The role of calcium in parotid amylase secretion evoked by excitation of cholinergic, ?- and?-adrenergic receptors

Abstract

1. The output of amylase from superfused mouse parotid segments was monitored by an on-line automated fluorometric method. 2. During exposure to Ca2+-free solution, containing the Ca2+-chelating agent EGTA, excitation of α-adrenoceptors or cholinergic receptors only resulted in a very small and transient increase in amylase output. Admission of Ca2+ during sustained stimulation caused a marked rise in amylase output which was sustained. 3. During exposure to Ca2+-free solution containing EGTA excitation of β-adrenoceptors caused the usual very marked rise in amylase output and the enhanced amylase secretion was sustained. Admission of Ca2+ during sustained isoprenaline stimulation only caused a small transient rise in amylase output. 4. The effect of ACh on amylase output varied with the extracellular Ca2+ concentration, being reduced at subnormal extracellular levels and enhanced during superfusion with fluid containing 20 mM Ca2+. 5. 5 mM Mn2+ acted as a stimulant of amylase secretion even in the presence of blocking agents for the cholinergic, α- and β-adrenergic receptor sites. The effect of Mn2+ was biphasic; an initial transient increase in amylase output followed by a slowly developing sustained increase in secretion. The initial response was abolished after pretreatment with EGTA in a Ca2+-free solution. 6. Adding Mn2+ (5 mM) just after addition of ACh had caused maximal amylase secretion resulted in an immediate reduction in amylase output. Adding Mn2+ and ACh simultaneously to the superfusion solution resulted in a response smaller than that expected for ACh alone. The effect of ACh during continued exposure to Mn2+ (5 mM) was greatly reduced compared to control conditions. Stimulation with Mn2+ during continued exposure to isoprenaline resulted in a marked transient increase in amylase output. 7. The action of stimulants exciting cholinergic and α-adrenergic receptors is entirely dependent on extracellular Ca2+ whereas the action of stimulants exciting adrenergic β-receptors is relatively independent of Ca2+. Mn2+ immediately inhibits ACh-evoked amylase secretion probably by reducing Ca2+-influx. Mn2+ is, however also a stimulant of amylase secretion probably acting by displacing membrane-bound cell Ca2+.