid Transport in Caterpillar and Larval Mosquito Alimentary Canal

Abstract

Two principal strategies are used to energize membranes in living organisms, a Na⁺ strategy and a voltage strategy. In the Na⁺ strategy a primary Na⁺/K⁺ ATPase imposes both Na⁺ and K⁺ concentration gradients across cell membranes with Na⁺ high outside and K⁺ high inside the cells. The Na⁺ gradient, Δ[Na⁺] is used to drive diverse secondary transporters. For example, in many animal cells Δ[Na⁺] drives Na⁺ inwardly coupled to H⁺ outwardly, mediated by Na⁺/H⁺ exchangers (NHEs). They provide the principal means by which metabolically produced acids are ejected from mammalian cells [70]. In the voltage strategy the electron transport system of prokaryotes or H⁺ V-ATPases of eukaryotes, impose a voltage gradient, ΔΨ, across biological membranes with the outside positive. The ΔΨ drives secondary (Na⁺ or K⁺)/nH⁺ antiport that is mediated by Na⁺/H⁺ antiporters (NHAs). The stoichiometry of NHEs is 1Na⁺ to 1H⁺ so they are independent of the membrane potential and are said to be electroneutral. The stoichiometry of NHAs is 1Na⁺ or K⁺ to more than 1H⁺ so they are driven both by the ion gradients and the membrane potential and are said to be electrophoretic. NHAs operate in the opposite direction from NHEs, moving nH⁺ inwardly and Na⁺ or K⁺ outwardly. ΔΨ also drives Na⁺- or K⁺-coupled nutrient amino acid uptake that is mediated by electrophoretic (Na⁺ or K⁺) amino acid symporters (NATs) [11]. In eukaryotic cells the primary sources of voltage gradients across plasma membranes have classically been considered to be K⁺, Na⁺, or other ionic diffusion potentials. Thus, K⁺ diffusion potentials dominate the resting potential and Na⁺ diffusion potentials dominate the action potential in squid axon and many other nerves. Only recently are ΔΨs generated by H⁺ V-ATPases becoming recognized as the energy source for electrophoretic transporters in animal cells [35, 65, 90]. The H⁺ V-ATPases translocate H⁺ outwardly across the cell membrane leaving their partner anion (gegenion) behind. Thus, they charge the capacitance of the membrane resulting in a transmembrane voltage, with the outside positive. The translocated H⁺s exchange with more numerous Na⁺s or K⁺s in the outside bulk solution, transforming the H⁺ electrochemical gradient to a Na⁺ or K⁺ electrochemical gradient which in turn drives Na⁺- or K⁺-coupled amino acid symport via a NAT into the cells. Membrane energization by H⁺ V-ATPases is accomplished by a five-phase system consisting of (1) the bulk solution inside the cells, (2) the inside solution/membrane interface, (3) the membrane, (4) the outside solution/membrane interface, and (5) the outside bulk solution [36, 49, 50].

The chapter is divided into five parts: (1) voltage-driven transporters and their terminology, (2) a summary of progress from the concept of “active K⁺ transport” through the discovery of portasomes and their role in the isolation of the so-called K⁺ pump to the cloning of its component H⁺ V-ATPase and K⁺/2H⁺ antiporter, (3) the cloning and localization of components of the H⁺ V-ATPase-Na⁺/H⁺ antiporter-NAT system of mosquito larval alimentary canal (AC), with emphasis on the cloning of the first, putatively electrophoretic, Na⁺/nH⁺ antiporter from Anopheles gambiae (AgNHA1), (4) attempts to characterize NHEs and NHAs heterologously in Xenopus oocytes, and (5) the incorporation of existing data into a qualitative model of the mosquito system for taking up amino acids while recycling H⁺, Na⁺, and K⁺ between lumen, cells, and hemolymph as well as generating longitudinal pH gradients in the absence of barriers along the AC of mosquito larvae.