Uptake of Methylamine byUlva rigida: Transport of Cations and Diffusion of Free Base

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Abstract
Methylamine1 is taken up rapidly by disks cut from fronds of Ulva rigida (‘Ulva lactuca’) and can be accumulated at concentrations several hundred times greater than those in the bathing medium. At pH 8.0 (the pH of sea-water) the relationship between influx and concentration is normally linear up to 0.1–0.3 mM, followed by a second, less steep linear phase, the slope of which decreases or increases with decreasing or increasing external pH. When pH is greater than 9.0, however, net uptake soon ceases. Methylamine influx is greatly reduced at low temperature, by low concentrations of ammonia and, depending on the length of storage of the material, by darkness. Influx is also greatly reduced when disks are pretreated in solutions containing ammonia and to a much lesser extent when they are pretreated in methylamine, imidazol or nitrate. Methylamine influx lowers intracellular K+, increases Cl− and has no effect on Na+. We suggest that the first linear phase of influx versus concentration reflects the operation of an amine cation porter that is rate-limited by diffusion of CH3NH3+ through the external unstirred layer, and that the second phase is due to diffusion of CH3NH2 into the tissue.