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1. The role of extracellular and intracellular Ca²⁺ in pancreatic enzyme secretion has been assessed by correlating the exchange of ⁴⁵Ca with amylase secretion in the isolated uncinate pancreas of baby rats.

2. The rate coefficient of ⁴⁵Ca efflux from pre-loaded glands declined continually (indicating that ⁴⁵Ca is retained in several different pools) and probably reflects changes in the concentration of cytoplasmic free ⁴⁵Ca, which is determined by the rate at which ⁴⁵Ca is released from intracellular organelles into the cytoplasm.

3. The rate coefficient of ⁴⁵Ca release was not influenced by extracellular Ca²⁺ or Mg²⁺ concentrations.

4. Cholecystokinin-pancreozymin (CCK-PZ) and acetylcholine accelerated the release of both ⁴⁵Ca and amylase in a dose-dependent fashion, even when extracellular Ca²⁺ was reduced to 0·1 mM, but did not affect the initial rate of ⁴⁵Ca uptake by the tissue.

5. In Ca²⁺-free media (containing 0·5 mM-EGTA) basal amylase secretion slowly declined and stimulated secretion was virtually abolished, but the accelerated release of ⁴⁵Ca was maintained.

6. These observations indicate that natural stimuli of pancreatic enzyme secretion alter ⁴⁵Ca distribution in the cell by a process which is independent of extracellular Ca²⁺ and which is associated with amylase secretion provided that the plasma membrane has not been depleted of Ca²⁺.

7. Secretin, glucagon and insulin did not influence ⁴⁵Ca release. Secretin slightly increased amylase secretion, but this may have been a washout effect.

8. Replacement of extracellular Na⁺ by Li⁺ increased the release of ⁴⁵Ca and amylase, but only in the presence of extracellular Ca²⁺. Li⁺-substitution also increased ⁴⁵Ca uptake. Thus, under special conditions, secretion may be stimulated when increased amounts of Ca²⁺ are made available from extracellular sources.

9. Hyperosmolarity (known to increase ⁴⁵Ca release in muscle) also accelerated ⁴⁵Ca release and amylase secretion.

10. 2,4-Dinitrophenol markedly accelerated ⁴⁵Ca efflux but did not stimulate amylase secretion, indicating that a rise in cytoplasmic Ca²⁺ will not initiate secretion if energy metabolism is impaired.

11. CCK-PZ slightly increased the rate coefficient of ⁴²K release, indicating a changed membrane permeability.

12. The stimulatory effects of CCK-PZ and acetylcholine were suppressed during Na⁺-substitution by Li⁺, suggesting that the Na⁺ concentration gradient across the membrane is important in secretion.

13. It is concluded that the primary action of CCK-PZ and acetylcholine may be to increase the influx of Na⁺ into the cell by changing membrane permeability. This in turn is responsible for the release of Ca²⁺ from intracellular stores (probably endoplasmic reticulum), leading to a rise in Ca²⁺ concentration close to the structures involved in enzyme secretion. Secretion then follows provided that ATP is available and the plasma membrane is not depleted of Ca²⁺.

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