Effect of endogenous and infused parathyroid hormone on plasma concentrations of recently administered45Ca

Summary The net rate of loss of⁴⁵Ca from plasma was monitored in normal, thyroparathyroidectomized (TPTX), and nephrectomized rats. Also studied was the effect of a 24 h intravenous infusion of PTH in TPTX rats. The⁴⁵Ca was injected after an overnight fast and 1 to 4 h prior to the time the first blood sample was obtained.In normal rats, the net rate of loss of⁴⁵Ca from plasma was slower than in TPTX rats for at least 12 h after radionuclide injection. Nephrectomy did not affect this difference between control and TPTX rats until 8 h after⁴⁵Ca injection or about 18 h after nephrectomy. At this time a reversal occurred and the net rate of plasma⁴⁵Ca loss in nephrectomized rats with parathyroids became faster than in TPTX nephrectomized rats.When PTH was infused at a constant rate (3 mU/g body weight/h) plasma calcium concentrations rose gradually for approximately 12 h after which a constant plasma calcium level was maintained with continued PTH infusion. During the period when plasma calcium concentrations were rising, the net rate of loss of⁴⁵Ca from plasma was slower than in TPTX controls infused only with saline. However, when plasma levels stabilized, plasma⁴⁵Ca disappearance rates changed such that the rate loss of⁴⁵Ca from plasma became faster in PTH-infused than in TPTX controls.It is concluded that PTH increases calcium efflux from the same compartment in bone which receives calcium from plasma, thereby returning some recently deposited⁴⁵Ca to plasma. In the non-equilibrated state, this returns additional⁴⁵Ca to plasma causing anet decrease in the rate of loss of⁴⁵Ca from plasma. However, in the equilibrated condition, when both calcium efflux from bone and influx into bone are in balance but at higher flux rates, PTH stimulates the diffusion of⁴⁵Ca into other bone compartments.