Impaired growth hormone-releasing hormone signal transduction in the dwarf (dw) rat is independent of a generalized defect in the stimulatory G-protein, Gs alpha.

The homozygous dwarf (dw) rat pituitary is characterized by a 95% reduction in GH content as well as a 75-80% reduction in the number of somatotrophs. The nature of the mutation responsible for this phenotype is unknown. Previous investigations from our laboratory indicate that dw somatotrophs exhibit decreased sensitivity and a reduced GH secretory response to GH-releasing hormone in vitro, accompanied by a decreased generation of cAMP. We hypothesized that dw rats have a defect in the pathway linking the GH-releasing hormone receptor to adenylyl cyclase and focused on the expression and function of the stimulatory G-protein, Gs alpha. When corrected for differences in pituitary size and cell number, GH mRNA content was reduced by 78% compared to that in controls. However, there was no difference in Gs alpha mRNA content or size in dw pituitaries. Similarly, there was no difference in the content or size of mRNA for the pituitary transcription factor pit-1 in dw pituitaries. Immunoblot analysis of pituitary membrane proteins using a Gs alpha-specific antibody revealed no differences in size, quantity, or relative distribution of Gs alpha peptides between control and dw pituitaries. In addition, cholera toxin effectively ribosylated dw Gs alpha, and there were no differences in the size, quantity, or relative distribution of ribosylated membrane proteins between dw and control pituitaries. Finally, to examine for mutations in other regions of the Gs alpha-coding sequence, we cloned the full-length Gs alpha cDNA from dw rat pituitaries by polymerase chain reaction. The sequence of this clone was identical to that of normal rat Gs alpha cDNA. These results indicate that 1) 52-kilodalton Gs alpha appears to predominate in both normal and dw pituitary; 2) the content, function, and sequence of Gs alpha in adult dw rats are normal; and 3) a generalized Gs alpha regulatory or structural mutation as the cause of the observed phenotype can be excluded. The results also demonstrate that there is no decrease in pit-1 expression in the adult dw pituitary.