TRANSIENT INCREASE IN CA-2+ INFLUX IN SACCHAROMYCES-CEREVISIAE IN RESPONSE TO GLUCOSE - EFFECTS OF INTRACELLULAR ACIDIFICATION AND CAMP LEVELS

Influx of Ca-45(2+) into Saccharomyces cerevisiae was measured under experimental conditions which enabled measurements of initial rate of transport across the plasma membrane, without interference by the vacuolar Ca2+ transport system. Addition of glucose or glycerol to the cells, after pre-incubation in glucose-free medium for 5 min, caused a rapid, transient increase in Ca-45(2+) influx, reaching a peak at 3-5 min after addition of substrate. Ethanol, or glycerol added with antimycin A, had no effect on Ca-45(2+) influx. We have shown previously that this increase is not mediated by an effect of the substrates on intracellular ATP levels. Changes in membrane potential accounted for only a part of the glucose-stimulated Ca-45(2+) influx. The roles of intracellular acidifcation and changes in cellular cAMP in mediating the effects of glucose on Ca-45(2+) influx were examined. After a short preincubation in glucose-free medium addition of glucose caused a decrease in the intracellular pH, [pH]i, which reached a minimum value after 3 min. A transient increase in the cellular cAMP level was also observed. Addition of glycerol also caused intracellular acidification, but ethanol or glycerol added with antimycin A had no effect on [pH]i. Artificial intracellular acidification induced by exposure to isobutyric acid or to CCCP caused a transient rise in Ca2+ influx but the extent of the increase was smaller than that caused by glucose, and the time-course was different. We conclude that intracellular acidification may be responsible for part of the glucose stimulation of Ca2+ influx. The role of the increase in cAMP level on Ca2+ influx was examined by measuring the effect of glucose and of artificial intracellular acidification on Ca2+ influx in a cyr 1 strain which lacks adenylate cyclase activity. In this strain, addition of glucose or isobutyric acid still led to a transient increase in Ca2+ transport. Therefore, we concluded that at least part of the increase in Ca2+ influx in response to glucose is cAMP-independent.