Modulation of calcitonin secretion by modification of calcium channels?

Voltage-dependent calcium channels (VDCC) regulating Ca++ influx through the cellular plasma membrane play a major role in the Ca(++)-induced calcitonin (CT) secretion. Using rat C-cells (rMTC 6-23 cell line), we have studied the effect of repetitive stimulation by either Ca++ (2 mM) or glucagon (10 microM) or epinephrine (10 microM) on CT secretion. Following a Ca(++)-induced initial rise, CT release declined to basal levels after about four hours despite high Ca++; addition of 10 microM glucagon to the "Ca++ desensitized C-cells" yielded the normal stimulatory effect of glucagon on CT release. Repetitive stimulation with glucagon showed a constant stimulatory action over an eight-hour period. In contrast, repetitive stimulation with 10 microM epinephrine caused an initial rise followed by a gradual decline of CT release over six hours. The observed desensitization of Ca(++)-induced CT secretion may be due to a modification of VDCC in C-cells. Whether or not the desensitization of epinephrine-induced CT release occurs independently of the regulation of VDCC remains unclear.