Addition of 1 mM-LaCl to 30 s prior to the initiation of Cauptake resulted in a dramatic inhibition of Cainflux to 7% of the control value. The lanthanide Gdand several other recognized Cachannel blockers (ruthenium red, nifedipine, methoxyverapamil) failed to inhibit Cainflux. Direct measurement of membrane potential (Δψ) with micro-electrodes revealed a La-induced depolarization of about 80 mV for 1 mM-Lain the presence of 1 mM-Ca. The depolarization is rapid and partially reversible on Lawashout. The concentration-dependence of the depolarization can be described by a rectangular hyperbola with a for La= 0.11 mM. The La-induced depolarization is Ca-sensitive, decreasing as external Caincreases. The inhibitory effect of Caalso exhibits a hyperbolic concentration-dependence, with a for Ca= 2.5 mM for depolarization induced by 1 mM-La. While the flux data suggest a direct effect of Laon Cauptake, the electrophysiological data imply additional effects of Laon the membrane. Three hypotheses were considered: (1) Lainteracts with Kchannels; (2) Laentry into cells carries a large depolarizing current; (3) Lainhibits the electrogenic H-pump. Hypothesis (1) was eliminated by experiments showing that depolarization occurs regardless of whether the equilibrium potential for Kis positive or negative of the resting value of Δψ. Hypotheses (2) and (3) remain possible, although Lainflux of the magnitude required to generate the observed depolarization seems very unlikely. We conclude that Lashould be deployed only with considerable caution as a blocker of plasma-membrane Cainflux in fungi.


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