Amylase activity, protein and urea in saliva of the red kangaroo ( Macropus rufus)

Abstract

Parotid and mandibular salivas, produced at flow rates of 0.055 ± 0.0038 to 4.45 ± 0.101 and 0.052 ± 0.0059 to 4.30 ± 0.072 ml min −1 respectively by intracarotid infusion of acetylcholine or methacholine, were Anatysed. During acetylcholine stimulation, the protein concentration ranged from 86.4 ± 10.17 to 235.0 ± 23.5 mg l −1 in parotid and from 102.9 ± 11.54 to 379.6 ± 38.01 mg l −1 in mandibular saliva. The minimum protein concentrations in parotid and mandibular salivas occurred at 0.5 and 1.0 ml min −1 respectively with the concentration rising progressively at lower and higher flow rates. Both glands maintained constant levels of protein during 90 min of sustained stimulation at constant flow rates of 0.5 and 2.0 ml min −1. The optimum pH for salivary amylase activity lay between 7.4 and 7.9 (mean 7.5–7.6). The amylase activity of parotid saliva ranged between 32.2 ± 3.81 and 94.5 ± 24.05 μkat l −1 and the activity/protein ratio was similar at all flow rates (mean, 0.39 ± 0.049 μkat amylase/mg protein). Amylase levels in arterial plasma were constant within experiments but varied from 8.0 ± 0.07 to 26.4 ± 0.24 μkat l −1 between experiments, and were always less than corresponding parotid values. Mandibular amylase activities were low, highly variable or often absent. Methacholine-evoked parotid saliva had lower protein concentrations but similar amylase/protein ratios to acetylcholine-stimulated saliva. Urea concentrations in arterial plasma were constant within experiments but varied from 6.71 ± 0.092 to 14.2 ± 0.14 mmol 1 −1 between experiments. In both salivas, urea was negatively correlated with flow rate, its parotid saliva/plasma ratio ranging from 1.04 ± 0.009 to 0.92 ± 0.015 and its mandibular saliva/plasma ratio ranging between 0.84 ± 0.055 and 0.08 ± 0.007. The low protein concentrations of both salivas are thus adaptations to their specialized functions, parotid saliva being a buffer in the forestomach as in ruminants, and mandibular saliva being used as an external coolant. The low protein levels also allow the concentration to remain constant in both salivas over long periods of parasympathetic stimulation, in contrast to other salivary glands. The parotid gland synthesizes amylase, whereas the mandibular gland may not. Parotid amylase levels are lower than in many eutherian species but the special conditions in the anterior digestive tract may make them significant. The parotid has higher urea permeability and lower fluid re-absorption than the mandibular gland, and is the main route for salivary transfer of urea into the digestive tract.