Handling of phosphate by a parotid gland (ovine)

The handling of phosphate by the sheep's parotid gland was investigated by examining the effects on the composition of saliva of phosphate depletion, secretomotor nerve stimulation, enrichment of phosphate in the blood supply, and parotid arterial infusion of parathyroid hormone (PTH) and calcitonin. Experiments were performed in anesthetized sheep after vascular isolation of the parotid gland. In phosphate depletion arterial plasma phosphate fell from 1.50 +/- 0.12 to 0.73 +/- 0.08 mM. In phosphate repletion salivary phosphate ion was concentrated 8- and 16-fold compared with parotid arterial and venous plasma, respectively. In depletion salivary phosphate concentration was unchanged at slow salivary flow rate but was significantly lower at high flow rate. Enrichment of phosphate in the arterial blood increased salivary phosphate concentration in both situations. Synthetic bPTH (1-34), highly purified bPTH (1-84), and bovine parathyroid extract were equipotent in increasing salivary phosphate concentration in phosphate repletion and had no effect in phosphate depletion. They also increased parotid blood flow. The rise in phosphate with PTH was conditional on the rise in blood flow, but the same rise in blood flow alone did not reproduce the effect of PTH on salivary phosphate. PTH increased work done and oxygen consumption by the gland but arterial phosphate enrichment did not. Increases in salivary phosphate were associated with decreases in salivary bicarbonate, and the sum [HCO-3] + [Cl-] + (HPO2-4] remained almost constant. Micropuncture studies have shown that ovine parotid ducts do not normally reabsorb phosphate and therefore PTH probably increases phosphate secretion in the endpieces.