Action of phosphorus on calcium release in isolated perfused rat tails

A bone perfusion system using isolated rat tails was developed to study the action of inorganic phosphorus (Pi) on calcium (Ca) release in bone. Rats were either pre-labelled with 45Ca to study calcium removal from bone or 45Ca was added to the perfusate to study Ca deposition into bone. When perfusate Pi was increased from 3.5 to 5.25 mg/dl, a marked inhibition of 45Ca release was demonstrated. 85Sr microspheres, used to demonstrate differences in the flow of perfusate between bone and soft tissue, showed that the Pi effect was not due to alterations in perfusate flow to bone. Increasing the perfusate Pi concentration from 3.5 to 7 mg/dl did not increase 45Ca deposition into bone, nor did it affect baseline or PTH-stimulated cyclic AMP production. The Pi effect was inhibited by reducing the perfusate temperature to 4 degrees C, by using an anerobic perfusate, adding iodoacetate to the perfusate, and finally, by pre-treating the rats with actinomycin D prior to the experiment. Thus, in this model an increase in the Pi concentration of the perfusate inhibits Ca release from bone but does not increase Ca uptake by bone. The inhibitory effect appears to be dependent on normal cell metabolism.