The effects of manganese, cobalt and calcium on amylase secretion and calcium homeostasis in rat pancreas

Abstract

SUMMARY1. 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 of45Ca 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 MCo2+, 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 x 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. 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 25 x 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 ...