Effects of acetylcholine, isoprenaline and forskolin on electrolyte and protein composition of rabbit mandibular saliva.

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1. The major purpose of this study was to investigate cellular regulation of the ductal transport processes in salivary glands which act to modify the electrolyte composition of primary saliva and cause it to become hypotonic. This was achieved using an isolated mandibular gland preparation by observing the effect of different stimuli on the electrolyte composition of saliva secreted at the same flow rate, on the assumption that these stimuli do not influence primary saliva composition. The effects of the same stimuli on the volume of primary fluid secretion and on protein secretion were also observed. Proteins were measured in total and as individual components after their separation by high-performance liquid chromatography. 2. Acetylcholine was used as a 'Ca2+-mobilizing' agonist (i.e. one which both elevates intracellular Ca2+ concentration and activates protein kinase C). Isoprenaline was initially used to elevate intracellular cyclic AMP concentration but was subsequently abandoned in favour of forskolin. 3. Acetylcholine was a very potent stimulus of primary fluid secretion. By contrast, isoprenaline and forskolin were essentially without effect, even when superimposed on acetylcholine stimulation. 4. As judged by saliva electrolyte composition, increasing the concentration of acetylcholine enhanced ductal absorption of Na+ and Cl- and secretion of K+ (and presumably HCO3-). Forskolin had the opposite effect: when superimposed on submaximal acetylcholine stimulation it caused saliva concentrations of Na+ and Cl- to remain high and K+ low (i.e. it inhibited ductal transport processes). The inhibitory effect of forskolin on ductal transport could be overcome by increasing the concentration of acetylcholine, and vice versa. 5. Acetylcholine, isoprenaline and forskolin each increased salivary protein secretion, although the kinetics of secretion differed. The spectrum of proteins secreted in response to the three stimuli was the same. The relative proportions of the individual proteins was influenced by the strength of stimulation (i.e. the proportions at high total protein output differed from those at low total protein output) but not apparently by the nature of the stimulus. 6. Thus, the three major secretory processes in the rabbit mandibular salivary gland respond differently to the two major signal transduction mechanisms. For primary fluid secretion, Ca2+ is stimulatory and cyclic AMP almost without effect; for ductal transport, Ca2+ is stimulatory and cyclic AMP inhibitory; and for protein secretion both Ca2+ and cyclic AMP are stimulatory.

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