Ca2+ not cyclic AMP mediates the fluid secretory response to isoproterenol in the rat mandibular salivary gland: Whole-cell patch-clamp studies

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Abstract
We have performed whole-cell patch-clamp studies on dispersed seccretory cells of the rat mandibular gland to determine how β-adrenergic stimulation causes fluid secretion. When the pipette contained a high K+ solution, the resting membrane potential averaged −33 mV±1.1 (SEM,n=34) and the clamped cell showed strong outward rectification. We monitored K+ and Cl− currents for periods of 15 min by recording the currents needed to clamp the cell potential at 0 and −80 mV, respectively. Isoproterenol (1–2 μmol/l) caused increases in the clamp current at 0 mV (the K+ current) and at −80 mV (the Cl− current) in about 80% of cases, although the responses were variable in size and time-course; the responses were indistinguishable from those induced by acetylcholine or the Ca2+ ionophore, A23187. The α-adrenergic antagonist, phentolamine (1–2 μmol/l), had no effect on the response, but the β-adrenergic antagonist, propranolol (10 μmol/l), blocked it completely. The isoproterenol response could not be mimicked by application to either surface of the cell membrane, of cyclic AMP (100 μmol/l), forskolin (1 or 20 μmol/l) or cholera toxin (2.5 μg/ml). However, increasing the Ca2+-chelating capacity of the pipette solution by raising its EGTA concentration from the customary 0.5 to 20 mmol/l, blocked the response to isoproterenol, suggesting that β-adrenergic agonists activate Cl− and K+ channels by raising cytosolic Ca2+. Since neomycin, which blocks phospholipase C, blocked the action of isoproterenol without impairing the cell responsiveness to A23187, it appears that isoproterenol, like muscarinic agonists, increased cytosolic Ca2+ via the phosphatidylinositol cycle.