Effects of gamma irradiation on the two mechanisms of Rb (K) uptake by Chlorella

Abstract

Chlorella has two mechanisms of K uptake, depending on concentration. Mechanism 1 operates at low concentration and may be responsible for net transport. Mechanism 2 is dominant at high concentration (above 1 × 10⁻³M) and may be attributed to the K/K exchange system. In either case, Rb, which has a more favourable radioisotope, substitutes for K. Both mechanisms are temperature sensitive, dependent on metabolism, and carrier-mediated. At low Rb concentration (3 × 10⁻⁵M), γ-irradiation (60 krad) reduced Rb uptake at 30 °, but increased it at 10 °. At high concentration (3 × 10⁻³M) uptake was stimulated by irradiation at both temperatures. With K-deficient cells the Rb net transport contributed to a higher degree to total Rb influx, compared with normal cells. On γ-irradiation of K-deficient cells, at low concentration a decrease of the uptake was observed at both 30 ° and 10 °, and at high concentration the stimulation was less pronounced than with normal cells at both temperatures. It is suggested that the K/K exchange system is stimulated, but the K (Rb) net transport system is injured by irradiation. The increased K efflux after irradiation is about equal to the stimulated K (Rb) influx via the exchange system. Thus it is not necessary to assume enlarged membrane permeability to explain the increase of the K efflux after γ-irradiation.