It has been reported that addition of glucose to cells of Saccharomyces cerevisiae grown on a sugar-free
medium causes a peak of intracellular CAMP levels. Also, it has been proposed that this effect might be
mediated by plasma membrane depolarization. However, here, we observed a hyperpolarizing effect of
glucose in S. cerevisiae and, in addition, no change in CAMP levels when depolarization was induced by
valinomycin in the presence of K+. In contrast, treatments that induced a rapid intracellular acidification
such as addition of the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone at pH 5.5 but
not at pH 8.0, extracellular pH shift from 8.5 to 3.5, and glucose itself, also increased the cyclic nucleotide.
Thus, our data strongly support the hypothesis that intracellular acidification mediates the effect of glucose
on CAMP levels.