The relationship between calcium exchange and enzyme secretion in the isolated rat pancreas

Abstract

1. The role of extracellular and intracellular Ca 2+ in pancreatic enzyme secretion has been assessed by correlating the exchange of 45 Ca with amylase secretion in the isolated uncinate pancreas of baby rats. 2. The rate coefficient of 45 Ca efflux from pre‐loaded glands declined continually (indicating that 45 Ca is retained in several different pools) and probably reflects changes in the concentration of cytoplasmic free 45 Ca, which is determined by the rate at which 45 Ca is released from intracellular organelles into the cytoplasm. 3. The rate coefficient of 45 Ca release was not influenced by extracellular Ca 2+ or Mg 2+ concentrations. 4. Cholecystokinin‐pancreozymin (CCK‐PZ) and acetylcholine accelerated the release of both 45 Ca and amylase in a dose‐dependent fashion, even when extracellular Ca 2+ was reduced to 0·1 m M, but did not affect the initial rate of 45 Ca uptake by the tissue. 5. In Ca 2+‐free media (containing 0·5 m M‐EGTA) basal amylase secretion slowly declined and stimulated secretion was virtually abolished, but the accelerated release of 45 Ca was maintained. 6. These observations indicate that natural stimuli of pancreatic enzyme secretion alter 45 Ca distribution in the cell by a process which is independent of extracellular Ca 2+ and which is associated with amylase secretion provided that the plasma membrane has not been depleted of Ca 2+. 7. Secretin, glucagon and insulin did not influence 45 Ca 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 45 Ca and amylase, but only in the presence of extracellular Ca 2+. Li+‐substitution also increased 45 Ca uptake. Thus, under special conditions, secretion may be stimulated when increased amounts of Ca 2+ are made available from extracellular sources. 9. Hyperosmolarity (known to increase 45 Ca release in muscle) also accelerated 45 Ca release and amylase secretion. 10. 2,4‐Dinitrophenol markedly accelerated 45 Ca efflux but did not stimulate amylase secretion, indicating that a rise in cytoplasmic Ca 2+ will not initiate secretion if energy metabolism is impaired. 11. CCK‐PZ slightly increased the rate coefficient of 42 K 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 Ca 2+ from intracellular stores (probably endoplasmic reticulum), leading to a rise in Ca 2+ 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 Ca 2+.