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S L Howell

Publications and source records attributed to S L Howell.

At least 37 records · Page 2Linked to original sources
PubMed1993

Arachidonic acid-induced insulin secretion from rat islets of Langerhans is not mediated by protein phosphorylation.

Arachidonic acid (AA) stimulated protein phosphorylation in electrically permeabilised islets, most notably of an islet protein of approximate molecular weight 18 kDa. This protein did not appear to be a substrate for cAMP-dependent protein kinase. The AA-induced protein phosphorylation was mediated by unmetabolised AA since the lipoxygenase inhibitor, nordihydroguaretic acid (NDGA), or the cyclooxygenase inhibitor, indomethacin, did not significantly reduce AA-induced phosphorylation. Although saturated fatty acids did not stimulate phosphorylation of islet proteins, a number of cis-unsaturated fatty acids, other than AA, induced 32P incorporation into an 18 kDa protein. However, some fatty acids which stimulated protein phosphorylation had no effect on insulin secretion in experiments where AA clearly stimulated insulin secretion. AA stimulated protein kinase C (PKC) activity extracted from islets but several fatty acids which induced protein phosphorylation had no significant effect on PKC activity in vitro. 50 nM staurosporine had no effect on AA-induced protein phosphorylation but this concentration of staurosporine markedly inhibited PKC activity. 200 nM staurosporine caused complete inhibition of the AA-induced phosphorylation without having any effect on AA-induced insulin secretion. These results suggest that AA and some other fatty acids can promote 32P incorporation into islet proteins, independently of PKC activation, and that AA-induced phosphorylation is not required for insulin secretory responses to AA.

Alkaloids↗
PubMed1993

Effects of midazolam on insulin secretion from isolated rat pancreatic islets of Langerhans.

Isolated, cultured rat pancreatic islets of Langerhans were incubated in medium containing glucose 2 or 20 mmol litre-1 and with glucose supplemented with midazolam 10, 100 or 1000 ng ml-1. Glucose 20 mmol litre-1 with the addition of noradrenaline 10 mumol litre-1 was used as a control. In contrast with noradrenaline, midazolam did not inhibit glucose-induced insulin secretion. Noradrenaline 10 mumol litre-1 inhibited secretion to basal values in response to glucose 20 mmol litre-1 (P < 0.001).

Animals↗
PubMed1993

Isoflurane inhibits insulin secretion from isolated rat pancreatic islets of Langerhans.

We have investigated the effects of isoflurane on insulin secretion in vitro from rat isolated islets of Langerhans and found a significant, dose-related and reversible inhibition of insulin secretion. Isoflurane 2% decreased insulin secretion stimulated by glucose 20 mmol litre-1 to basal, nonstimulated values. In other studies to identify the stage in the stimulus secretion pathway for insulin at which the anaesthetic may exert an inhibitory action, we have stimulated insulin release using glyceraldehyde and a phorbol ester. Insulin secretion induced by these secretagogues was also blocked by isoflurane. This suggests that the inhibitory effect of the anaesthetic agent may be at a site distal to stimulation of insulin secretion by glyceraldehyde and phorbol esters.

Anesthesia, Inhalation↗
PubMed1992

Ca2(+)-induced insulin secretion from electrically permeabilized islets. Loss of the Ca2(+)-induced secretory response is accompanied by loss of Ca2(+)-induced protein phosphorylation.

Increasing the cytosolic Ca2+ concentration of electrically permeabilized rat islets of Langerhans caused rapid increases in insulin secretion and in 32P incorporation into islet proteins. However, the secretory responsiveness of permeabilized islets was relatively transient, with insulin secretion approaching basal levels within 20-30 min despite the continued presence of stimulatory concentrations of Ca2+. The loss of Ca2(+)-induced insulin secretion was accompanied by a marked reduction in Ca2(+)-dependent protein phosphorylation, but not in cyclic AMP-dependent protein phosphorylation. Similarly, permeabilized islets which were no longer responsive to Ca2+ were able to mount appropriate secretory responses to cyclic AMP and to a protein kinase C-activating phorbol ester. These results suggest that prolonged exposure to elevated cytosolic Ca2+ concentrations results in a specific desensitization of the secretory mechanism to Ca2+, perhaps as a result of a decrease in Ca2(+)-dependent kinase activity. Furthermore, these studies suggest that secretory responses of B-cells to cyclic AMP and activators of protein kinase C are not dependent upon the responsiveness of the cells to changes in cytosolic Ca2+.

Adenosine Triphosphate↗
PubMed1992

Arachidonic acid induces phosphorylation of an 18 kDa protein in electrically permeabilised rat islets of Langerhans.

Arachidonic acid (AA) was shown to induce concentration-dependent, calcium-independent, in situ phosphorylation of a protein of approximate molecular weight 18 kDa in electrically permeabilised rat islets of Langerhans. This protein did not appear to be a substrate for protein kinase C (PKC) since stimulation of PKC by 4 beta phorbol myristate acetate (4 beta PMA) did not result in 32P incorporation into an 18 kDa protein, and since AA-induced phosphorylation was observed in islets in which PKC had been down-regulated by prolonged exposure of islets to 4 beta PMA. These results suggest that AA stimulates protein phosphorylation by a mechanism other than PKC activation.

Animals↗
PubMed1992

Nitric oxide is not involved in the initiation of insulin secretion from rat islets of Langerhans.

The involvement of nitric oxide as an intracellular messenger in the control of insulin secretion from pancreatic Beta cells was studied in rat islets of Langerhans by measuring: (i) nitric oxide generation in response to physiological insulin secretagogues; (ii) the effects of inhibitors of nitric oxide synthesis on insulin secretory responses to physiological secretagogues, and on insulin synthesis; (iii) changes in islet cyclic guanosine monophosphate in response to secretagogues; (iv) the effects of exogenous cyclic guanosine monophosphate and dibutyryl cyclic guanosine monophosphate on insulin secretion from electrically permeabilised islets and from intact, respectively. These studies produced no evidence that nitric oxide generation is required for the initiation of insulin secretion by common secretagogues. However, the results of our experiments suggest that the generation of nitric oxide may be involved in long-term, glucose-dependent increases in cyclic guanosine monophosphate content of islet cells, although the physiological relevance of these changes requires further investigation.

Amino Acid Oxidoreductases↗
PubMed1992

Insulin secretion and protein phosphorylation in PKC-depleted islets of Langerhans.

Protein kinase C (PKC)-dependent phosphorylation of endogenous substrates was measured in electrically permeabilised rat islets of Langerhans. The PKC-activating phorbol ester, 4 beta-phorbol myristate acetate (PMA), caused a slow but prolonged increase in insulin secretion from permeabilised islets, which was accompanied by increased 32P incorporation into several islet proteins of apparent M.W. 30-50 kDa. Depletion of islet PKC by prolonged exposure to PMA abolished subsequent secretory and phosphorylating responses to the phorbol ester. However, PKC-depleted islets did not show diminished responses to glucose, suggesting that PKC-mediated phosphorylation of these proteins is not essential for nutrient-induced insulin secretion.

Animals↗
PubMed1992

Arachidonic acid-induced insulin secretion from rat islets of Langerhans.

There is growing evidence that arachidonic acid (AA) and/or its metabolites may be involved in the control of insulin secretion. We have now investigated the effect of AA on insulin secretion from rat islets, and the possible involvement of protein kinase C (PKC) in this process. Exogenous AA stimulated insulin secretion from intact islets at a substimulatory concentration of glucose (2 mM), but did not further enhance glucose-induced (20mM) insulin secretion. AA-induced insulin secretion was temperature dependent. The secretory responses seen at 37 degrees C were totally abolished by reducing the incubation temperature to less than or equal to 34 degrees C. AA-induced insulin secretion was not dependent upon extracellular Ca2+ and was potentiated by omission of Ca2+ or bovine serum albumin from the media. PKC in rat islets can thus be stimulated by AA, but the stimulation of PKC is not required for AA-induced insulin secretion.

Animals↗
PubMed1991

Activation of protein kinase C is not required for glyceraldehyde-stimulated insulin secretion from rat islets.

Glyceraldehyde-induced insulin release from rat islets of Langerhans was not affected following down-regulation of protein kinase C (PKC) by prolonged exposure to the tumour-promoting phorbol ester, 4 beta-phorbol myristate acetate (PMA). Glyceraldehyde did not cause translocation of islet PKC under conditions in which PMA stimulated redistribution of enzyme activity. These results indicate that activation of PKC is not required for glyceraldehyde stimulation of insulin secretion from normal rat islets.

Animals↗
PubMed1991

Activation of protein kinase C is essential for sustained insulin secretion in response to cholinergic stimulation.

Insulin secretion from isolated rat islets of Langerhans is enhanced by cholinergic agonists, such as carbachol (CCh), in the presence of a stimulatory concentration of glucose. Depletion of islet protein kinase C activity by prolonged exposure to a tumour-promoting phorbol ester did not prevent the initial secretory response to CCh, but markedly reduced the duration of CCh-induced elevated secretory rates. These results suggest that the major action of PKC is in maintaining rather than initiating the insulin secretory response to cholinergic agonists.

Animals↗
PubMed1991

Protein kinase C and the regulation of insulin secretion from pancreatic B cells.

Protein kinase C (PKC) has been identified in islets of Langerhans and insulin-secreting tumour cells. Diacylglycerols (DAGs, the endogenous PKC activators) are generated in response to insulin secretagogues, although nutrient and non-nutrient secretagogues generate DAGs of different compositions and of different potencies as PKC activators. Exogenous activators of PKC stimulate insulin secretion from B cells, but attempts to define a physiological role for PKC by using inhibitors of this enzyme have produced ambiguous results. However, in studies using PKC-depleted B cells the loss of PKC activity does not inhibit glucose-induced insulin secretion, but markedly reduces responses to cholinergic agonists. These observations are supported by measurements of PKC activation which suggest that the enzyme is activated by cholinergic agonists, but not by nutrient secretagogues. Currently available experimental evidence therefore suggests that activation of PKC is not essential for nutrient-induced insulin secretion, but is required for the expression of a normal secretory response to cholinergic neurotransmitters.

Animals↗
PubMed1990

Glucose-stimulated insulin secretion is not dependent on activation of protein kinase A.

The involvement of cyclic AMP-dependent protein kinase A (PKA) in the exocytotic release of insulin from rat pancreatic islets was investigated using the Rp isomer of adenosine 3',5'-cyclic phosphorothioate (Rp-cAMPS). Preincubation of electrically permeabilised islets with Rp-cAMPS (1 mM, 1 h, 4 degrees C) inhibited cAMP-induced phosphorylation of islet proteins of apparent molecular weights in the range 20-90 kDa, but did not affect basal (50 nM Ca2+) nor Ca2(+)-stimulated (10 microM) protein phosphorylation. Similarly, Rp-cAMPS (500 microM) inhibited both cAMP- (100 microM) and 8BrcAMP-induced (100 microM) insulin secretion from electrically permeabilised islets without affecting Ca2(+)-stimulated (10 microM) insulin release. In intact islets, Rp-cAMPS (500 microM) inhibited forskolin (1 microM, 10 microM) potentiation of insulin secretion, but did not significantly impair the insulin secretory response to a range of glucose concentrations (2-20 mM). These results suggest that cAMP-induced activation of PKA is not essential for either basal or glucose-stimulated insulin secretion from rat islets.

Animals↗
PubMed1990

Proinsulin processing in electrically permeabilized rat islets of Langerhans.

Proinsulin conversion to insulin was studied using electrically permeabilized rat islets of Langerhans. Using high-performance liquid chromotography separation of radiolabelled islet proteins, we have demonstrated that conversion was dependent upon temperature, sensitive to pH and regulated by MgATP. Ammonium acetate was used to collapse the granular pH gradient, over a pH range of 3.5-7.4. Conversion was optimum at pH 4.5-5.5 and was reduced, but not abolished, at pH 7.4. Ca2+ (10 microM) and 4 beta-phorbol 12-myristate 13-acetate (500 nM), which are insulin secretagogues in permeabilized islets, caused no significant stimulation of conversion.

Acetates↗
PubMed1989

The role of protein kinase C in cholinergic stimulation of insulin secretion from rat islets of Langerhans.

The role of the Ca2+/phospholipid-dependent protein kinase C (PKC) in cholinergic potentiation of insulin release was investigated by measuring islet PKC activity and insulin secretion in response to carbachol (CCh), a cholinergic agonist. CCh caused a dose-dependent increase in insulin secretion from cultured rat islets at stimulatory glucose concentrations (greater than or equal to 7 mM), with maximal effects observed at 100 microM. Short-term exposure (5 min) of islets to 500 microM-CCh at 2 mM- or 20 mM-glucose resulted in redistribution of islet PKC activity from a predominantly cytosolic location to a membrane-associated form. Prolonged exposure (greater than 20 h) of islets to 200 nM-phorbol myristate acetate caused a virtual depletion of PKC activity associated with the islet cytosolic fraction. Under these conditions of PKC down-regulation, the potentiation of glucose-stimulated insulin secretion by CCh (500 microM) was significantly decreased, but not abolished. CCh stimulated the hydrolysis of inositol phospholipids in both normal and PKC-depleted islets, as assessed by the generation of radiolabelled inositol phosphates. These results suggest that the potentiation of glucose-induced insulin secretion by cholinergic agonists is partly mediated by activation of PKC as a consequence of phospholipid hydrolysis.

Animals↗
PubMed1989

Time-course of Ca2+-induced insulin secretion from perifused, electrically permeabilised islets of Langerhans: effects of cAMP and a phorbol ester.

The pattern of insulin secretion from electrically permeabilised islets was studied in a perifusion system. Increases in intracellular Ca2+ stimulated insulin secretion in a dose-related manner, but the secretory response to Ca2+ was only transient, with permeabilised islets becoming rapidly insensitive to a stimulatory concentration of Ca2+. Cyclic AMP and the protein kinase C activator, phorbol 12-myristate 13-acetate (PMA), both stimulated Ca2+-induced insulin secretion from perifused permeabilised islets by increasing the maximum secretory response to Ca2+, and by maintaining elevated rates of secretion when the permeabilised islets had become insensitive to the stimulatory effects of Ca2+. These results suggest that cAMP and PMA may act partly by modifying the magnitude of the secretory response to Ca2+, and also by Ca2+-independent effects on the secretory mechanism.

Animals↗