Exogenous Ca, at concentrations up to 5 mM, induced partial germ tube formation in (= ) in media normally supporting growth in the yeast-like phase. The calmodulin inhibitors calmidazolium (R24571) and trifluoperazine (TFP), and the Caionophore, A23187, suppressed germ tube formation in germ-tube-inducing medium without affecting yeast-like growth. R24571 was the most effective inhibitor, giving almost complete suppression at 3 μM. Addition of excess Ca(up to 5 mM) did not reverse the inhibitory action of R24571 and only ∼ 10% of yeast-like cells formed germ tubes on addition of Cain the presence of 20 μM-TFP or 15 μM-A23187. Intracellular cAMP increased on incubation with R24571 and A23187, possibly as a result of inhibition of the cAMP phosphodiesterase. The exogenous supply of the calcium-binding agents methylhydroxybenzoate (MHB) and EGTA also suppressed germ tube formation under inducing conditions. These results confirm an involvement of Cain the yeast-mycelium transition of . Yeast-like cells and germ tubes of exhibited metabolism-dependent Cauptake which was reduced in the absence of glucose, or by the presence of KCN, the ATPase inhibitors '-dicyclohexylcarbodiimide (DCCD) and diethylstilboestrol (DES), and the protonophoric uncoupler DNP, indicating dependence on the electrochemical proton gradient across the plasma membrane generated by the H-ATPase. Germ tubes exhibited greater sensitivity to inhibitors of Cauptake than yeast-like cells, while Cauptake was competitively inhibited by Mg, Mnand Zn. R24571 and A23187 inhibited Cauptake by germ tubes although TFP stimulated uptake in comparison to control cells. Cauptake by both cell types conformed to Michaelis-Menten kinetics at concentrations below ∼ 200 μM but deviated strongly above this concentration. Kinetic analysis of Cauptake by yeast-like cells and germ tubes, at Caconcentrations below 100 μM, revealed that both cell types possessed Catransport systems of similar specificity, with values ranging between ∼ 15 and 25 μM, although germ tubes always exhibited greater Cauptake than yeast cells under similar experimental conditions, possibly a consequence of increased vacuolar compartmentation.


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