Aquaporin-9 is the main pathway in mouse liver import of glycerol

Background: Glycerol released from adipocytes flows to the liver where its utilization in supplying hepatocyte gluconeogenesis is rate-limited by the permeation step. An aquaglyceroporin, AQP9, has been often linked to liver uptake of glycerol.However, the truthfulness of this postulation and the potential existence of additional pathways of glycerol import by hepatocytes have never been verified directly. Here, we define the molecular identity and extent of liver glycerol transport andevaluate the correlationbetweenhepatic AQP9 level and glycerol permeability inAQP9+/+ wild type mice in different nutritional states and circulating insulin levels. Materials and methods: In male C57BL/6J wild type or AQP9 knockout mice, were used. Levels of AQP9 mRNA and protein were evaluated by RT-PCR and immunoblotting/immunocytochemistry, respectively. Glycerol permeability (Pgly), Arrhenius activation energy (Ea) and inhibition of glycerol transport were assessed by stopped flow light scattering. Results: Facilitated diffusion of glycerol into hepatocyteswas indicated by the lowArrhenius activationenergy (3.5 kcalper mole) and strong inhibition exerted by phloretin, anAQP9 blocker.While fasting markedly increased hepatic AQP9, a straight parallelism was seen both in quantitative and time-space terms between glycerol permeability and AQP9 protein in AQP9+/+ mice kept in fed or fasted/refed states. The highest glycerol permeability, at 18-h fasting, coincided with the highest percent of phloretin inhibition (63%). Besides being markedly lower than inAQP9+/+ mice the liver Pgly of the AQP9 / mice did not increase during fasting. Conclusions: These results prove experimentally AQP9 plays a major functional significance rolefor AQP9 in maximizing liver glycerol import during states requiring increased glucose production. Refining the understanding of liver AQP9 in metabolic and energy balance may reveal helpfulunravel novel for therapeutic purposesstrategies in several metabolic disorders.