ACTIVATION OF CA-2+ INFLUX BY METABOLIC SUBSTRATES IN SACCHAROMYCES-CEREVISIAE - ROLE OF MEMBRANE-POTENTIAL AND CELLULAR ATP LEVELS

Influx of Ca2+ into cells of Saccharomyces cerevisiae was measured under non-steady-state conditions, which enable measurements of the initial rate of transport across plasma membranes without interference by the vacuolar Ca2+ transport system. Removal of glucose from the incubation medium led to inactivation of Ca2+ influx within 5 min. Readdition of glucose led to a transient increase in the rate of Ca2+ transport, reaching a peak after 3-5 min. A second increase was observed 60-80 min later. To examine whether the first transient activation of Ca2+ influx by glucose was mediated by membrane hyperpolarization, influx of 45Ca2+ was 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 Ca2+ influx were plotted against values of ΔΨ. Two different slopes were obtained, depending upon whether the metabolic substrate was present or absent. Ca2+ influx in the presence of the metabolic substrates was always higher than expected by their effect on ΔΨ. Glycerol plus antimycin A did not affect Ca2+ influx. It was concluded that metabolized substrates activate Ca2+ influx not only by effects on ΔΨ but also by additional mechanism(s). Since no simple correlation between Ca2+ influx and intracellular ATP levels was observed, it was concluded that ATP levels do not affect the initial rates of Ca2+ transport across the plasma membrane of S. cerevisiae.