A radioimmunoassay for rat calcitonin was developed, which used a heterologous system that involved labelled human calcitonin, an antibody developed against purified human calcitonin and purified or synthetic rat calcitonin as a standard. The sensitivity of the assay was 0·3 ng rat calcitonin/ml plasma. The levels of calcitonin in the plasma of pregnant rats were studied in the last week of gestation and compared with the levels found in the plasma of control virgin female rats. The level of calcitonin in the plasma after 17·5 days of gestation was already higher than that in the control rats, it increased further up to 19·5 days and decreased at 21·5 days. Infusion of calcium into pregnant rats for 1 h after 21·5 days of gestation induced a sixfold increase in the level of calcitonin in the plasma after 30 min, and a threefold increase after 1 h.
The level of calcitonin in the plasma of fed and unfed newborn rats was measured 2, 6, 12 and 16 h after birth. At each stage, the concentration of calcitonin was higher in fed than in unfed animals. Stomach gavage of fat (triglycerides) in fasted rats born 2 h earlier stimulated secretion of calcitonin; administration of pentagastrin or glucagon had no effect on the level of calcitonin in the plasma. Subcutaneous injection of a calcium load to fasted newborn rats (2 hours old) increased the level of calcitonin in the plasma after 30 min.
These results demonstrate that during gestation calcitonin has a comparable role in rats to that already found in ruminants. They also substantiate the part played by the fat present in milk as a stimulus (through some unknown gastrointestinal hormones) for secretion of calcitonin during suckling, and the involvement of calcitonin secretion in the regulation of nutrient absorption in the newborn rat.
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