Somatostatin, the growth hormone release inhibitory hormone (Brazeau, Vale, Burgus, Ling, Butcher, Rivier & Guillemin, 1973), NH2-Ala-Gly-Cys-Lys-Asn-Phe-Phe-Trp-Lys-Thr-Phe-Thr-Ser-Cys-Co2H, is the first hypothalamic hormone shown unequivocally to exert a negative control over pituitary secretion. Its inhibition of growth hormone release has been confirmed in normal and diabetic men (Hansen, Ørskov, Seyer-Hansen & Lundbaek, 1973).
The present study was undertaken to determine whether somatostatin can also alter prolactin release. Cultures of secreting pituitary cells offer the least ambiguous model for such a question, because of the enormous population of viable secreting units, the expectation of direct interaction between receptor and hormone, and a level of basal secretion sufficiently high to show a range of changes by radioimmunoassay. Monolayer cultures of enzymically dispersed cells were prepared from whole pituitaries of normal Charles River CD male rats, using Eagle's minimal essential medium for growth (Grant, Clark & Rosanoff, 1973). After test