A CRYPTIC PEPTIDE (160-169) OF THYROTROPIN-RELEASING-HORMONE PROHORMONE DEMONSTRATES BIOLOGICAL-ACTIVITY INVIVO AND INVITRO

TRH is synthesized as a precursor peptide containing five copies of the sequence Gln-His-Pro-Gly, QHPG, flanked by paired basic amino acids, and linked by other peptides. We tested one cryptic peptide, PPT (160-169, SFPWMESDVT), as a possible physiological regulator of pituitary activity in vivo. Male rats were cannulated (jugular) and received a single dose of either PPT or TRH (10(-8)-10(-6) m). PPT caused no consistent effects on either TSH or PRL secretion, while TRH stimulated the secretion of both hormones. However, PPT stimulated a dose-dependent increase in both pituitary TSHbeta and PRL mRNA content at 240 min similar to TRH. In primary cultures of rat pituitaries, PPT stimulated a maximum 4-fold increase in TSHbeta mRNA and a 2-fold increase in PRL mRNA in 4 h, while TRH increased both TSHbeta and PRL mRNA approximately 3-fold. Again, PPT had no significant effect on TSH or PRL secretion into the medium. Thus, PPT appears to be a physiological regulator of both TSH and PRL synthesis, but, unlike TRH, does not act as a secretagogue.