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Influence of flow rate and aldosterone administration on mandibular salivary composition in the koala: (Phascolarctos cinereus)

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Abstract

The possibility of adaptations of mandibular salivary function to a diet of eucalypt leaves and for evaporative cooling was investigated in anaesthetized koalas. Salivary composition, at a range of flow rates up to maximum and during aldosterone administration, was measured in saliva evoked by intravenous infusion of carbamylcholine chloride. Maximum fluid secretion rates per gram tissue (230 ± 18·2 μl/min/g gland) were similar to mandibular glands of other species and about four times the rates/g gland reported for the parotid gland of koalas. Salivary concentrations of Na (6·1 ± 0·91 to 67·7 ± 3·51 mmol·l−1), Cl (7·2 ± 0·71 to 53·0 ± 2·82 mmol·l−1), HCO3 (8·6 ± 0·81 to 19·4 ± 2·13 mmol·l−1) and the osmolality (26·7 ± 2·34 to 132·9 ± 5·47 mosmol.kg−1) were positively correlated with salivary flow rate. The concentrations of urea (1·87 ± 0·034 to 0·71 ± 0·099 mmol·l−1), K (13·0 ± 0·90 to 6·25 ± 0·482 mmol·l1-), Mg (33·6 ± 2·53 to 10·1 ± 4·68 pmol·l−1), H+ (246·2 ± 55·78 to 62·1 ± 8·01 &#x006e;̈mol·l−1) and PO4 (3·46 ± 0·350 to 1·13 ± 0·044 mmol·l−1) were negatively related to flow rate. Total protein (411 ± 44·4 to 656 ± 146·4 mg·l−1) and total Ca (356 ± 45·2 to 467 ± 61·6 μmol·l−1) concentrations were not correlated with flow but were highly correlated with one another. With salivary amylase activity being absent and plasma amylase activity being very low indicating low production rates by other tissues, the koala is likely to have limited ability to digest starch. Intravenous infusion of aldosterone at 5 μg·h−1 for 90 rnin followed by 50 μg·h−1 for 105 min demonstrated that the gland responded rapidly to changes in endogenous mineralocorticoid levels. Mean salivary Na/K ratio before aldosterone administration was 6·9± 201, the ratio had fallen significantly (P < 0·05) after 75–90 min infusion and had fallen to 0·71 ± 0·172 by the last 30 min of infusion.

It was concluded that the mandibular gland of the koala, with its relatively high secretory capacity and its sensitivity to stimulation, would have the major influence on the composition of mixed saliva during low-flow reflex secretion. Additionally, because the gland is responsive to acute changes in mineralocorticoids and produces a markedly hypotonic saliva, the mandibular is better adapted than the parotid gland to be the primary source of saliva for evaporative cooling.

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