| Abstract: | 1. Calcium movements in the isolated rabbit pancreas and in rabbit pancreas fragments have been studied with the aid of 4 5 Ca2+. 2. Addition of 4 5 Ca2+ to the incubation medium of the isolated rabbit pancreas results in an immediate appearance of isotope in the secreted fluid reaching a constant specific activity in 30 min. The absolute activity in the secreted fluid is 30-40% of that in the incubation medium. 3. Addition of 10(-5) M carbachol after 2 h preincubation with 4 5 Ca2+ results in enzyme secretion accompanied by calcium release. There is also an increase in 4 5 Ca2+ secretion, but this is maximal 10 min after the protein and total calcium peaks. 4. Partial removal of 4 5 Ca2+ from the bathing medium, before stimulation, reduces the increase in 4 5 Ca2+ secretion nearly proportionally. 5. 3H]Mannitol, added to the bathing medium, appears in the secreted fluid and behaves upon carbachol stimulation similarly to 4 5 Ca2+. 6. Upon repeated stimulation with 10(-5) M acetylcholine, a 4 5 Ca2+ peak appears, even in virtual absence of enzyme secretion. In this case the peak coincides with a small total calcium peak. 7. Efflux studies of rabbit pancreas fragments, preloaded with 4 5 Ca2+, show a carbachol-stimulated 4 5 Ca2+ efflux in addition to a release of amylase. 8. These studies indicate that there are three calcium movements in rabbit pancreas which can all be influenced by cholinergic agents: (a) an extracellular route for calcium and other small molecules and ions; (b) a calcium release across the apical membrane along with the enzymes, originating from a pool which does not freely exchange with 4 5 Ca2+ in the bath; (c) a calcium flux across the serosal membrane, which involves calcium exchanging freely with 4 5 Ca2+ from the bath. The third flux is thought to result from an increase in cytoplasmic calcium, which may be involved in the stimulus-secretion coupling of pancreatic enzyme secretion. |
