Effect of dietary calcium on secretion of calcitonin and gastrin in rats with medullary thyroid carcinoma.

Abstract

Rats transplanted with medullary thyroid carcinoma (MTC) were divided into 3 groups which received for 34 weeks diets with low (0.09%), medium (0.40%) or high (1.80%) calcium content with a calcium/phosphate ratio 2:1. After 6 weeks the different calcium regimens produced corresponding variations in serum calcium and inverse changes in serum magnesium. No alterations in serum proteins were observed. Serum immunoreactive calcitonin (iCT) was unaltered until week 28 of the experiment, but thereafter increased rapidly by about 4-fold. No difference in serum iCT was observed between the 3 groups at any time. Immunoreactive serum gastrin was reduced in rats on medium and low calcium diet compared to those on a high calcium diet. After 28 weeks, when serum iCT was rising, gastrin values fell in rats on high calcium diet to levels approaching those in the other groups. With increased serum iCT (weeks 28-34) serum gastrin concentrations were low and equal in the 3 groups despite different serum calcium concentrations. Electronmicroscopic examination of tumors from the various diet groups did not reveal significant ultrastructural differences. low, medium and high calcium intake gave corresponding changes in serum calcium. Diet-induced hypercalcaemia was not normalized by elevated serum iCT. In contrast, serum immunoreactive gastrin maintained by high calcium diet was suppressed by hypercalcitonaemia.