Abstract
Abstract— Saxitoxin and tetrodotoxin at low concentrations (10−7‐10−8 M) exerted similar inhibitory effects on the increase in lactate production and the redistrjbution of Na+ and K+ that normally accompany electrical stimulation of rat cerebral cortical slices. In contrast, the toxins exerted dissimilar effects on the production of lactate in response to low concentrations of Ca2+ in the medium. Inhibition by tetrodotoxin occurred at a higher concentration of Ca2+ and was significantly greater than that produced by saxitoxin at concentrations of Ca2+ below 0.75 mM. These differences were not related to differential effects on the redistribution of Na+ and K+ under such conditions. The toxins had different effects on Ca2+ influx. Tetrodotoxin, but not saxitoxin, inhibited the influx of Ca2+ in the absence of electrical stimulation. The influx of Ca2+ increased when electrical pulses were applied and tetrodotoxin inhibited this increase, whereas saxitoxin potentiated influx of Ca2+ during stimulation. Our results suggest that metabolic responses to conditions that increase excitability are not governed solely by changes in the distribution of Na+ and K+. The differential effects of the toxins on Ca2+ fluxes suggest that one site of Ca2+ entry during electrical stimulation may be functionally independent of Na+ entry.
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