Effects of parathyroid hormone and parathyroid hormone-related protein on the rates of absorption of magnesium, calcium, sodium, potassium and phosphate ions from the reticulo-rumen of sheep.

Not only parathyroid hormone (PTH) but also parathyroid hormone-related protein (PTHrP) may play a role in calcium and phosphate homeostasis in ruminants. In five trained sheep, each with a large rumen cannula, the isolated rumen wash technique was used to measure electrolyte absorption rates from the rumen. After two control periods of measurement, 20 micrograms PTH(1-34) (n = 4), or PTHrP(1-34) (n = 5), was injected intravenously as a loading dose, followed by an infusion of 0.67 micrograms min-1 over 2 h. Both PTH(1-34) and PTHrP(1-34) significantly increased the absorption rates of calcium and phosphate from the reticulo-rumen. The increases in the absorption rates of magnesium, sodium and potassium observed were not significant. The same technique was also used to demonstrate that increasing the intraruminal calcium concentration from 1 to 4 mmol l(-1) caused corresponding increases in the net rate of absorption of both calcium and inorganic phosphate from the reticulo-rumen. Rumen epithelium was taken from four sheep and mounted in Ussing chambers so that fluxes of calcium could be measured in both directions using 45Ca. It was found that the addition of 100 ng ml-1 PTH(1-34) or PTHrP(1-34) to the serosal side increased the net calcium flux rates across the ruminal epithelium. It is concluded that both PTH and PTHrP can influence calcium and phosphate homeostasis in sheep not only by their recognized actions on bone and kidney but also on the absorption of these ions from the forestomachs.