1. An insulin-producing cell line, RINm5F, derived from a rat insulinoma was studied. 2. The cellular content of immunoreactive insulin was 0.19 pg/cell, which represents approx. 1% of the insulin content of native rat beta-cells, whereas that of immunoreactive glucagon and somatostatin was five to six orders of magnitude less than that of native alpha- or delta-cells respectively. 3. RINm5F cells released 7-12% of their cellular immunoreactive-insulin content at 2.8 mM-glucose during 60 min in Krebs-Ringer bicarbonate buffer. 4. Glucose utilization was increased by raising glucose from 2.8 to 16.7 mM. There was, however, no stimulation of immunoreactive-insulin release even when glucose was increased from 2.8 to 33.4 mM. A small stimulation of release was, however, found when glucose was raised from 0 to 2.8 mM. 5. Glyceraldehyde stimulated the release of immunoreactive insulin in a dose-dependent manner. 6. At 20 mM, leucine or arginine stimulated release at 2.8 mM-glucose. 7. Raising intracellular cyclic AMP by glucagon or 3-isobutyl-1-methylxanthine stimulated release at 2.8 mM-glucose with no additional stimulation at 16.7 mM-glucose. 8. Stimulation of immunoreactive-insulin release by K+ was dose-related between 2 and 30 mM. Another depolarizing agent, ouabain, also stimulated release. 9. Adrenaline (epinephrine) inhibited both basal (2.8 mM-glucose) release and that stimulated by 30 mM-K+. 10. Raising Ca2+ from 1 to 3 mM stimulated immunoreactive-insulin release, whereas a decrease from 1 to 0.3 or to 0.1 mM-Ca2+ lowered the release. 11. These findings could reflect a relatively specific impairment in glucose handling by RINm5F cells, contrasting with the preserved response to other modulators of insulin release.

Insulin secretion was studied using adult rat intact islets, dissociated isolated islet cells, and isolated islet cells that had been allowed to reaggregate. The three preparations were maintained for 2 h in tisue culture medium at 37 C in order to allow for equilibration after the isolation procedures and to permit restoration of contacts between the reaggregated cells. The isolated cells appeared well preserved at the ultrastructural level. Evidence for intimate contacts between the reaggregated cells was demonstrated by the reappearance of gap junctions between adjacent cells. Basal insulin release (2.8 mM glucose) during 30 min was elevated in the isolated cell suspension but was restored to the level found in intact islets when isolated cells from the same population were reaggregated. The elevated basal release did not appear to be due to cell damage since there was a commensurate increase in the rate of insulin biosynthesis relative to intact islets. Although there was a marked stimulation of insulin release from the intact islets at 16.7 mM glucose, the response was smaller in the reaggregated cells and in the isolated cell suspension. All three preparations responded to elevated cAMP levels evoked by glucagon in the presence of 16.7 mM glucose. Similar results were obtained when insulin release was studied in various preparations derived from newborn rat pancreatic endocrine cells. Thus, basal insulin release was elevated in isolated cells, whereas cultures with cell contacts displayed lower basal insulin release. Taken together, the restoration of lower basal insulin release and the parallel appearance of contacts between the reaggregated cells suggest that these two phenomena are interrelated. Thus, cell contacts may be important in regulating basal insulin release. Whether such regulation is a consequence merely of cell association or of direct communication between cells remains to be established.

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