Summary: The time-course of Ca influx into yeast cells was measured under non-steady-state conditions obtained by preincubating the cells in a Ca-free medium containing glucose and buffer. Two components were distinguished: a saturable component which reached a steady-state after about 40 s of Ca uptake and a linear increase in cellular Ca starting after 60-90s. Using differential extraction methods it was determined that after 20s of uptake, Ca was localized in the cytoplasmic pool and in bound form with no Ca in the vacuole. After 3 min most of the cellular Ca was concentrated in the vacuole and in bound form. The initial rate of Ca uptake under non-steady-state conditions thus measured Ca transport across the plasma membrane without interference by vacuolar uptake. The effect of membrane potential (δψ) on this transport was investigated in cells depleted of ATP. A high δψ was produced by preincubating the cells with trifluoperazine (TFP) and subsequently washing the cells free from TFP. Substantial Ca influx was measured in the absence of metabolic energy in cells with a high δψ. Below a threshold value of −69·5 mV the logarithms of the initial rate of Ca influx and of the steady-state level of the first component were linear with respect to δψ. It is suggested that Ca influx across the plasma membrane is mediated by channels which open when δψ is below a threshold value. The results indicated that Ca influx across the plasma membrane was driven electrophoretically by δψ.


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