Activation of alpha-2-adrenoceptors results in an increase in F-actin formation in HIT-T15 pancreatic B-cells

Abstract

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.