| Abstrakt: |
In dispersed rat parotid gland acinar cells, the beta-adrenergic agonist (-)-isoproterenol, but not its stereoisomer (+)-isoproterenol, induced a transient 1.6-fold (at maximum stimulation, 2 x 10(-4) M) increase in cytosolic free calcium ([Ca2+]i) within 9 s, which returned to resting levels (approximately 190 nM) by 60 s. This [Ca2+]i response was not altered by chelating extracellular Ca2+ with [ethylenebis(oxyethylenenitrilo)]tetraacetic acid (EGTA) and could be completely blocked by the beta-adrenergic antagonists propranolol (beta 1 + beta 2) and ICI 118,551 (beta 2) but not by atenolol (beta 1). The muscarinic-cholinergic agonist carbachol (at maximum stimulation, 10(-5) M) induced a 3-4-fold elevation in [Ca2+]i within 6 s, which slowly returned to resting levels by 8-10 min. The peak carbachol [Ca2+]i response was not substantially altered by the addition of EGTA to the extracellular medium. However, if the cells were first stimulated with isoproterenol in the EGTA-containing medium, the peak carbachol response was decreased approximately 54%. When carbachol was added to cells in the presence of high extracellular calcium, at the isoproterenol-stimulated [Ca2+]i peak, the resulting [Ca2+]i level was equal to that achieved when carbachol was either added alone or added after propranolol and isoproterenol. 8-Bromo-cyclic AMP induced a [Ca2+]i response similar to that elicited by isoproterenol, which was not additive to that by carbachol. Carbachol induced a approximately 3.5-fold increase in inositol trisphosphate (IP3) production in parotid cells within 30 s. 8-Bromo-cAMP, N6,O2′-dioctanoyl-cAMP, and isoproterenol consistently induced a significant stimulation in IP3 production. The half-maximal concentration of isoproterenol required for [Ca2+]i mobilization and IP3 production was comparable (approximately 10(-5) M). Isoproterenol-induced IP3 formation was blocked by propranolol. The data show that in rat parotid acinar cells, beta-adrenergic stimulation results in IP3 formation and mobilization of a carbachol-sensitive intracellular Ca2+ pool by a mechanism involving cAMP. This demonstrates an interaction between the cAMP and phosphoinositide second messenger systems in these cells. |
