1. The effect of La3+ on amylase release and Ca2+ fluxes in mouse pancreatic fragments in vitro was studied. 2. Amylase release was increased by 0·1 m M‐La3+ and progressively inhibited by 1·0–10 m M‐La3+. Non‐stimulated and bethanecol stimulated secretion were altered in an identical manner. Inhibition of amylase release was rapid and reversible. 3. Uptake of 45Ca2+ was multiphasic with equilibrium with stable Ca2+ still not complete after 2 hr. La3+ (10 m M) limited uptake of 45Ca2+ to the extracellular space and slightly decreased total Ca2+ content. Lower concentrations of La3+ affected 45Ca2+ uptake and total Ca2+ content in a biphasic manner which paralleled effects on amylase release. 4. La3+ restricted washout of 45Ca2+ to isotope in the extracellular space and abolished the bethanecol‐stimulated increase in 45Ca2+ efflux. 5. Uptake of 45Ca2+ into intracellular space, as measured by the ‘lanthanum’ method, was not affected by bethanecol. 6. Tissue ultrastructure and Na+ and K+ content were not affected by La3+. 7. It is concluded that an influx of extracellular Ca2+ is not important for triggering of secretion and that La3+ may inhibit amylase release by acting on the release process rather than on Ca2+ influx.
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