EFFECTS OF CHOLINERGIC STIMULATION AND ALDOSTERONE ADMINISTRATION ON SALIVARY PAROTID SECRETION IN THE BRUSHTAIL POSSUM, TRICHOSURUS-VULPECULA

Parotid salivation was stimulated by infusion of bethanechol chloride into anaesthetized brushtail possums to ascertain maximal flow rates, salivary composition and dietary adaptations of salivary function. Sec retion rates for one gland ranged from 5.3 +/- 0.16 to 84.3 +/- 3.20 m u l/min (2.4 +/- 0.07 to 37.8 +/- 1.43 mu l/min per kg body weight). S alivary osmolality (160.8 +/- 15.39 to 248.2 +/- 8.70 mosmol/kg) and t he concentrations of Na (63.1 +/- 10.93 to 124.1 +/- 5.52 mmol/l) and HCO3 (19.5 +/- 3.41 to 89.0 +/- 3.19 mmol/l) were positively correlate d with flow rate. The concentrations of urea (7.8 +/- 0.64 to 4.6 +/- 0.33 mmol/l), PO4 (2.6 +/- 0.25 to 0.96 +/- 0.10 mmol/l), H+ (46.2 +/- 16.30 to 9.0 +/- 1.51 nequiv/l), K (21.4 +/- 3.73 to 13.7 +/- 2.33 mm ol/l), Ca (3.7 +/- 0.53 to 2.2 +/- 0.27 mmol/l) and Mg (0.3 +/- 0.07 t o 0.04 +/- 0.007 mmol/l) fell with increasing how rate. The relations between flow rate and amylase activity (22.6 +/- 10.47 to 10.5 +/- 3.6 8 mu kat/l), protein concentration (2.7 +/- 0.61 to 1.3 +/- 0.28 g/l), and Cl concentration (62.1 +/- 6.88 to 50.6 +/- 6.37 mmol/l) were inc onsistent between experiments. Salivary Na/K ratios were not decreased by infusion of aldosterone (2.2-22.2 nmol/h for 5 h), showing that th e gland of Na-replete possums is unresponsive to short-term increases in mineralocorticoids. The low salivary amylase activity, less than th at of kangaroos, presumably reflects the interaction of evolutionary h istory of the possum with its natural low-starch diet. The excretion c urves and concentrations of major electrolytes in possum saliva more c losely resemble those found in parotid saliva of potoroine marsupials than those of the kangaroo parotid and thus support the conclusion tha t evolution by kangaroos of parotid saliva, characteristic of foregut fermenters, started after the Macropodoidea split away from other dipr otodonts, not before.