The overall objectives of the proposed project is to investigate and clarify the pharmacological and physiological interrelationships between calcium and the secretion and effects of calcemic and gastrointestinal (GI) hormones. The goal is to relate the findings to calcium homeostasis and gastrointestinal function in health and disease. Primary emphasis will be placed on secretion, synthesis and, actions of calcitonin (CT), gastrin, and parathyroid hormone (PTH), but studies will also involve several other gastrointestinal hormonal peptides (e.g., cholecystokinin, somatostatin and bombesin). Problems to be investigated include: 1. In vivo in the pig: a) Effects of various GI hormones, esp. gastrin, bombesin and somatostatin, on secretion of CT and the mechanisms involved. b) Possible physiological significance of effects of PTH, CT, somatostatin, and bombesin, on antral gastrin secretion and the mechanisms involved. In vivo in the rat: a) Application of an immunoassay for rat PTH and its use in conjunction with that for rat CT to explore potential GI factors affecting release of calcemic hormones during feeding - esp. in suckling baby rats. b) Examination of possible detrimental skeletal consequences of CT removal in early life in the rat. 3. Evaluation of concurrent secretion of rat CT and PTH in vitro to elucidate mechanisms involved in biosynthesis and release of PTH and CT and to study putative secretagogues and endogenous factors which may influence secretion of CT and PTH.