Phosphorus homeostasis in sheep. III.* Relationship between the amount of salivary phosphorus secreted and the quantities of phosphorus excreted via the urine and faeces

Abstract

The interrelationships between the urinary and faecal pathways for phosphorus excretion in sheep have been examined under conditions allowing direct proportional control of the major portion of endogenous phosphorus secretion to the gut which is contributed by the parotid salivary glands. Two rumen-fistulated Merino wethers were fitted with permanent bilateral re-entrant parotid duct cannulas which allowed continuous collection and return of parotid saliva. During experimental periods, a proportion of the phosphorus secreted by the parotid glands during control periods was returned to the rumen in artificial saliva, and the remaining quantity not returned in this manner was infused intravenously over a corresponding time period. Diversion of the secreted parotid salivary phosphorus from the rumen to the blood led to an increase in urinary phosphorus excretion which was proportional to the fractional amount of secreted phosphorus withheld from the rumen. Phosphorus balance remained unaltered, which indicated a virtual substitution between the two excretory pathways. The increased urinary excretion of phosphorus is attributed partly to increased plasma phosphorus levels and partly to a reduced efficiency of renal tubular reabsorption of phosphorus. No direct evidence for any hormonal action could be seen. The data show that the pathway of excretion of phosphorus can be determined quantitatively by the rate of endogenous phosphorus secretion into the gut. This mechanism is of fundamental importance among factors contributing to phosphorus homeostasis in sheep. ____________________ *Part II, Aust. J. Agric. Res., 25: 485 (1974).