Factors affecting the secretion of phosphate in parotid saliva in the sheep and goat

The relationship between the concentration of phosphate in plasma and parotid saliva was studied in six conscious sheep and a goat, either intact or thyroparathyroidectomized (t.x.p.t.x.), under conditions designed to minimize marked fluctuations in flow rate of saliva. A linear relationship between acutely induced changes in plasma phosphate concentration and the phosphate level in saliva has been demonstrated in both intact and t.x.p.t.x. animals. Dietary phosphorus depletion caused adaptation of salivary phosphate concentration so that less was secreted at a given concentration of plasma phosphate. Attention is drawn to the similarity between this phenomenon and that already described for the proximal renal tubule. Parathyroid hormone (PTH) was shown to reduce the salivary phosphate concentration with little or no effect on phosphataemia. The administration of 1,25-dihydroxycholecalciferol (1,25(OH)2CC) also caused a reduction in salivary phosphate concentration despite hyperphosphataemia and hypercalcaemia. It is suggested that salivary phosphate concentration can be influenced directly by the concurrent level of plasma phosphate but that this relationship can be modified by the circulating concentration of 1,25(OH)2CC and indirectly by PTH via increased production of 1,25(OH)2CC.