Abstract

The hormone-stimulated secretion of α-amylase (EC 3.2.1.1) from superfused rat parotid tissue has been measured using an automated, continuous system. Stimulation of parotid slices with muscarinic or alpha adrenergic agonists produces a rapid initial elevation of the rate of secretion, though the amount of α-amylase secreted is less than with stimulation by the beta adrenergic agonist isoproterenol. Isoproterenol-stimulated secretion begins with a slower rate of initial output than does stimulation with either the alpha adrenergic agonist phenylephrine or the muscarinic agonist carbachol. If any of these three agonists is allowed to superfuse the tissue continually, α-amylase output reaches a peak rate, then declines over a period of time until it returns to basal rate. Thus continuous stimulation with phenylephrine rapidly produces refractoriness, which is absolute within 20 to 30 min. This refractoriness is specific, since carbachol and isoproterenol, agonists acting through receptors not originally involved, are still active. Continuous stimulation with carbachol or isoproterenol also leads to refractoriness, though the process takes longer than that seen with phenylephrine and needs one to two hours to be completed. Other experiments have shown that the simultaneous stimulation of α-amylase secretion with phenylephrine and carbachol is no more effective than either agonist alone for peak α-amylase secretion. However when α-amylase secretion is stimulated with carbachol plus isoproterenol, more enzyme is released with a greater initial secretion rate than when either agonist is used alone. Further, if isoproterenol stimulation pulses are superimposed upon the continuous superfusion of carbachol, nonspecific refractoriness is seen; carbachol diminishes the ability of isoproterenol to stimulate α-amylase secretion. These forms of secretion refractoriness correlate well with the pharmacology of cyclic nucleotide refractoriness that we have previously reported. However, the ability of cyclic nucleotides to mediate α-amylase secretion is not firmly established. Cyclic AMP may be a second messenger mediating α-amylase secretion. Cyclic GMP, on the other hand, may not play any role in muscarinically- or adrenergically-stimulated α-amylase secretion. Indeed, though sodium azide raises the cyclic GMP concentration in parotid slices, it has no effect on α-amylase secretion. This lack of effect is not due to a nonspecific inhibition of secretion. Azide produces little inhibition of carbachol-stimulated α-amylase secretion or cyclic nucleotide accumulation.