The Tegument of the Human Parasitic Worm Schistosoma mansoni as an Excretory Organ: The Surface Aquaporin SmAQP Is a Lactate Transporter
Figure 4
A. SmAQP, when expressed in X. laevis oocytes, can transport several solutes. SmAQP transports lactate, in addition to mannitol, fructose and L-alanine but not glucose. The substrates were added to a final concentration of 1 mM and accumulation of radiolabeled substance traces on the cells was measured after 1.5 minute (n = 16–18 oocytes). B. The transport of lactate is pH dependent. Solute diffusion coefficient permeability for lactate increased in a pH dependent manner (6.4±0.9×10−6 cm/s pH 7.4 and 20.1±3.5×10−6 cm/s pH 6.3), whereas for mannitol it was unchanged (1.5±0.2×10−6 cm/s pH/.4 and 1.7±0.2×10−6 cm/s pH 6.3) (n = 15–17). C. The transport of lactate is saturable. SmAQP has low affinity for lactate and follows Michaelis-Menten kinetics (n = 14–24). D. Lactate moves through a pore. The energy of activation (Ea) was estimated by plotting the temperature dependent transport (at 4, 15 and 25°C) as an Arrhenius plot. The slope of the linear regression was −1.57±0.15 K (n = 9–15). In all cases, values obtained from control oocytes, not-expressing SmAQP have been subtracted to generate the data shown. Data represent the mean ± SE generated in at least 2 independent experiments.