Insulin and the pancreas

Multiphasic action of glucose and alpha-ketoisocaproic acid on the cytosolic pH of pancreatic beta-cells. Evidence for an acidification pathway linked to the stimulation of Ca2+ influx

Glucose stimulation raises the pHi of pancreatic beta-cells, but the underlying mechanisms are not well understood. We have now investigated the acute effects of metabolizable (glucose and the mitochondrial substrate alpha-ketoisocaproic acid, KIC) and nonmetabolizable (high K+ and the K-ATP channel blocker tolbutamide) insulin secretagogues on the pHi of pancreatic beta-cells isolated from normal mice, as assessed by BCECF fluorescence from single cells or islets in the presence of external bicarbonate. The typical acute effect of glucose (22-30 mM) on the pHi was a fast alkalinization of approximately 0.11 unit, followed by a slower acidification. The relative expression of the alkalinizing and acidifying components was variable, with some cells and islets displaying a predominant alkalinization, others a predominant acidification, and others yet a mixed combination of the two. The initial alkalinization preceded the [Ca2+]i rise associated with the activation of voltage-sensitive Ca2+ channels. There was a significant overlap between the glucose-evoked [Ca2+]i rise and the development of the secondary acidification. Depolarization with 30 mM K+ and tolbutamide evoked pronounced [Ca2+]i rises and concomitant cytosolic acidifications. Blocking glucose-induced Ca2+ influx (with 0 Ca2+, nifedipine, or the K-ATP channel agonist diazoxide) suppressed the secondary acidification while having variable effects (potentiation or slight attenuation) on the initial alkalinization. KIC exerted glucose-like effects on the pHi and [Ca2+]i, but the amplitude of the initial alkalinization was about twice as large for KIC relative to glucose. It is concluded that the acute effect of glucose on the pHi of pancreatic beta-cells is biphasic. While the initial cytosolic alkalinization is an immediate consequence of the activation of H+-consuming metabolic steps in the mitochondria, the secondary acidification appears to originate from enhanced Ca2+ turnover in the cytoplasm. The degree of coupling between glucose metabolism and Ca2+ influx as well as the relative efficacies of these processes determines whether the acute pHi response of a beta-cell (or of a tightly coupled multicellular system such as an islet of Langerhans) is predominantly an alkalinization, an acidification, or a mixed proportion of the two.

Effect of diabetes and time after in vivo insulin administration on ketogenesis and gluconeogenesis in isolated rat hepatocytes

1. Diabetic rats had elevated rates of gluconeogenesis and ketone body production. 2. Four hours after insulin administration the rate of gluconeogenesis returned to normal and the rate of ketogenesis decreased with certain substrates, but did not return to normal. 3. Eight hours after insulin treatment ketogenesis from all substrates was significantly reduced, but still significantly higher than normal.

Nutrient induced insulin release from an insulinoma derived B-cell line

The present study describes the effect of nutrient secretagogues upon insulin release from an insulinoma derived B-cell line RINm5F. Glucose failed to stimulate insulin release, it did however increase the rate of glucose oxidation by some 16 fold. Glyceraldehyde stimulated insulin release in a concentration dependent manner, maximum stimulation occurring at 20 mmol/l. The response to glyceraldehyde was suppressed by calcium channel blockade and calmodulin inhibition. Insulin release was also stimulated by the amino acids leucine and arginine, maximum stimulation for both amino acids occurring at 15 mmol/l. The present study confirms the usefulness of this cell line as a model for the study of insulin secretion.

Centrally mediated hypoglycemic effect of insulin: apparent involvement of specific insulin receptors

We have studied the involvement of central nervous system (CNS) insulin receptors in mediating the central hypoglycemic effect of insulin by using insulin derivatives modified at regions of the hormone necessary for receptor reactivity and peripheral bioactivity. Acetylation or succinylation of the 3 free amino groups of insulin at positions A1, B1 and B29 resulted in a corresponding decrease in lipogenic activity in isolated rat adipocytes, with concentrations of hormone required to produce half the maximal effect (ED50) being 0.15 ng/ml, 3 ng/ml and 50 ng/ml for native insulin, acetyl3 insulin and succinyl3 insulin, respectively. Moreover, the modified insulins exhibited diminished hypoglycemic effect following central administration in mice, with the doses needed for suppression of plasma glucose to 50% of basal levels being 1 microgram, 10 micrograms and 25 micrograms for native insulin, acetyl3 insulin and succinyl3 insulin, respectively. Because binding of insulin derivatives to CNS receptors can be predicted from their peripheral bioactivity, the present finding of parallel decrements in lipogenic activity in vitro and central hypoglycemic effect in vivo, following modification of insulin at regions implicated in receptor activation, is consistent with the view that insulin exerts its central effect on plasma glucose by interacting with specific CNS receptor sites which are closely related to the peripheral insulin receptors.

Selective effects of secretagogues on insulin secretion: a mathematical model

A mathematical model of the secretion of insulin from pancreatic islets of Langerhans is proposed. Previously proposed mathematical models of insulin secretion have dealt solely with glucose-stimulated release, and not with the more complex patterns of secretion (such as "off-responses") in response to other secretagogues or combinations of secretagogues. We conclude that an off-response is consistent with a compartmental model; and the facilitating effect of a constant concentration of glucose on leucine-stimulated insulin secretion is consistent with a selective effect of glucose on a single compartment of the model.

Inhibition of parathyroid hormone release from dispersed bovine cells by ouabain

Ouabain, at concentrations of 3 X 10(-4)-10(-3) M, inhibited secretion of PTH approximately 50% in a freshly dispersed bovine parathyroid cell preparation. This inhibition was found with both unstimulated and secretagogue-stimulated PTH release. Reductions in PTH secretion were found at all concentrations of Ca++ tested between 0.3 mM and 2.0 mM and in the presence of the divalent cation chelators EDTA and EGTA, indicating that extracellular Ca++ is not an absolute requirement for the inhibition. The ouabain inhibition did not appear to be mediated through changes in either adenylate cyclase activity or total cellular cAMP, implying a locus distal to the generation of this cyclic nucleotide. The data suggest that transmembrane potential and/or distribution of monovalent cations across the plasma membrane is important in the maintenance of PTH secretion. The mechanisms involved in this control do not appear to involve extracellular Ca++ directly.

Insulin secretion by a transplantable rat islet cell tumour

Investigation of the subcellular and molecular components of insulin secretion has been made difficult by the small quantities of material available. The recent development of a transplantable rat islet cell tumour of high insulin content and state of differentiation suggested a system more amenable to analysis. To validate the tumour as a model of secretion we have studied its release of insulin. In acute experiments in vitro immunoreactive insulin release was increased by leucine, glucagon, theophylline and dibutyryl cyclic AMP, though not by glucose. Leucine (20 mmol/l) plus theophylline (5 mmol/l) caused an abrupt, sustained and rapidly reversible stimulation of two- to fivefold. The response was inhibited by antagonists of cellular oxidative phosphorylation (cyanide, 2,4-dinitrophenol, antimycin A), calcium flux (EGTA, verapamil, Mg2+), calmodulin (trifluoperazine), microtubules (vinblastine, colchicine) and by adrenaline and somatostatin. These findings suggest that the tumour secretes insulin by an exocytotic mechanism similar to that of normal islet tissue.

Studies on the mechanism of renin release from rat kidney slices: calcium, sodium and metabolic inhibition

1. The coincident release of renin and lactic dehydrogenase (LDH) from rat renal cortical tissue slices was studied during calcium depletion, metabolic inhibition, and with the addition of ouabain (1 mM) to the incubation medium.2. The results indicate that although LDH accumulated in the medium during incubation, the pattern was dissimilar to that of renin. Ouabain significantly inhibited renin release in calcium containing medium, but had no effect on LDH release. Renin release was potentiated in calcium ;free' media, while calcium depletion reduced the release of LDH.3. The addition of potassium cyanide (2 mM) significantly inhibited the release of renin from these tissue slices. Cyanide was ineffective, however, when administered in calcium ;free' medium.4. At reduced incubation temperatures (5 degrees C) the release of both renin and LDH were significantly reduced.5. Medium sodium depletion caused a significant inhibition of renin release. The simultaneous removal of calcium from the medium did not restore renin release to control levels.6. These results are not consistent with the hypothesis that spontaneous renin release during calcium depletion and metabolic inhibition is a result of cell enlargement and increased membrane permeability. On the other hand, the in vitro release of renin during these experiments appeared to be inversely related to the intracellular concentration of calcium.

Effect of synthetic C-terminal fragments of hGH on glucose oxidation by isolated islets

The effect of polypeptides containing the human growth hormone sequence 177--191 on the glucose metabolism of isolated islets of Langerhans has been investigated. It has been found that such peptides accelerate the incorporation of hydrogen at carbon atom 5 into water while accelerating the flux (1-14C oxidation) through the pentose phosphate shunt and inhibiting the oxidation of [6-14C]glucose. The latter inhibition was found not to be due to inhibition of the pyruvate dehydrogenase complex, and it was further found that the previously demonstrated potentiation of glucose-induced insulin release by the peptides was independent of energy provision because none of the glyceraldehyde, pyruvate, or leucine could substitute for glucose. In view of the fact that the insulinogogue effect of this peptide is to potentiate the glucose stimulatory effect on islets, these phenomena may well be unrelated.

Preparation and characterization of plasma membrane-enriched fractions from rat pancreatic islets

Methods have been developed for the isolation on a semi-micro scale of a plasma membrane-enriched fraction from rat islets of Langerhans. An important feature of these experiments is the use of 125I-labeled wheat germ agglutinin as a specific probe for plasma membrane-containing fractions. The partly purified plasma membrane fraction had a density in sucrose of about 1.10 and was enriched in the activities of 5'-nucleotidase, alkaline phosphatase, sodium-potassium, and magnesium-dependent ATPase and adenylate cyclase. It contained only very low levels of acid phosphatase, cytochrome c oxidase, insulin, and RNA. Further purification was hampered by the relatively small amounts of fresh plasma membrane material that could be obtained from 16-24 rats in each experiment. When islets were prelabeled with radioactive fucose, the plasma membrane-enriched fraction contained radioactivity at a four- to fivefold higher specific acivity than the whole islet homogenate. Sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis of plasma membrane-enriched fractions pooled from several experiments revealed a distinctive pattern of protein bands as compared with other less pure fractions. With respect to rapidity, apparent specificity, and easy reversibility of the labeling of the plasma membrane fraction, 125I-wheat germ agglutinin provides a highly useful tool for the detection of microgram quantities of plasma membrane components which should be applicable to many other systems as well.

A threshold distribution hypothesis for packet storage of insulin and its mathematical modeling

Phases of insulin release were studied in the perfused pancreas during a variety of glucose stimulation patterns. Patterns included staircase stimulations, constant prolonged single steps, restimulations, and ramp functions. Except at low concentrations, prolonged single steps of glucose elicited early spikes of insulin and a slowly rising second phase. Total insulin in the initial spikes increased with higher glucose concentrations. However, the time-related pattern of these spikes was similar in all cases; ratios of initial secretion rate to total insulin released were constant. Total insulin released in this early phase approximated a sigmoidal function of glucose concentration; mathematical differentiation of this function gave a skewed bell-shaped distribution curve. Staircase stimulations caused insulin to be released as a series of transient spikes which did not correlate with the increment of glucose but rather to the available insulin for a given glucose concentration minus that released in previous steps. The sum of total insulin released as spikes in a staircase series leading to a given glucose concentration was the same as when that concentration was used as a single step. Interrupted prolonged glucose infusions indicated the second phase of insulin release could prime the pancreas and that the first and second phases were interrelated. When glucose was perfused as ramp functions of slow, increasing, concentration, phasic response disappeared.A previous two-compartmental model was expanded to include a threshold or sensitivity distribution hypothesis. This hypothesis proposes that labile insulin is not stored in a homogeneous form but as packets with a bell-shaped distribution of thresholds to glucose. These packets respond quickly when their threshold levels to glucose are reached or exceeded. Data from single step stimulations were utilized for constructing a mathematical model which simulated satisfactorily the various stimulation patterns.

Studies on the secretion of newly synthesized proinsulin and insulin from isolated rat islets of Langerhans

Islets of Langerhans isolated from rat pancreas were incubated at 37 degrees C(95% O(2)/5% CO(2)) in buffered medium containing 1.0 mg/ml glucose and leucine (3)H for 1 hr (1st hr), washed, and incubated for an additional hr (2nd hr) in low glucose medium (0.5-1.0 mg/ml) containing unlabeled leucine. A portion of the islets was then extracted with acid-ethanol and the remainder were transferred to medium containing 3.0 mg/ml glucose and incubated for 2 hr (3rd and 4th hr) at 37 degrees C. The medium was exchanged at 30-min intervals and portions of the islets were extracted at the 3rd and 4th hr. The total amounts and specific activities of the proinsulin and insulin in the islet extracts and medium samples were determined after fractionation on Biogel P-30 columns in 3 M acetic acid. Maximal release of newly synthesized insulin occurred between the 3rd and 4th hr of incubation, confirming the results of Howell and Taylor (Biochem. J.102: 922. 1967). The high glucose medium increased the secretion of insulin approximately three to fourfold. The ratio of the specific activities of the insulin in the medium to that in the islets was about 1/1 during incubation in low glucose, but it increased to 2.5/1 during incubation with high glucose. The peak occurred at the 3rd hr, i.e., 1 hr after exposure to high glucose. The ratio of labeled proinsulin to insulin was slightly lower in the medium than in the islets. Addition of sufficient cycloheximide after the 1st hr to inhibit protein synthesis did not inhibit these responses. The specific activity of the proinsulin in the medium was about the same as that in the islets, and both were about 10-fold higher than the specific activity of the insulin. High glucose did not alter the proinsulin specific activity, which tended to decline throughout the period of observation. With cycloheximide present, the concentration of proinsulin in the islets steadily declined while the specific activity of proinsulin remained high, indicating that the proinsulin pool is small and is turning over rapidly. In terms both of amount and radioactivity proinsulin amounted to 6-7% on a molar basis of the insulin in both the medium and the islets. Addition of dibutyryl cyclic 3',5'-adenosine monophosphate (DBCAMP) (0.002 M) with high glucose during the postlabeling period slightly increased the rate of insulin secretion (133% of control) but did not significantly alter the other parameters. The results suggest that while newly synthesized insulin and proinsulin may be preferentially secreted to a slight degree, about 90% of the insulin released during 3 hr in response to glucose, or to glucose and DBCAMP, is derived from pre-existing granule stores. There were no indications of the existence of independent or nongranule pathways of insulin or proinsulin secretion.