SUMMARY: Influx of Cainto cells of was measured under non-steady-state conditions, which enable measurements of the initial rate of transport across plasma membranes without interference by the vacuolar Catransport system. Removal of glucose from the incubation medium led to inactivation of Cainflux within 5 min. Readdition of glucose led to a transient increase in the rate of Catransport, reaching a peak after 3-5 min. A second increase was observed 60--80 min later. To examine whether the first transient activation of Cainflux by glucose was mediated by membrane hyperpolarization, influx of Cawas measured in the presence and absence of metabolic substrates (glucose, glycerol, and glucose plus antimycin A) in cells hyperpolarized to different values of membrane potential (Δψ). Logarithms of the rate of Cainflux were plotted against values of Δψ. Two different slopes were obtained, depending upon whether the metabolic substrate was present or absent. Cainflux in the presence of the metabolic substrates was always higher than expected by their effect on Δψ. Glycerol plus antimycin A did not affect Cainflux. It was concluded that metabolized substrates activate Cainflux not only by effects on Δψ but also by additional mechanism(s). Since no simple correlation between Cainflux and intracellular ATP levels was observed, it was concluded that ATP levels do not affect the initial rates of Catransport across the plasma membrane of .


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