The effects of a liquid diet on initial and sustained, stimulated parotid salivary secretion and on parotid structure in the rat.

Adult male rats were maintained on a nutritionally adequate liquid diet, or laboratory chow and water (controls), for 7 days. They were then anaesthetized and parotid flow was recorded after isoprenaline or pilocarpine stimulation, each collected over two, timed sampling periods--an initial 5 min and a subsequent 15 min. The isoprenaline-induced flow rates in liquid diet rats were reduced to 45 and 30% of those in control rats for the first and subsequent samples respectively (p less than 0.02). After pilocarpine stimulation there were no significant differences in the first samples, but in the subsequent samples the flow rate in liquid diet rats was reduced to 54% of that in controls (p less than 0.001). The parotid gland weights were reduced by 35% in liquid diet rats compared to controls (p less than 0.05). On computerized planimetric analysis, parotids (from rats that had not been given secretagogues) had significantly smaller mean acinar areas, mean acinar profile perimeters and mean acinar transection diameters after liquid feeding (p less than 0.001). The findings support the notion that there is a functional reserve capacity available in atrophied glands to support the relatively fast flow that occurs on initial stimulation but which becomes exhausted during sustained stimulation. This exhaustion occurs more severely and more rapidly in the low-volume, protein-rich saliva elicited by sympathomimetic stimulation than in the high-volume, low-protein saliva formed in response to parasympathomimetic stimulation.