Abstract
Parotid slices respond to norepinephrine with a rapid but transient accumulation of cyclic AMP. Continued exposure of the slices to norepinephrine results in a loss of responsiveness to subsequent application of norepinephrine (refractoriness or desensitization).Refractoriness could not be overcome by exposing the slices to a supramaximum concentration of norepinephrine. The suppressed response was clearly seen at 15 min after the first exposure to norepinephrine, but about 60 min were required for reaching complete suppression. The refractoriness also depended on the concentration of agonists, and their order of potency correlated well with their agonistic activity. The induction of desensitization was β-agonist specific, and was blocked by β-adrenergic blocking agents such as atenolol and alprenolol. The recovery from desensitization was observed by removing norepinephrine from the medium. The recovery was not evident until 15 min after washing the slices, but was nearly complete after 1 hr. Although parallel changes were observed between the initial increase in cyclic AMP level and the degree of desensitization by norepinephrine, the induction of desensitization does not seem to be mediated by cyclic AMP. The mechanism underlying this phenomenon does not appear to involve the activation of phosphodiesterase, the formation of an inhibitory substance in the medium or an increase in the rate of excretion of cyclic AMP. Changes in the ATP level by norepinephrine did not always correlate with the degree of refractoriness.No significant changes in the amount of [3H]-dihydroalprenolol binding were found in the membrane fraction prepared from the slices incubated with norepinephrine.These results suggest that the induction of desensitization may be mediated by modulating the coupling process between the receptor and adenylate cyclase.
