Leslie L. Bennett

Effect of pulse administration of glucose or glucagon on insulin secretion in vitro

Abstract The isolated perfused pancreas of the rat was modified as follows from that previously used in this laboratory: Albumin buffer was substituted for rat plasma as perfusate, permitting controlled addition of agents with no interference from unknown plasma components; pulse experiments were performed in which agents were rapidly added and complete pancreatic eluates were collected at 30-sec. intervals, allowing measurement of the time relationship between stimulation and insulin response during a single passage of an agent through the pancreas. After addition of glucose, insulin secretion was prompt, occurring within 30 sec. Levels followed the rise and fall of glucose and notably there was no pancreatic memory, insulin secretion terminating 30 sec. after completion of the glucose pulse. Glucagon, containing glucagon-I 131 as an isotopic marker, directly stimulated insulin secretion in the absence of any added glucose. Insulin secretion occurred within 30 sec. after the glucagon entry in the pancreas and paralled glucagon concentration. As with the glucose-stimulated pancreas, there was no pancreatic memory, insulin secretion decreasing to basal levels within 30 sec. after the glucagon pulse was cleared. Glucagon did not affect the chromatographic integrity of simultaneously infused insulin-I 131 . Casein, an inhibitor of “insulinase”, did not duplicate the effect of glucagon. Theophylline, an inhibitor of cyclic AMP degradation, stimulated secretion in the absence of glucose. It was concluded that insulin secretion promptly responds to changes in glucose concentration and there is no significant pancreatic memory. Glucagon can also stimulate insulin secretion by a mechanism not requiring glucose or a sparing effect on insulin degradation, but possibly requiring the production of cyclic AMP.

Reversal of somatostatin inhibition of insulin secretion by calcium

Abstract Somatostatin infusion inhibits insulin secretion by perfused rat pancreases. This inhibition is reversed by elevating Ca++ from its control 4.6 meq/l to 8 meq/l or 11 meq/l.

Direct inhibition of insulin secretion by synthetic somatostatin

Summary Synthetic somatostatin has been found tosignificantly inhibit glucose-induced insulin secretion by the perfused rat pancreas. This inhibition was observed to be rapidly reversible.

IN VITRO STUDIES SUGGESTING A TWO-COMPARTMENTAL MODEL FOR INSULIN SECRETION

SUMMARY Our previous studies with the in vitro perfused pancreas of the rat have shown that insulin release in response to a constant stimulation is multiphasic. Response to glucose or tolbutamide is characterized by a rapid release of insulin followed by a rapid decrease in secretion rate. During this later phase, the pancreas can be relatively refractive to further stimulation. After 15 min, glucose but not tolbutamide produces a second rise in the rate of insulin release which increases during subsequent perfusion. When, after prolonged glucose stimulation, the pancreas is rested and then restimulated with a glucose pulse, a super-normal secretion of insulin occurs. Thus glucose produces either a refractory or hypersensitive state in the pancreas depending on the phase studied. If glucose concentration is increased slowly from 0 to 250 mg per 100 ml, the multiphasic response no longer occurs. A mathematical model was devised which closely duplicated the various multiphasic patterns of insulin release. The model assumes two compartments of stored insulin which are concentration related. The smaller labile compartment is more sensitive to agents stimulating insulin release and is the primary site of insulin secretion. The model also includes an action of agents such as glucose to increase the production of theoretical potentiator which in turn activates the provision of newly available insulin to the small compartment. Examination of the constants derived from the model for perfused rat pancreas shows: (1) the small compartment is only a few % of total extractable insulin; (2) exchange rates between the large and the small compartment are slow, thus a rapid stimulation of insulin release results in a partial depletion of the small compartment; (3) glucose and tolbutamide probably act on the same small compartment though not necessarily by the same mechanism; (4) the rate of non-specific degradation of the potentiator is slow, thus for some time after stopping glucose, addition of insulin to the small compartment continues (producing a hypersensitive pancreas). The model suggests, but does not prove, the novel hypothesis that newly provided insulin directly enters the small labile compartment and is the insulin preferentially excreted. It is also easily modified to incorporate our previons suggestion of a stimulator-induced feedback inhibition.

INSULIN SECRETION FROM THE ISOLATED PANCREAS IN ABSENCE OF INSULINOGENESIS: EFFECT OF GLUCOSE.

Summary The effect of dinitrophenol (DNP) on glucose stimulation of the release of immunologically measurable insulin was studied in the isolated perfused pancreas of the rat. In a 3-hour perfusion at 5 × 10−4 m DNP, protein synthesis, as measured by incorporation of valine-C14 into total pancreatic protein, was completely inhibited. The release of insulin by glucose, however, was unimpaired. Results indicate that glucose (or a product resulting from glucose metabolism) can stimulate insulin secretion directly and that a prior stimulation of insulinogenesis from amino acids is not required.

In vitro enhancement of insulin secretion by growth hormone

Abstract Using the isolated perfused hamster pancreas bovine growth hormone at a concentration of 7.5 ug per ml was recycled for 1 hour prior to stimulation by glucose. Under these conditions the total insulin secreted was roughly doubled compared to the controls during 1 hour of glucose stimulation.

The effects of infusing quinidine sulfate and potassium chloride, separately and combined, on conduction times of the dog heart

Abstract Normal dogs were infused with potassium chloride solution and with quinidine sulfate solution separately and in combination. Concentrations of serum potassium and of plasma quinidine were determined. Effects upon P waves, together with intra-atrial, atrioventricular, and intraventricular conduction times, were determined. Both quinidine and hyperkalemia produce prolongation of all three parameters of conduction velocity within the heart. These effects are neither additive nor synergistic.

Preparation of a Clinically Active Adrenocorticotropic Hormone (ACTH) in Good Yield from Sheep Pituitary Glands.

Summary A method is described for the preparation in high yield, from whole sheep pituitary glands, of purified ACTH, substantially free from growth, gonadotropic, thyrotropic, antidiuretic and pressor activities. Clinical and laboratory evidence indicates that ACTH prepared by this method is therapeutically and metabolically effective in patients with rheumatoid arthritis.