Interaction between substance P and gastrin-releasing peptide on thyrotropin secretion by rat pituitary in vitro

The effect of substance P (SP) on thyrotropin (TSH) secretion is controvers ial. In this study we evaluated the effect of SP on TSH secretion by hemipi tuitaries of 9-month-old Wistar rats in vitro and its interaction with gast rin-releasing peptide (GRP) at equimolar concentrations (1 mu M and 10 mu M ). TSH release was measured under basal conditions and 30 min after incubat ion in the absence or presence of SP, GRP or both peptides. Pituitary TSH c ontent was also measured in the pituitary homogenate after incubation. SP a t both concentrations caused a significant (P<0.05) increase in TSH secreti on compared with all other groups, which was approximately 60% (1 mu M) and 85% (10 mu M) higher than that of the control group (23.3 +/- 3.0 ng/ml). GRP at the lower concentration did not produce a statistically significant change in TSH secretion, whereas at the concentration of 10 mu M it produce d a 50% reduction in TSH. GRP co-incubated with substance P completely bloc ked the stimulatory effect of SP at both concentrations. Pituitary TSH cont ent decreased in the SP-treated group compared to controls (0.75 +/- 0.03 m u g/hemipituitary) at the same proportion as the increase in TSH secretion, and this effect was also blocked when GRP and SP were co-incubated. In con clusion, in an in vitro system, SP increased TSH secretion acting directly at the pituitary level and this effect was blocked by GRP, suggesting that GRP is more potent than SP on TSH secretion, and that this inhibitory effec t could be the predominant effect in vivo.