Influence of the calcium ionophores A23187 and X537A on calcitonin secretion from the isolated perfused porcine thyroid

The present study examined the influence of the calcium ionophores A23187 and X537A on the calcitonin (CT) secretory process. The isolated perfused porcine thyroid was used to evaluate ionophore effects on CT secretion and thyroid slices were used to measure 45Ca uptake. Both A23187 and X537A enhanced the rate of CT release from the perfused thyroid. A23187 at a concentration of 19 microM (10 micrograms/ml) produced a maximal increase in CT secretion of 325% above control levels. X537A at a concentration of 16 microM (10 micrograms/ml) produced a peak rise in CT release of approximately 2000% over control levels. The CT secretory response to A23187 was found to be completely calcium dependent; however, the secretory response to X537A was partially, but not completely dependent upon the presence of perfusate calcium. The results demonstrate that these calcium ionophores are very potent CT secretagogues which vary considerably in their calcium dependency.