The effects of manganese, cobalt and calcium on amylase secretion and calcium homeostasis in rat pancreas
Authors: Argent B. E. , Case R. M. , Hirst Frances C. 
Published: December 2014

1. Mn2+ evoked an atropine‐resistant secretion of amylase from the isolated pancreas of the young rat. The lowest effective concentration of Mn2+ was 10−3
m. The response to 10−2
m‐Mn2+ was biphasic, an initial peak being followed by a slow sustained rise in amylase output. The maximal effect of 10−2
m‐Mn2+ was to double the basal rate of amylase secretion after 70 min incubation.

2. Co2+ (10−2
m) also stimulated amylase secretion. The maximal rate, about three times the basal value, was attained after 20 min incubation. Atropine partially inhibited this effect.

3. Ca2+ (10−2
m) evoked an atropine‐resistant amylase secretion similar in both magnitude and time course to the sustained phase observed with 10−2
m‐Mn2+.

4. Mn2+ (10−4‐10−2
m) also increased the rate of 45Ca efflux from the gland. Maximal efflux rates were attained after 30 min incubation and thereafter declined to basal values. A small increase was also observed with 10−2
m‐Co2+, but not with 10−2
m‐Ca2+. The effect of Co2+ was almost completely abolished by atropine.

5. Reducing the extracellular Ca2+ concentration from 2·5 × 10−3 to 10−5
m did not reduce amylase secretion in response to 10−2
m‐Mn2+, but secretion was abolished in a Ca2+‐free medium containing EGTA. The increase in 45Ca efflux rate evoked by Mn2+ was inversely related to the extracellular Ca2+ concentration.

6. Mn2+ (10−2
m) increased the concentration of cyclic 3′,5′‐guanosine monophosphate (cyclic GMP) within the pancreas. Also, Mn2+ accumulated within the cellular pool of the gland. The time course of both these effects was similar to the time course of 45Ca efflux.

7. Mn2+ displaced Ca2+ bound to isolated pancreatic microsomal membranes. The cation‐binding sites on these membranes probably have a higher affinity for Mn2+ than Ca2+.

8. We conclude that Mn2+ stimulates enzyme secretion by displacing membrane‐bound Ca2+, the resulting increase in cytosolic Ca2+ concentration activating the secretory mechanism.

9. Mn2+ partially inhibited amylase secretion stimulated by optimal doses of either acetylcholine (ACh) or caerulein. Maximal inhibition (about 60%) occurred with 10−3
m‐Mn2+ (i.e. the lowest concentration required to stimulate secretion in the absence of secretagogues). Decreasing the extracellular Ca2+ concentration reduced the inhibitory effect of Mn2+.

10. When glands were exposed to ACh and Mn2+ simultaneously, the time required for inhibitory effects to develop was inversely related to the dose of ACh and the concentration of Mn2+.

11. Mn2+ did not alter the acceleration of 45Ca efflux evoked by ACh or by caerulein in a medium containing 2·5 × 10−3
m‐Ca2+. However, under conditions of Ca2+ deprivation ACh‐stimulated 45Ca efflux was greatly enhanced.

12. Mn2+ reduced the total amount of Ca2+ accumulated into the cellular pool of the pancreas after 60 min incubation, but had no effect on the initial, rapid phase of Ca2+ uptake.

13. The effects of Mn2+ on the relationship between ACh dose, amylase release and the extracellular Ca2+ concentration suggest that the inhibitory actions of Mn2+ cannot be explained by a simple, competitive interaction with the stimulant or with extracellular Ca2+. However, the time course of inhibition is consistent with a requirement for Mn2+ to accumulate within the acinar cells.

14. Mn2+ partially inhibited amylase secretion stimulated by hyperosmolarity and also increased the 45Ca efflux rate under these conditions.

15. Our results are not consistent with Mn2+ exerting its inhibitory effect on secretagogue‐stimulated enzyme secretion solely by blocking Ca2+ influx from the extracellular space. We conclude that inhibition probably depends on the ability of Mn2+ to displace Ca2+ from binding sites involved in secretion, presumably coupled with a reduced ability of Mn2+ to replace Ca2+ in the secretory process.