Somatostatin modulation of glucagon secretion and its importance in human glucose homeostasis

Gap junction coupling and islet delta-cell function in health and disease

The pancreatic islets contain beta-cells and alpha-cells, which are responsible for secreting two principal gluco-regulatory hormones; insulin and glucagon, respectively. However, they also contain delta-cells, a relatively sparse cell type that secretes somatostatin (SST). These cells have a complex morphology allowing them to establish an extensive communication network throughout the islet, despite their scarcity. Delta-cells are electrically excitable cells, and SST secretion is released in a glucose- and K_ATP-dependent manner. SST hyperpolarises the alpha-cell membrane and suppresses exocytosis. In this way, islet SST potently inhibits glucagon release. Recent studies investigating the activity of delta-cells have revealed they are electrically coupled to beta-cells via gap junctions, suggesting the delta-cell is more than just a paracrine inhibitor. In this Review, we summarize delta-cell morphology, function, and the role of SST signalling for regulating islet hormonal output. A distinguishing feature of this Review is that we attempt to use the discovery of this gap junction pathway, together with what is already known about delta-cells, to reframe the role of these cells in both health and disease. In particular, we argue that the discovery of gap junction communication between delta-cells and beta-cells provides new insights into the contribution of delta-cells to the islet hormonal defects observed in both type 1 and type 2 diabetes. This reappraisal of the delta-cell is important as it may offer novel insights into how the physiology of this cell can be utilised to restore islet function in diabetes.

The Liver-α-Cell Axis and Type 2 Diabetes

Both type 2 diabetes (T2D) and nonalcoholic fatty liver disease (NAFLD) strongly associate with increasing body mass index, and together these metabolic diseases affect millions of individuals. In patients with T2D, increased secretion of glucagon (hyperglucagonemia) contributes to diabetic hyperglycemia as proven by the significant lowering of fasting plasma glucose levels following glucagon receptor antagonist administration. Emerging data now indicate that the elevated plasma concentrations of glucagon may also be associated with hepatic steatosis and not necessarily with the presence or absence of T2D. Thus, fatty liver disease, most often secondary to overeating, may result in impaired amino acid turnover, leading to increased plasma concentrations of certain glucagonotropic amino acids (e.g., alanine). This, in turn, causes increased glucagon secretion that may help to restore amino acid turnover and ureagenesis, but it may eventually also lead to increased hepatic glucose production, a hallmark of T2D. Early experimental findings support the hypothesis that hepatic steatosis impairs glucagon's actions on amino acid turnover and ureagenesis. Hepatic steatosis also impairs hepatic insulin sensitivity and clearance that, together with hyperglycemia and hyperaminoacidemia, lead to peripheral hyperinsulinemia; systemic hyperinsulinemia may itself contribute to worsen peripheral insulin resistance. Additionally, obesity is accompanied by an impaired incretin effect, causing meal-related glucose intolerance. Lipid-induced impairment of hepatic sensitivity, not only to insulin but potentially also to glucagon, resulting in both hyperinsulinemia and hyperglucagonemia, may therefore contribute to the development of T2D at least in a subset of individuals with NAFLD.

The somatostatin receptor in human pancreatic β-cells

The peptide hormone somatostatin (SST) is produced in the brain, the gut, and in δ-cells in pancreatic islets of Langerhans. SST secretion from δ-cells is stimulated by glucose, amino acids, and glucagon-like peptide-1. Exogenous SST strongly inhibits the secretion of the blood glucose-regulating hormones insulin and glucagon from pancreatic β-cells and α-cells, respectively. Endogenous SST secreted from δ-cells is a paracrine regulator of insulin and glucagon secretion, although the exact physiological significance of this regulation is unclear. Secreted SST binds to specific receptors (SSTRs), which are coupled to Gi/o proteins. In both β- and α-cells, activation of SSTRs suppresses hormone secretion by reducing cAMP levels, inhibiting electrical activity, decreasing Ca²⁺ influx through voltage-gated Ca²⁺ channels and directly reducing exocytosis in a Ca²⁺ and cAMP-independent manner. In rodents, β-cells express predominantly SSTR5, whereas α-cells express SSTR2. In human islets, SSTR2 is the dominant receptor in both β- and α-cells, but other isoforms also contribute to the SST effects. Evidence from rodent models suggests that SST secretion from δ-cells is dysregulated in diabetes mellitus, which may contribute to the metabolic disturbances in this disease. SST analogues are currently used for the treatment of hyperinsulinism and other endocrine disorders, including acromegaly and Cushing's syndrome.

Infusion of graded concentrations of somatostatin in man: pharmacokinetics and differential inhibitory effects on pituitary and islet hormones

The present study was undertaken to determine the plasma levels of somatostatin-like immunoreactivity (SLI) during constant infusion of graded concentrations of synthetic somatostatin-14 (S-14); to determine the half-life (t1/2) and metabolic clearance rate (MCR) of SLI; to correlate the plasma SLI levels with the degree of inhibition of pituitary and islet hormone secretion and to establish whether the plasma SLI levels capable of inhibiting pituitary and islet hormone secretion fall into the physiological range. Four normal subjects on separate occasions were each infused with saline or S-14 (25,50 and 75 micrograms/h) at a constant rate for 2 1/2 h. Thirty min following the infusions, TRH (200 micrograms) and arginine (0.5 g/kg) were given i.v. Blood samples were drawn every 15 min for measurement of GH, TSH, insulin, glucagon and SLI (by RIA of acid-ethanol extracted plasma) and at rapid intervals for 10 min after stopping the infusions for measurement of SLI disappearance. During S-14 infusions, plasma SLI rose rapidly, reached a plateau from 15-150 min and declined rapidly on cessation of the infusions with a mean t 1/2 of 2.72 +/- 0.45 min. Mean plateau SLI levels were: 149 +/- 3 pg/ml (25 micrograms/h), 465 +/- 35 pg/ml (50 micrograms/h), and 1244 +/- 71 pg/ml (75 micrograms/h). SLI was cleared rapidly but the MCR exhibited a dose-dependent decrease from 3225 +/- 699 ml/min for the 25 micrograms infusion to 1249 +/- 241 ml/min for the 75 micrograms/h infusion (P less than 0.05). The 25 micrograms/h infusion dose produced near-maximal suppression of GH secretion and inhibited insulin secretion but not TSH or glucagon secretion. The intermediate dose significantly inhibited GH, TSH, and insulin but not glucagon whereas the 75 micrograms/h infusion suppressed all four hormones. In six normal subjects endogenous plasma SLI rose from a basal value of 32.5 +/- 4.9 pg/ml to 75.5 +/- 9.0 pg/ml following ingestion of a mixed meal. This level was 50% of that resulting from the 25 micrograms/h infusion and which suppressed GH almost completely. We concluded that: Infused S-14 is cleared rapidly and decays with a short t 1/2; S-14 inhibits its own MCR; The somatotrophs are the most sensitive to S-14 inhibition, followed by the thyrotrophs and the B-cells (approximately equally) followed by the A-cells; Fluctuations in plasma SLI occurring physiologically may influence GH and possibly other S-14 sensitive cells by an endocrine mechanism.

A super active cyclic hexapeptide analog of somatostatin

The cyclic hexapeptide, cyclo (Pro-Phe-D-Trp-Lys-Thr-Phe), I, has been shown to have the biological properties of somatostatin. We now report structure-activity studies which optimize the potency of this cyclic hexapeptide series with the synthesis of cyclo (N-Me-Ala-Tyr-D-Trp-Lys-Val-Phe), II, which is 50-100 times more potent than somatostatin for the inhibition of insulin, glucagon and growth hormone release. The hydroxyl group of tyrosine is seen to lend a 10-fold enhancement to the potency. Potency also is found to be correlated with hydrophobicity. II is found to improve the control of postprandial hyperglycemia in diabetic animals when given in combination with insulin. The analog is found to be quite stable in the blood and in the gastrointestinal tract, but the bioavailability after oral administration is only 1-3%. The biological properties and long duration of II should allow clinical evaluation of the inhibition of glucagon release as an adjunct to insulin in the treatment of patients with diabetes.

Hemodynamic effects of somatostatin in insulin-dependent diabetic subjects

The aim of the present study was to evaluate possible hemodynamic effects of somatostatin in insulin-dependent diabetic subjects. For this purpose, 7 insulin-requiring juvenile-onset diabetics were submitted to a short-term infusion of cyclic somatostatin (250 micrograms/h, over 2 h) or saline in randomized order. Somatostatin infusion resulted in a progressive and significant decrease in heart rate, stroke volume, cardiac index and velocity circumferential fiber; on the other hand, left ventricular ejection time was augmented by somatostatin. None of these effects was seen in the saline control study. We conclude that somatostatin exerts a negative inotropic effect in insulin-dependent diabetes.