Acute and long-term effects of gastrin on muscle and bone calcium uptake in rats

It has previously been proposed that the hypocalcemic action of gastrin was mediated by a gastric peptide gastrocalcin which enhanced bone uptake of calcium (Ca). The present study, thus, aimed to reconfirm the effect of gastrin on muscle and bone Ca uptake and to evaluate a possible direct effect of gastrin on tissue Ca uptake. First, we showed that human and rat gastrin (100 mu g/100 g body weight (bw), s.c.) acutely increased uptake of intragastrically administered Ca-45 by the tibia and femur. Interestingly, human gastrin was more effective and also enhanced Ca-45 uptake by the gastrocnemius muscle. In order to find out whether gastrin could act directly on bone, 10 mu g of rat gastrin/100 g bw and Ca-45 were injected into the right femoral artery supplying the right leg while the left leg received normal saline and an equal amount of Ca-45. At 30 min, the right gastrocnemius muscle and femur were found to contain 168% and 162%, respectively, more Ca-45 than the left leg. In contrast, daily administration of 25 mu g of rat gastrin/100 g bw, s.c., for 2 weeks had no effect on muscle or bone total Ca content while the content of Ca-45 given on days 12 and 13 was lower in the tibia and femur of gastrin-treated rats. It could be concluded that gastrin transiently enhanced Ca uptake by muscle and bone. However, two-week treatment with gastrin appeared to accelerate the bone turnover rate i.e., enhanced uptake and release of calcium, so that there were no net changes in bone total Ca content at the end of the experimental period.