ACTIVATION OF BETA-ADRENOCEPTORS DOES NOT CAUSE ANY CHANGE IN CYTOSOLIC CA2-CELLS( DISTRIBUTION IN RAT PAROTID ACINAR)

The effects of the beta-adrenoceptor agonist isoproterenol on the dist ribution of cytosolic Ca2+ concentrations were studied with digital im aging microscopy in fura-2-loaded rat parotid acinar cells. At concent rations < 10 mu M, isoproterenol did not cause any measurable change i n cytosolic Ca2+ concentration ([Ca2+](i)). Monitoring of [Ca2+](i) in selected areas of the acinar cells failed to show that stimulation wi th isoproterenol causes a localized rise in [Ca2+](i) at the apical re gion close to the lumen. As the maximum response of amylase exocytosis is observed at 0.1 or 1 mu M isoproterenol [Tanimura, A., Matsumoto, Y., Tojyo, Y., 1990. Evidence that isoproterenol-induced Ca2+-mobiliza tion in rat parotid acinar cells is not mediated by activation of beta -adrenoceptors. Biochim. Biophys. Acta, 1055, pp. 273-277], the data o btained here indicate that the isoproterenol-induced amylase exocytosi s is not accompanied by Ca2+ mobilization. The high concentration (100 mu M) of isoproterenol caused a small but significant increase in [Ca 2+](i), particularly in the apical region. This response was completel y attenuated by the alpha-adrenoceptor antagonist phentolamine, but no t by the beta-adrenoceptor antagonist propranolol, indicating that the isoproterenol-induced increase in [Ca2+](i) resulted from an activati on of alpha-adrenoceptors. Further, the effect of cyclic AMP on Ca2+ r elease from intracellular Ca2+ stores was studied in saponin-permeabil ized acinar cells using the lipophilic Ca2+ indicator Calcium Green C- 18. Cyclic AMP had no effect on the Ca2+ release, while the same acina r cells responded strongly to inositol 1,4,5-trisphosphate. This resul t does not support the hypothesis that cyclic AMP directly stimulates Ca2+ mobilization in rat parotid acinar cells. (C) 1998 Elsevier Scien ce B.V. All rights reserved.