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The dwarf (dw/dw) rat differs from all other rodent models of GH deficiency in that its pituitary prolactin (PRL) content is normal or even increased. We have now studied this throughout postnatal development, using a combination of immunocytochemistry, RIA and fluorescence-activated cell sorting (FACS) and analysis. Compared with normal Albino Swiss (AS) rats, adult dw/dw rats showed a profound reduction in pituitary GH content accompanied by increased PRL content, significantly so in females (AS vs dw/dw; P&lt;0.01). Somatotroph hypoplasia was evident in the adult dw/dw rats, with most GH(+ve) cells showing weak immunostaining, whereas many more strongly stained PRL cells were evident in pituitary sections from dw/dw rats. Facs analysis confirmed both somatotroph hypoplasia and relative lactotroph hyperplasia in dw/dw rats at all ages studied (9-144 days); the difference in somatotrophs increased with age whereas the difference in lactotrophs declined with age. At 9 days, the percentage of lactotrophs was 10-fold higher in dw/dw rats than in AS rats. Young dw/dw rats also had a higher proportion of mammosomatotrophs than AS rats, although this difference disappeared as the mammosomatotroph proportions increased with age in both strains. GHRH released GH from both dw/dw and as cells maintained in culture for 5 days. The sensitivity to GHRH and the amount of GH released was lower in the dw/dw cultures, mostly explained by their fewer GH cells and lower initial GH content. GHRH increased cAMP in as but not in dw/dw cultures, even when these were greatly enriched for dw/dw somatotrophs by FACS sorting prior to culture. These results suggest that GHRH-induced cAMP stimulation is required for trophic effects on GH synthesis and somatotroph proliferation, but is not required for GHRH-stimulated GH release. The inverse changes in somatotroph and lactotroph numbers suggest that the dw/dw mutation disturbs the mechanism that specifies and retains appropriate numbers of somatotrophs in their differentiated state, and results in a higher proportion of the remaining cells progressing to lactotrophs. The dw/dw phenotype is thus not confined to somatotrophs.

Increased lactotrophs despite decreased somatotrophs in the dwarf (dw/dw) rat: a defect in the regulation of lactotroph/somatotroph cell fate?