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S L Howell

Publications and source records attributed to S L Howell.

At least 73 records · Page 4Linked to original sources
PubMed1986

Restoration of the A cell response to glucose in isolated rat islets of Langerhans.

This study investigated the modulatory effects of forskolin, phorbol 12-myristate 13-acetate (PMA) and arginine on pancreatic glucagon secretion in response to changes in glucose concentrations. Glucose, on its own (0, 5, 9 and 18 mM), did not modify glucagon secretion from A cell-rich isolated rat islets of Langerhans. In the presence of 20 microM forskolin, glucagon release was stimulated dose-dependently on lowering the external glucose concentration to 0 mM. Sensitivity to glucose was achieved in the presence of either PMA or arginine; both agents also significantly enhanced glucagon release at all glucose concentrations tested. The response of the B cells in these experiments were as expected from the available literature. These results indicate that the endogenous rate of glucagon secretion in the isolated islet preparation was minimal and was insensitive to glucose, sensitivity of the A cells to glucose could be restored by either arginine or agents which alter the concentration or activity of proposed cellular second messengers.

Animals↗
PubMed1986

The characteristics of beta-adrenergic binding sites on pancreatic islets of Langerhans.

The sympathetic nervous system is believed to play a part in the control of insulin release from the pancreatic islets of Langerhans. Stimulation of alpha-adrenoceptors is thought to inhibit the release of insulin whereas stimulation of beta-adrenoceptors enhances insulin release. The present experiments were conducted to establish the existence of beta-adrenergic receptors on guinea-pig and rat islet cells and to quantify them using the selective beta-adrenergic ligands [3H]dihydroalprenolol (DHA) and [125I]cyanoiodopindolol (CYP). Guinea-pig islets had 62 fmol beta-adrenoceptors/mg protein using [3H]DHA, corresponding to 43,700 binding sites/cell and 25 fmol beta-adrenoceptors/mg protein using [125I]CYP, corresponding to 17,400 sites/cell. Rat islet cells were found to have 4.6 fmol beta-adrenoceptors/mg protein using [125I]CYP, corresponding to 7200 sites/cell. Adenylate cyclase activation exhibited a positive dose-response relationship when exposed to the beta-adrenoceptor agonist isoprenaline, with a maximum response (190 +/- 21% above basal) at 10 mumol isoprenaline/l. This response was abolished with 1 mumol/l of the beta-adrenergic antagonist l-alprenolol. Insulin secretion in the presence of 10 mmol glucose/l, but in the absence of the alpha-adrenoceptor blocker phentolamine, was not affected by 10 mumol isoprenaline/l. However, perifusion experiments showed that secretion of insulin from isolated rat islets in the presence of 10 mmol glucose/l was significantly increased (332%) by 10 mumol isoprenaline/l in the presence of 10 mumol phentolamine/l. These results suggest that binding of selective radiolabelled ligands occurs to beta-adrenergic receptors on the B cell surface of the islets of Langerhans, and that these receptors are functionally coupled to insulin secretion through modulation of adenylate cyclase activity.

Adenylyl Cyclases↗
PubMed1985

Effects of Ca2+ and a phorbol ester on insulin secretion from islets of Langerhans permeabilised by high-voltage discharge.

Isolated rat islets of Langerhans permeabilised by high-voltage discharge secreted insulin in response to elevations in Ca2+ over the range 100 nM to 10 microM Ca2+. The phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA), had no effects on insulin secretion in the absence of Ca2+. In the presence of Ca2+ concentrations of greater than 10 nM, TPA produced dose-related shifts in the Ca2+-activation curve to lower Ca2+ concentrations, together with marked increases in the maximum secretory response to Ca2+. These results suggest that, in islets, the activation of protein kinase C is important in modulating both the sensitivity of the exocytotic mechanism to intracellular Ca2+, and the magnitude of the insulin secretory response.

Animals↗
PubMed1985

Assessment of the antidiabetic activity of epicatechin in streptozotocin-diabetic and spontaneously diabetic BB/E rats.

(-)-Epicatechin has previously been suggested to rapidly reverse alloxan diabetes in rats. We have assessed the therapeutic value of the compound in two further animal models of insulin-dependent diabetes mellitus, namely streptozotocin-diabetic rats and the spontaneously diabetic BB/E rat. There was no indication of a reversal of established diabetes in either the streptozotocin-diabetic or the spontaneously diabetic BB/E rats. Moreover, epicatechin also failed to halt the progression of the disease in prediabetic BB/E rats. Earlier claims of the potential use of epicatechin as an antidiabetic agent must therefore be treated with some caution.

Administration, Oral↗
PubMed1985

Effects of flavonoids on insulin secretion and 45Ca2+ handling in rat islets of Langerhans.

The effects of some flavonoids, a group of naturally occurring pigments one of which has been claimed to possess antidiabetic activities, on insulin release and 45Ca2+ handling have been studied in isolated rat islets of Langerhans. Insulin release was enhanced by approximately 44-70% when islets were exposed to either (-)epicatechin (0.8 mmol/l) or quercetin (0.01-0.1 mmol/l); others such as naringenin (0.1 mmol/l) and chrysin (0.08 mmol/l) inhibited hormone release by approximately 40-60%. These effects were observed only in the presence of 20 mmol glucose/l. Quercetin (0.01 mmol/l) and (-)epicatechin (0.8 mmol/l) both inhibited 45Ca2+ efflux in the presence and absence of extracellular Ca2+. In the presence of 20 mmol glucose/l both the short-term (5 min) and steady-state (30 min) uptake of 45Ca2+ were significantly increased by either quercetin or (-)epicatechin. These results suggest that the stimulatory compounds such as quercetin and (-)epicatechin may, at least in part, exert their effects on insulin release via changes in Ca2+ metabolism.

Animals↗
PubMed1984

Effects of monensin on metabolism of isolated rat islets of Langerhans.

Monensin, a univalent ionophore, is a carboxylic acid produced by Streptomyces cinnamonensis. It will complex various alkali-metal ions, but most readily binds Na+. Because of interest in the possible role of Na+ in the regulation of insulin secretion, we examined its effects on several aspects of the metabolism of isolated rat islets of Langerhans. The ionophore inhibited glucose-stimulated insulin release in a concentration-dependent manner, completely inhibiting secretion evoked by 20 mM-glucose at concentrations as low as 0.1 microM in static incubations. In perifusion experiments, both phases of insulin release were equally affected. Monensin (0.1 microM) had no significant effect on glucose oxidation as measured by the generation of 14CO2 from [14C]glucose. Monensin increased the rate of 22Na+ efflux from preloaded islets and net 22Na+ uptake over 30 min, in the absence of changes in islet volume or extracellular space. The ionophore increased the Rb+/K+ permeability of islet cells, as shown by its inhibition of 86Rb+ retention and stimulation of 86Rb+ efflux. At 0.1 microM, monensin abolished glucose-stimulated 45Ca2+ uptake by islets during 5 min incubations, and stimulated 45Ca2+ efflux from preloaded islets perifused with Ca2+-free medium, even in the complete absence of extracellular Na+. Studies of the uptake of 14C-labelled 5,5-dimethyloxazolidine-2,4-dione showed that 0.1 microM-monensin increased net intracellular pH from 7.05 to 7.13. 7 Monensin has widespread, complex, effects on the secretory responses and ion handling by the B cells, which are difficult to interpret in terms solely of actions as a Na+ ionophore.

Animals↗
PubMed1984

Effects of monensin on biosynthesis and intracellular processing of proinsulin.

Monensin, a specific sodium ionophore, has been shown to reduce glucose-induced proinsulin biosynthesis by 30% and to completely inhibit the intracellular conversion of proinsulin to insulin. Autoradiography of monensin-treated cells demonstrated the presence of large quantities of newly synthesized proteins in amorphous vesicles close to the Golgi complex of B cells. The results suggest profound effects of monensin on biosynthesis and intracellular processing of proinsulin.

Animals↗
PubMed1984

Effects of epicatechin on rat islets of Langerhans.

Plants containing epicatechin (a flavonoid) have been used to treat diabetes mellitus in Indian medicine. The present study reports effects of this compound on isolated islets of Langerhans. The flavonoid (1 mM) was found to increase insulin secretion from isolated rat islets of Langerhans in the presence of either 2 or 20 mM glucose, in static incubations, or in perifusion. The increase in insulin secretion mediated by epicatechin was both ATP- and temperature-dependent. Ultrastructural studies showed no deleterious changes in the structure of the B-cells after 5 days of exposure to the compound. Intraperitoneal injection of 30 mg/kg body wt of epicatechin twice daily for 4 days increased the islet insulin content by 30%. Secretion of insulin from islets isolated from epicatechin-injected rats was significantly increased when exposed to 20 mM glucose in comparison with water-injected controls. Furthermore, islets of adult rats cultured with 5.5 mM glucose for 4 days showed a significant increase in DNA synthesis in the presence of 0.05 mM epicatechin. These results suggest direct effects of epicatechin on various aspects of islet function.

2,4-Dinitrophenol↗
PubMed1983

Effect of dynorphin on insulin and somatostatin secretion, calcium uptake, and c-AMP levels in isolated rat islets of Langerhans.

Dynorphin-[1-13], at concentrations of 5.8 X 10(-12) to 5.8 X 10(-9) M, stimulated insulin secretion from isolated islets of Langerhans of the rat, in medium containing 6 mM glucose. Higher concentrations of dynorphin had no significant effect on secretion. Dynorphin (5.8 X 10(-9) M) was unable to initiate insulin release from islets in the presence of 2 mM glucose, or to increase insulin secretion further in the presence of 20 mM glucose or 6 and 12 mM glyceraldehyde. Dynorphin-induced insulin secretion from islets was blocked by verapamil (5 microM) or by chlorpropamide (72 microM), but not by a mu opiate receptor antagonist, naloxone (0.11 microM), or by ICI 154129, a specific antagonist for the delta receptor (0.25 microM). Dynorphin had no effect on islet somatostatin secretion, under conditions in which insulin secretion was greatly stimulated. Glucose (20 mM) and glyceraldehyde (6 and 12 mM) significantly increased both insulin and somatostatin secretion. Dynorphin (5.8 X 10(-9) M) increased 45Ca2+ uptake into islets, and also increased intracellular islet c-AMP levels. These changes persisted when higher concentrations of dynorphin were used. These results suggest that (1) dynorphin is a very potent stimulus for insulin secretion; (2) dynorphin does not affect somatostatin secretion in static incubations of islets, in the same way as does glucose and glyceraldehyde; (3) dynorphin's effects may involve increased calcium ion movement and can be blocked by verapamil; (4) dynorphin can also increase islet c-AMP, and could thereby modulate the responsiveness of other secretagogues; (5) the actions of dynorphin on insulin secretion are not mediated by delta or mu opiate receptors in islets.

Animals↗
PubMed1982

Regulation of insulin secretion from islets of Langerhans rendered permeable by electric discharge.

1. High-voltage electric discharge has been used to increase the permeability of B-cells of isolated islets of Langerhans to facilitate studies of the effects of normally impermeable substances on insulin secretion. 2. The application of an intense electric field increased the [(14)C]sucrose space of the islets from 37.8+/-3.1% to 86.2+/-5.2% of their total volume as assessed by (3)H(2)O content. The cells remained permeable for at least 40min. 3. Ultrastructural studies showed no deleterious changes in the structure of the B-cells after discharge. 4. Insulin secretion from normal islets was unaffected by increasing the medium [Ca(2+)] from 10nm to 10mum. In the islets that had been rendered permeable by discharge, insulin secretion was significantly increased under these conditions, without any alteration in the release of lactate dehydrogenase, a cytoplasmic marker enzyme. 5. Studies of the dynamics of insulin release during perifusion showed that the response to increased (10mum) Ca(2+) concentration was rapid and sustained over a period of at least 13min. 6. Secretion responses to Ca(2+) in perifusion established that maximum release in permeabilized islets occurs at approx. 1mum-Ca(2+) and half-maximum release occurs at approx. 0.6mum-Ca(2+). 7. The study of the effect of agents that interfere with the microtubular microfilamentous system in B-cells using a perifusion system revealed that cytochalasin B caused a considerable increase, whereas vinblastine sulphate caused a significant inhibition, in insulin release in response to 1mum-Ca(2+). 8. This technique should facilitate the study of the role of normally impermeable ions and metabolic intermediates in the regulation of insulin secretion.

Animals↗
PubMed1982

Actomyosin interactions with insulin-storage granules in vitro.

Interactions between actomyosin and insulin storage granules isolated from rat islets of Langerhans have been examined in a simple system in vitro, which allows comparison of the sedimentation of the granules in the presence of absence of actomyosin in various conditions. Actomyosin altered granule-sedimentation rates in a manner consistent with the binding of the granules of actomyosin filaments. This interaction was enhanced by addition of ATP (1.5 mM) but unaltered by addition of CaCl2, by calmodulin or by calmodulin in the presence of 10 microM-CaCl2. Addition of EGTA (0.1 mM), cyclic AMP (10 microM) of cytochalasin B (10 microgram/ml) were also without effects in these conditions. Pre-incubation of granules with phospholipase c did not affect granule-actomyosin interaction. Ultrastructural studies showed close contacts between the membranes of the granules and actomyosin filaments. The results indicate the possibility that actomyosin might provide the motile force for granule translocation during the insulin secretory process.

Actomyosin↗
PubMed1982

Effects of taxol and nocodazole on insulin secretion from isolated rat islets of Langerhans.

Taxol, a promotor of microtubule polymerization, and nocodazole, which induces microtubule depolymerization, used at concentrations known to be specific for these effects in other cell types, were each shown to inhibit glucose-stimulated insulin secretion from isolated rat islets of Langerhans. These findings suggest that the dynamic regulation of microtubule polymerization-depolymerization in pancreatic B cells may be important for insulin secretion via the microtubule-microfilamentous system.

Alkaloids↗
PubMed1982

A procedure for the purification of somatotrophs isolated from rat anterior pituitary glands using Percoll density gradients.

We have used fractionation on density gradients of Percoll to separate the cell types in the rat anterior pituitary gland and to produce a purified preparation of somatotrophs. The method differs from those described previously which used, for example, albumin or Ficoll gradients, in being more rapid and avoiding low temperatures, and therefore gives cells with improved viability. Anterior pituitary glands from male rats were dispersed with trypsin to produce 1.5 x 10 (6) -2.0 x 10 (6) cells/gland. These were fractionated on hyperbolic density gradients of Percoll. Two bands of cells containing somatotrophs were detected, one of which (band A; density 1.075-1.082 g/cm3) contained approximately 90% somatotrophs, whereas the other (band B; density 1.055-1.068 g/cm3) contained about 70% somatotrophs mixed with other cells, especially lactotrophs. Cells in band A appeared more responsive to secretagogues than those in band B; growth hormone secretion was stimulated markedly by cyclic AMP derivatives and prostaglandin E2, and inhibited by somatostatin. Such purified somatotrophs are well suited to biochemical studies on the mechanism of the control of growth hormone secretion.

Adrenocorticotropic Hormone↗