Methylamine influx into Hydrodictyon has been measured with [l4C]methylamine. The influx increases with rising external pH up to about pH 8. Between pH 8 and 9 influx remains quite constant, with a further increase above pH 9. Influx is light-dependent, temperature-sensitive, and is decreased by NH4+. During short-term influx (less than 4 h) metabolism of methylamine appears negligible. Prolonged influx results in CH3NH3+ accumulation, and efflux of K+, Na+, and H+. There is no effect on Cl− influx. Methylamine decreases the membrane electric p.d. by 60–120 mV at external concentrations of 0·2–1·0 mM. The results indicate that, below pH 9, methylamine enters the cell almost entirely as CH3NH3+. It is suggested that a passive electrogenic uniporter is involved, and that by analogy uniport of NH4+ may also be expected in Hydrodictyon. The results are discussed in relation to the evidence for uptake of CH3NH3+ and NH4+ by other plant cells.
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SMITH et al. (1978) studied this question.