Functional analysis of the glucose transporters-1α, -6, and -13.1 expressed by zebrafish epithelial cells

The hexose supply and subsequent metabolism are crucial for the operations of the iono- and osmoregulatory mechanisms in fish, but how hexose is transported and supplied to cells of the ionoregulatory epithelia is unknown. Three zebrafish glucose transporters (zGLUTs), zGLUT1a, -13.1, and -6, were previously found to respectively be expressed by ionocytes (Na ⁺-K⁺-ATPase-rich, Na⁺-Cl^- cotransporter-expressing, and H⁺-ATPase-rich cells) and adjacent energy-depositing cells [glycogen-rich (GR) cells] in zebrafish skin and gills (32). The present study aimed to test if the transport kinetics of these three zGLUTs differ, and if the transport functional differences are of physiological relevance to the respective functions of epithelial cells. The three zGLUTs expressed by Xenopus laevis oocytes revealed different D-glucose transport kinetics; zGLUT13.1 showed the lowest Michaelis constant (K_m), whereas zGLUT6 had the highest K_m and maximal velocity. In morpholino injection experiments, translational knockdown of zGLUT1a and -13.1, respectively, impaired Cl^-/Ca²⁺ and Na⁺/Ca ²⁺ uptake, but loss-of-function of zGLUT6 did not cause a significant effect on ion uptake functions in zebrafish. Based on these results, zGLUT1a and -13.1 appear to be superior to zGLUT6 in competing for glucose under a situation of low blood glucose due to extensive energy consumption, whereas, in a high blood glucose situation, zGLUT6 is able to absorb the excess glucose for energy deposition. The timely and sufficient supply of energy to ionocytes so that they can carry out ion regulation is definitely a more important event than storing energy in GR cells, particularly when acute environmental change disturbs the ion balance in zebrafish.