EFFECT OF SOMATOSTATIN ON THE RELEASE OF GONADOTROPINS IN MALE-RATS

Since somatostatin, the growth hormone release-inhibiting hormone, has inhibitory actions in many cell types and is delivered to the anterio r pituitary gland via the hypophysial portal vessels, as well as being synthesized by cells within the gland, we tested the hypothesis that it might inhibit the release of gonadotropins from anterior pituitarie s in vitro. Consequently, the effect of somatostatin on gonadotropin r elease from incubated anterior pituitaries of male rats with and witho ut the stimulatory action of luteinizing hormone-releasing hormone (LH RH) was studied. After a preincubation period of 1 hr, hemipituitaries from adult male rats were incubated in fresh Krebs-Ringer bicarbonate (KRB) buffer in a Dubnoff incubator with an atmosphere of 95% O-2-5% CO2 at 37 degrees C for 3 hr, Incubation with somatostatin (10(-6), 10 (-7), and 10(-8) M) had no effect on basal release of either LH or fol licle-stimulating hormone (FSH), However, somatostatin (10(-6)-10(-8) M) suppressed LHRH (1.7 x 10(-8) M)-induced release of LH (P < 0.01 to P < 0.0001), but not FSH, Furthermore, somatostatin antiserum (1:1000 ) had no significant effect on basal LH or FSH release, whereas incuba tion with the antiserum plus LHRH (1.7 x 10(-9) or 1.7 x 10(-8) M) inc reased LH (P = 0.015 and P = .005, respectively), but not FSH release, In summary, our results suggest that somatostatin exerts a physiologi cally significant inhibitory effect on LH but not FSH release in the p resence of LHRH in vitro. Presumably, somatostatin is secreted in vitr o by pituitary cells since not only have anterior pituitaries of rats been shown to contain somatostatin, but also somatostatin mRNA, Somato statin then diffuses to the LH gonadotropes, where it exerts its inhib itory action, However, the release of somatostatin is insufficient to alter basal in vitro release, On the other hand, at least at the conce ntrations employed, there was no significant effect either of somatost atin or the antiserum to alter basal or stimulated FSH release.