An interpretation of acute changes in plasma 45Ca following parathyroid hormone administration to...

Acute changes in plasma calcium and 45Ca were studies 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 plasma 45Ca 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 h 45Ca"), 18 h later ("18 h 45Ca") and more than 6 days later ("6 day 45Ca"). 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 plasma 45Ca concentrations. This increase was the same in the "1 h 45Ca" and the "18 h 45Ca" group. (3) Plasma 45Ca rose at least temporarily following PTH injection in the "18 h 45Ca" group. (4) The 45Ca rise following PTH was always greater in "fed" than in "fasted" groups. (5) Plasma 45Ca specific activities (S.A.) tended to rise in the "6 day 45Ca" group and to fall in the "18 h 45Ca" group, following PTH injection. However, the 45Ca S.A. was always higher in fed than fasted groups. (6) In a few experiments in which 32P was injected with 45Ca, specific activity changes in plasma 45Ca following PTH injection were not accompanied by similar changes in 32P 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.