Activation of alpha-2-adrenoceptors results in an increase in F-actin formation in HIT-T15 pancreatic B-cells.
1. Alpha-2-adrenoceptor agonists, such as noradrenaline, are potent inhibitors of insulin secretion, and it has been suggested that they control a late step in the pathway of exocytosis. We have investigated whether this could be related to a change in the extent of actin polymerization in the pancreatic B-cell, since actin microfilaments are implicated in regulating the access of secretory granules to the plasma membrane prior to exocytosis. 2. Cultured HIT-T15 pancreatic B-cells responded to noradrenaline with an increase in F-actin content, as judged by a rise in the fluorescence output after probing of the cells with phalloidin (a toxin which binds specifically to F-actin) conjugated to rhodamine. The response to noradrenaline was rapid, dose-dependent and sustained and could be reproduced by the highly selective alpha-2-agonist UK14,304. Examination of HIT-T15 cells by fluorescence microscopy after treatment with rhodamine-phalloidin, revealed a significant localization of F-actin immediately adjacent to the plasma membrane. The pattern of F-actin distribution in the cells was not altered dramatically by noradrenaline, although the intensity of staining close to the plasma membrane appeared to be slightly reduced. 3. The increase in F-actin content induced by noradrenaline and UK14,304 was inhibited significantly by the alpha-2-antagonist idazoxan but not by the alpha-1-selective antagonist prazosin. Pretreatment of HIT-T15 cells with pertussis toxin did not lead to any direct alteration in F-actin content, although the toxin significantly modified the responses induced by noradrenaline and UK14,304. In each case, cells incubated for 24 h with pertussis toxin responded to the alpha-2-agonist with an enhanced fluorescence output, indicating that F-actin levels had increased still further. This did not correlate with any gross change in the distribution of F-actin as judged by fluorescence microscopy. 4. The results demonstrate that alpha-2-adrenoceptors are coupled to control of actin polymerization in HIT-T15 cells. They suggest that regulation of F-actin formation could be a component of the mechanism by which alpha-2-agonists mediate inhibition of insulin secretion.
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Department of Biological Sciences, Keele University, U.K.
el-Mansoury
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Morgan
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D000199:Actins / Q000096:biosynthesis* / Q000235:genetics; D007516:Adenoma, Islet Cell / Q000473:pathology; D058647:Adrenergic alpha-2 Receptor Agonists; D058669:Adrenergic alpha-2 Receptor Antagonists; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology*; D001704:Biopolymers; D000068438:Brimonidine Tartrate; D003572:Cytochalasins / Q000494:pharmacology; D004146:Dioxanes / Q000494:pharmacology; D004347:Drug Interactions; D005089:Exocytosis; D005456:Fluorescent Dyes; D005786:Gene Expression Regulation / Q000187:drug effects*; D019329:Idazoxan; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects* / Q000378:metabolism; D009363:Neoplasm Proteins / Q000096:biosynthesis / Q000235:genetics; D009638:Norepinephrine / Q000494:pharmacology; D037342:Pertussis Toxin; D011224:Prazosin / Q000494:pharmacology; D011810:Quinoxalines / Q000494:pharmacology; D018341:Receptors, Adrenergic, alpha-2 / Q000502:physiology; D014407:Tumor Cells, Cultured; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D000199:Actins; D058647:Adrenergic alpha-2 Receptor Agonists; D058669:Adrenergic alpha-2 Receptor Antagonists; D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D001704:Biopolymers; D003572:Cytochalasins; D004146:Dioxanes; D005456:Fluorescent Dyes; D007328:Insulin; D009363:Neoplasm Proteins; D011810:Quinoxalines; D018341:Receptors, Adrenergic, alpha-2; D010566:Virulence Factors, Bordetella; C031218:cytochalasin A; D000068438:Brimonidine Tartrate; D037342:Pertussis Toxin; D009638:Norepinephrine; D011224:Prazosin; D019329:Idazoxan