Prior exposure of isolated perifused rat islets to 20 mM glucose or 10 mM glyceraldehyde amplifies their subsequent insulin secretory response to 10 mM glucose. The involvement of phosphoinositide turnover in the induction of this "time-dependent potentiation" (TDP) was investigated. In islets in which inositol-containing phospholipids were prelabeled with myo-[2-3H]inositol, the addition of 20 mM glucose augments the efflux of 3H. This effect persists for approximately 50 min after the cessation of stimulation. Direct measurements of labeled inositol phosphate accumulation confirmed that this increase in 3H efflux is primarily the result of a persistent increase in phosphoinositide (PI) hydrolysis and not due to the slow efflux and/or degradation of performed [3H]inositol phosphates. The duration of the increase in 3H efflux parallels the duration of TDP. Mannoheptulose abolishes both the increase in 3H efflux evoked by 20 mM glucose and TDP. The omission of extracellular calcium plus 0.5 mM ethylene glycol-bis(beta-aminoethylether)-N,N,N',N'-tetraacetic acid also abolishes both of these effects of high glucose. D-Glyceraldehyde (10 mM) addition to 3H-inositol-prelabeled islets results in an acute efflux of 3H, a persistent efflux after removal of the D-glyceraldehyde from the perifusion medium, and the induction of TDP. Similar to the results obtained with high glucose, the return of 3H efflux rates to prestimulatory values is accompanied by the abolition of TDP. These results suggest that events associated with persistent stimulant-induced increases in phosphoinositide hydrolysis may participate in the induction and maintenance of TDP.

Preincubation of collagenase-isolated rat islets for 150 min with 100 U/ml purified human interleukin 1 (IL-1) altered their ability to secrete insulin. Whereas basal release rates with 4 mM glucose were comparable in control and IL-1-treated islets, both the first and second phases of release in response to 20 mM glucose were significantly reduced from IL-1-treated tissue. IL-1 pretreatment also impaired the secretory response to the combination of 100 nM cholecystokinin plus 7 mM glucose. However, the secretory response to 10 mM alpha-ketoisocaproate was comparable in control and IL-1-pretreated islets. Reducing the IL-1 exposure time to 60 min was accompanied by an augmented first phase of release to 20 mM glucose. Second phase secretion was diminished. The use of glucose measured after the perifusion was similar in control and IL-1-treated islets. Similar to other compounds that adversely impact on beta-cell viability, the inhibitory effect of IL-1 on release may presage a cytotoxic action of monokine.

The effects of sulfated cholecystokinin (CCK-8S) and glucose on insulin secretion and polyphosphoinositide (PPI) metabolism were studied in isolated rat islets. Both agonists stimulate PPI hydrolysis, inositol phosphate accumulation, 3H efflux from [3H]inositol-prelabeled tissue, and 45Ca efflux from prelabeled cells. However, the effects of CCK-8S on PPI metabolism are considerably greater than those of glucose. Furthermore, the effects of CCK-8S on PPI and Ca2+ metabolism are observed whether islets are incubated in either 2.75 or 7 mM glucose, but CCK-8S only stimulates insulin secretion (a biphasic response) when the higher glucose concentration is present. Addition of 1 microM forskolin to islets incubated in media containing 2.75 mM glucose does not influence basal insulin secretion but sensitizes the islets to the action of CCK-8S. In the presence of forskolin, CCK-8S induces a very marked first phase but no second phase of insulin secretion. We postulate that CCK-8S acts in this tissue via receptor-linked PPI hydrolysis, leading to an inositol trisphosphate-induced Ca2+ efflux. These receptor-mediated effects of CCK-8S are not altered either by the ambient glucose concentration or the cAMP content of the islets, but these two factors determine the responsiveness of the islets (in terms of insulin secretion) to a given CCK-8S signal.

The C-terminal eight-amino acid derivative of CCK, sulfated on the tyrosine residue (CCK8S), stimulated a dose-dependent biphasic pattern of insulin secretion from isolated perifused islets in the presence of 7 mM glucose. It was without any effect if glucose were absent from the medium or maintained at 4 mM. The response to CCK8S was readily reversible and dependent on the presence of extracellular calcium. While CCK8S did not increase glucose usage rates above those noted with 7 mM glucose alone, inclusion of the metabolic inhibitor 2-deoxyglucose lowered glucose usage rates to values obtained with 3-5 mM glucose and abolished the influence of CCK8S on insulin output. Removal of the metabolic inhibitor restored the secretory response. N-Acetylglucosamine (15 mM) or glyceraldehyde (2.5 mM) substituted for glucose and permitted CCK8S to evoke secretion. The nonsulfated eight-amino acid derivative of CCK, CCK8, provoked insulin secretion in the presence of 7 mM glucose, but only at 10-100 times greater levels than CCK8S. CCK4 (1 microM) did not influence insulin output in the presence of 7 mM glucose. On an equimolar basis, CCK8S was significantly more effective than gastric inhibiting polypeptide in augmenting insulin output. The results support a role for CCK8S in the regulation of insulin levels in vivo.

Prior, short-term exposure of isolated perifused islets to cholecystokinin (CCK8S) sensitizes them to subsequent glucose stimulation. This sensitization effect develops quickly and persists long after the removal of CCK8S from the perifusion medium. Continued binding of CCK8S to its receptor on the beta-cell and the increase in glucose metabolism noted with glucose stimulation are essential for the full expression of this response. This sensitization process may play an integral role in the postulated incretin effect of the peptide.