Secretion rates and composition of parotid saliva in the koala (Phascolarctos cinereus)

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Abstract

Parotid salivation was stimulated in anaesthetized koalas by intravenous infusion of carbamylcholine chloride to ascertain flow rates, composition and possible adaptations of salivary function to a diet of eucalypt leaves. Maximum fluid secretion rates per gram tissue (62.0 ± 4.89 μl/g gland/min) were very low but, because the gland is relatively large, maximum flow rates (1.14 ± 0.105 ml min−1) were comparable with other herbivores when scaled for animal size. Salivary concentrations of protein (1.34 ± 0.165 to 3.35 ± 0.298 g 1−1), Na (42.0 ± 4.19 to 142.9 ± 1.78 mmol 1−1), Ca (0.97 ± 0.152 to 2.21 ± 0.300 mmol 1−1), HCO3 (14.3 ± 2.18 to 78.4 ± 2.48 mmol 1−1), Cl (10.6 ± 1.29 to 58.6 ± 1.16 mmol 1−1) and the osmolality (107.0 ± 7.67 to 273.9 ± 1.31 mosmol kg−1) were positively correlated with salivary flow rate. The concentrations of urea (4.22 ± 0.345 to 1.71 ± 0.386 mmol 1−1), K. (25.1 ± 3.04 to 8.92 ± 0.71 mmol 1−1), Mg (206.1 ± 42.78 to 18.3 ± 0.64 μmol 1−1), H+ (273.6±51.23 to 17.5 ± 1.16 μmol 1−1) and PO4(29.6 ± 2.78 to 4.3 ± 0.39mmol 1−1) were negatively correlated with flow rate. The functional significance of the high buffer capacity of the saliva is unclear as koalas do not foregut ferment. The saliva lacked amylase activity entirely and plasma amylase activity was very low indicating low production rates by other tissues, both of which are probable adaptations to the low starch levels of the eucalypt diet. Salivary Na/K ratios were unaltered by aldosterone infusion (3 h at 5 and 50 μg h−1) which was interpreted as an indication that the koala gland, like the macropod parotid, requires long exposure to high mineralocorticoids to sensitize the gland.

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