title: Ca2+ not cyclic AMP mediates the fluid secretory response to isoproterenol in the rat mandibular salivary gland: whole-cell patch-clamp studies.
abstract: We have performed whole-cell patch-clamp studies on dispersed secretory cells of the rat mandibular gland to determine how beta-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 mumol/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 alpha-adrenergic antagonist, phentolamine (1-2 mumol/l), had no effect on the response, but the beta-adrenergic antagonist, propranolol (10 mumol/l), blocked it completely. The isoproterenol response could not be mimicked by application to either surface of the cell membrane, of cyclic AMP (100 mumol/l), forskolin (1 or 20 mumol/l) or cholera toxin (2.5 micrograms/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 beta-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.
authors.affiliation: Department of Physiology, University of Sydney, NSW, Australia.
authors.forename: D I
authors.initials: DI
authors.lastname: Cook
authors.forename: M L
authors.initials: ML
authors.lastname: Day
authors.forename: M P
authors.initials: MP
authors.lastname: Champion
authors.forename: J A
authors.initials: JA
authors.lastname: Young
mesh_terms: D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D002772:Cholera Toxin / Q000494:pharmacology; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000502:physiology*; D007545:Isoproterenol / Q000494:pharmacology*; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D011943:Receptors, Adrenergic, beta / Q000502:physiology*; D013363:Submandibular Gland / Q000187:drug effects* / Q000378:metabolism
chemical_list: D011943:Receptors, Adrenergic, beta; D005576:Colforsin; D000001:Calcimycin; D002772:Cholera Toxin; D000242:Cyclic AMP; D007545:Isoproterenol; D000109:Acetylcholine; D002118:Calcium