Acute changes in plasma calcium and⁴⁵Ca were studied in young adult male thyroparathyroidectomized (TPTX) rats injected with moderate doses of parathyroid hormone (PTH). For plasma calcium changes, comparison was made between rats fasted or fed prior to PTH injection. For plasma⁴⁵Ca changes, the effect of the time of administration of the radionuclide was also studied; this included rats injected with PTH 1 h after radionuclide (“1 h⁴⁵Ca”), 18 h later (“18 h⁴⁵Ca”) and more than 6 days later (“6 day⁴⁵Ca”). The results can be summarized as follows: (1) Plasma calcium changes were greater when PTH was injected into “fed” rather than into “fasted” rats. (2) PTH always produced a relative increase (compared to controls tested concurrently) in plasma⁴⁵Ca concentrations. This increase was the same in the “1 h⁴⁵Ca” and the “18 h⁴⁵Ca” groups. (3) Plasma⁴⁵Ca rose at least temporarily following PTH injection in the “18 h⁴⁵Ca” group. (4) The⁴⁵Ca rise following PTH was always greater in “fed” than in “fasted” groups. (5) Plasma⁴⁵Ca specific activities (S.A.) tended to rise in the “6 day⁴⁵Ca” group and to fall in the “18 h⁴⁵Ca” group, following PTH injection. However, the⁴⁵Ca S.A. was always higher in fed than fasted groups. (6) In a few experiments in which³²P was injected with⁴⁵Ca, specific activity changes in plasma⁴⁵Ca following PTH injection werenot accompanied by similar changes in³²P specific activity.

These results could not be adequately explained by PTH effects on bone resorption, but the data supported the postulate that PTH controls plasma calcium concentrations by increasing transport of calcium through the osteocyte-lining cell (osteoblast) bone cell complex from the bone fluid compartment to the ECF.