Proinsulin synthesis in islets of Langerhans from spiny mice (Acomys cahirinus). Comparison with rats and mice.

Proinsulin synthesis in response to different concentrations of glucose has been studied in islets of Langerhans of acomys, rat and mouse. The response to increased glucose was small in acomys when compared with rat or mouse. The time course of the effect of glucose on proinsulin synthesis was also studied in sequential 15 min periods up to 60 min. With 5.5 mM glucose the rate of 3H-leucine incorporation for acomys islets were greater over the periods 15-30, 30-45 and 45-60 min after the addition of glucose to the islets than at 0-15 min. By contrast rat islets exhibited a greater glucose-induced stimulation between 15-30 and 30-45 min relative to the 0-15 min period and a further increase of the incorporation rate between 45 and 60 min. With 27.5 mM glucose acceleration of incorporation relative to the first 15 min period occurred at 15-30 min; it was then stable up to 60 min for islets both from acomys and from rat. Incorporation into total protein was much greater in acomys than in rat or mouse: thus in 2.75 mM glucose, acomys islets incorporated 5 and 19 times more 3H-leucine into total protein than did islets from rat or mouse respectively. In 27.5 mM glucose the incorporation into total protein of the islets from acomys was 4 and 13 times greater. Islet content of insulin was similar for all three species. No significant changes in ATP content were observed in response to changes in glucose concentration from 2.75 to 27.5 mM. The results demonstrate a decreased responsiveness of proinsulin synthesis to glucose in acomys and are discussed in terms of the known decreased sensitivity of insulin release in this animal.

Effects of streptozotocin in vitro on proinsulin biosynthesis, insulin release and ATP content of isolated rat islets of Langerhans.

Proinsulin synthesis, insulin release and intracellular ATP concentrations were measured in isolated rat islets of Langerhans under control conditions in vitro incubation and after treatment with several concentrations of streptozotocin for different periods of time. It was found that streptozotocin inhibited proinsulin synthesis, as well as insulin release, in a time and concentration dependent manner. The characteristics of the inhibition of these two processes were similar in general terms, but one dissimilarity was noted, i.e. after 60 min exposure to a high concentration of streptozotocin, proinsulin synthesis was inhibited more than insulin release. ATP content was reduced by high concentrations of streptozotocin, but it was found that proinsulin synthesis and insulin release could be inhibited without any effect on ATP content by a low (6.22 mM) concentration of streptozotocin. The effect of streptozotocin on proinsulin synthesis was judged to be the result of a target specificity for the B-cell rather than a specific effect on proinsulin relative to total protein synthesis.

Calcium-induced insulin release in monolayer culture of the endocrine pancreas. Studies with ionophore A23187.

The role of Ca2+ on insulin release has been studied by the use of ionophore A23187. The ionophore complexes divalent cations and permits Ca2+ entry into cells by acting as a carrier in the plasma membranes. Cultured cells obtained by enzymatic digestion of pancreases from newborn rats were studied on the 3rd day of culture. With Ca2+ in the incubation medium the ionophore induced sustained insulin release even in the absence of glucose. Optimal effects of the ionophore were observed at 3 and 10 mug per ml in the presence of 0.3 to 1.0 mM Ca-2+. Under these conditions the insulin release was greater than that caused by 16.7 mM glucose. A graded response was observed to changes in Ca-2+ concentration from 0.1 to 1.0 mM Ca-2+. Higher Ca-2+ concentrations caused a large amount of insulin to be released promptly, but the release was not sustained. Mg-2+ and Sr-2+ were not found to substitute for Ca-2+. Ba-2+ at 0.3 mM stimulated insulin release even in the absence of ionophore. Cyclic adenosine 3':5'-monophosphate was able to increase ionophore-induced insulin release. The alpha-adrenergic effect of epinephrine to inhibit insulin release was not observed in the presence of Ca-2+ and the ionophore, and a stimulatory effect of epinephrine was seen. This unusual stimulatory effect of epinephrine was blocked by propranolol indicating a beta-adrenergic mechanism for epinephrine. It is concluded that Ca-2+, which plays an essential role in the stimulus-secretion coupling, can alone initiate and cause sustained insulin release.