Relationship of blood concentrations of calcium, phosphate, gastrin and calcitonin to the onset of feeding in the rat.

Daily fluctuations in plasma calcium concentrations in rats trained to a closely regulated feeding pattern have been compared to corresponding plasma gastrin and calcitonin concentrations. The time period studied was that extending from 4 hr prior to the start of the feeding. Both plasma calcium and phosphate levels fedd prior to the start of the feeding period and remained low at least for the first 2 hr of feeding. This pattern was also observed in rats in which food was withheld for 2 hr past the regular feeding time. Plasma 45Ca and 32P concentrations (radionuclide injected at least one week prior to sampling) did not follow the pattern of their stable counterparts. Instead, these values rose or remained constant until after feeding had commenced, after which they fell precipitously. Both plasma calcitonin and gastrin levels rose rapidly after the start of the feeding period. The primary point of emphasis is that calcitonin secretion was produced in these rats by an intestinal related stimulus and not by a rise in plasma calcium concentration.