CHRONIC TREATMENT WITH ESTROGEN UP-REGULATES EXPRESSION OF SST(2) MESSENGER-RIBONUCLEIC-ACID (MESSENGER-RNA) BUT DOWN-REGULATES EXPRESSION OF SST(5) MESSENGER-RNA IN RAT PITUITARIES

We previously showed that 17 beta-estradiol (E-2) induces the somatost atin (SRIF) responsiveness of the rat anterior pituitary, which leads to inhibition of PRL secretion through E-2-dependent SRIF receptors. T o examine receptor subtypes regulated by E-2, we determined the messen ger ribonucleic acid (mRNA) levels of all subtypes using a semiquantit ative RT-PCR and characterized pituitary membrane receptors using subt ype-preferential SRIF analogs. Most of the SRIF receptor subtype mRNAs were sst, and sst(2A) in ovariectomized rat pituitaries [sst(5)/glyce raldehyde 3-phosphate dehydrogenase (GAPDH) = 1.4 x 10(-2), sst(2A)/GA PDH = 0.4 x 10(-2)]. The expression pattern of the subtypes in ovariec tomized rat pituitaries was similar to that of normal male and female rat pituitaries, although the mRNA levels of sst(5) and sst(2A) in mal e rat pituitaries were higher than in females. chronic administration (4 weeks) of E-2 to the ovariectomized rats increased mRNA expression of sst(2A), sst(2B), sst(3), and sst(1) and drastically decreased expr ession of sst,; the transcripts of sst, isoforms constituted 87% of to tal SRIF receptor subtype mRNAs (sst(2A)/GAPDH = 1.2 x 10(-2)), wherea s the sst(5) mRNA level was less than 1%. Receptor-binding studies rev ealed that in pituitaries from both ovariectomized rats and male rats, heterogeneous receptor types, probably sst(5) and sst(2), were expres sed, whereas receptors from E-2-treated rat pituitaries mostly exhibit ed characteristics of the sst(2) subtype. The results demonstrated tha t sst(5) and sst(2A) were the major subtypes expressed in normal rat p ituitaries with a sex-dependent difference and that whereas E-2 up-reg ulates the expression of sst(2) isoforms, it down-regulates the expres sion of sst(5), suggesting roles for these subtypes in the control of pituitary functions.