Decrease in Gs protein expression may impair adenylate cyclase activation in old kidneys.

The possibility that alteration in stimulatory guanine nucleotide-binding protein (Gs) expression may contribute to the blunting of renal parathyroid hormone (PTH)-stimulated adenylate cyclase in aged rats was examined. Using rat cDNA probe, we identified a Gs alpha-subunit (Gs alpha) of 1.9 kb. Age did not alter the size of Gs alpha mRNA. The level of Gs alpha mRNA [normalized to poly(A)+ RNA] was decreased 23%, which was consistent with our previous report that Gs alpha protein decreased in senescence. In contrast, mRNA level of Gi alpha 2 increased with age. Level of beta-actin mRNA did not change with age. Nuclear runoff assay was performed to determine the transcription rate of Gs mRNA. Synthesis of poly(A)+ RNA and total RNA was reduced 39% and 37%, respectively, in nuclei prepared from old kidney, which suggested a general decline in RNA synthesis capacity in old rats. Our results also showed the transcription rate of Gs alpha mRNA in aged rats was reduced 89%, a decrease far exceeding that observed for total RNA or poly(A)+ RNA. We concluded that the decrease in steady-state level of Gs alpha mRNA was specific and probably was due to a reduction in the transcription activity. Thus alteration in Gs transcription may contribute, at least in part, to the impaired renal adenylate cyclase activation in aged rats.