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Abstract
Addition of 1 mM-LaCl3 to Neurospora crassa 30 s prior to the initiation of 45Ca2+uptake resulted in a dramatic inhibition of Ca2+influx to 7% of the control value. The lanthanide Gd3+and several other recognized Ca2+channel blockers (ruthenium red, nifedipine, methoxyverapamil) failed to inhibit Ca2+influx. Direct measurement of membrane potential (Δψ) with micro-electrodes revealed a La3+-induced depolarization of about 80 mV for 1 mM-La3+in the presence of 1 mM-Ca2+. The depolarization is rapid and partially reversible on La3+washout. The concentration-dependence of the depolarization can be described by a rectangular hyperbola with a K 0.5 for La3+= 0.11 mM. The La3+-induced depolarization is Ca2+-sensitive, decreasing as external Ca2+increases. The inhibitory effect of Ca2+also exhibits a hyperbolic concentration-dependence, with a K 0.5 for Ca2+= 2.5 mM for depolarization induced by 1 mM-La3+. While the flux data suggest a direct effect of La3+on Ca2+uptake, the electrophysiological data imply additional effects of La3+on the membrane. Three hypotheses were considered: (1) La3+interacts with K+channels; (2) La3+entry into cells carries a large depolarizing current; (3) La3+inhibits the electrogenic H+-pump. Hypothesis (1) was eliminated by experiments showing that depolarization occurs regardless of whether the equilibrium potential for K+is positive or negative of the resting value of Δψ. Hypotheses (2) and (3) remain possible, although La3+influx of the magnitude required to generate the observed depolarization seems very unlikely. We conclude that La3+should be deployed only with considerable caution as a blocker of plasma-membrane Ca2+influx in fungi.
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