CALCITONIN IS A PHYSIOLOGICAL INHIBITOR OF PROLACTIN SECRETION IN OVARIECTOMIZED FEMALE RATS

Calcitonin (CT) inhibits secretion of PRL when administered intravenou sly in rats and humans. It also inhibits PRL release from cultured rat anterior pituitary (AP) cells. Recent evidence suggests that CT-like immunoreactive peptide is synthesized and released from the AP gland. However, its physiological role in the regulation of PRL secretion has not been understood. Present studies tested the role of endogenous pi tuitary CT (pit-CT) in the regulation of PRL secretion in, vivo by pas sive immunization. In the first group of experiments, ovariectomized ( ovx) adult female rats were administered either preimmune or anti-salm on CT (sCT) serum, and their serum PRL levels were analyzed at various time points up to 3 h. A second group of experiments examined the eff ects of anti-sCT serum and dopamine on PRL release from cultured rat A P cells. In the next group of experiments, the regional distribution o f pit-CT secretion was examined in different sections of the AP gland. In the last set, CT-like activity of AP extract was tested in neonata l rat kidney cells, which respond to CT with an increase in cAMP accum ulation. These experiments also tested whether anti-sCT serum reduces AP extract-induced increase in cAMP accumulation. The results suggest that anti-sCT serum dramatically increased serum PRL levels (by 5-fold ) of ovx rats within 30 min of administration. The serum PRL levels de clined gradually after the peak. However, a significant increase in se rum PRL levels was maintained by the anti-sCT serum for the duration o f the experiment. The antiserum also induced a significant increase in PRL release from cultured AP cells when added in the presence or abse nce of dopamine. The distribution profile of pit-CT within the AP glan d suggests that the release of pit-CT immunoreactivity was significant ly greater in the inner sections, and anti-sCT serum also caused great er increase in PRL release in these sections. However, outer sections also secreted pit-CT at lower concentrations. Finally, AP extract and sCT stimulated cAMP accumulation in neonatal rat kidney cells, and ant i-sCT serum significantly reduced AP extract-induced cAMP accumulation . These results demonstrate that pit-CT is an important regulator of t onic PRL secretion in female rats and can potently inhibit PRL secreti on even in the presence of dopamine.