Stimulation of secretion of pig thyrocalcitonin by gastrin and related hormonal peptides

The secretion of thyrocalcitonin (TC) was measured in young pigs by radioimmunoassay of thyroid venous effluent blood. Systemic iv injection of synthetic pentagastrin and tetragastrin and of native porcine gastrin in doses of 0.034–3.4 nmoles/kg rapidly increased TC concentrations 3–20-fold in a dose-related fashion. Pancreozymin/cholecystokinin (PZ/CCK, 7.3–12 Crick U/kg) also stimulated TC secretion, while betazole (2 mg/kg) and pancreatic glucagon (68–204 μg/kg) were essentially ineffective. Prolonged infusion of pentagastrin caused initial stimulation of TC secretion, but thereafter TC secretion fell progressively despite continued infusion. Administration of pentagastrin or gastrin directly into the thyroid artery at dose levels ineffective systemically stimulated secretion of TC, indicating a direct action on the thyroid gland. Since tetragastrin as well as pentagastrin stimulated TC secretion and since the tetrapeptide sequence comprises the carboxyl terminus of both gastrin and PZ/CCK and is the active core responsible for hormonal activity on the gastrointestinal tract, it is likely that the tetragastrin sequence constitutes the portion of both gastrin and PZ/CCK responsible for increasing TC secretion. The results are consistent with our earlier hypothesis that gastrin or a related gastrointestinal hormone may play a physiological role in stimulating secretion of TC.