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

The possibility that alteration in stimulatory guanine nucleotide-binding protein (G_s) 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 G_s α-subunit (G_sα) of 1.9 kb. Age did not alter the size of G_sα mRNA. The level of G_sα mRNA [normalized to poly(A)⁺ RNA] was decreased 23%, which was consistent with our previous report that G_sα protein decreased in senescence. In contrast, mRNA level of G_iα2 increased with age. Level of β-actin mRNA did not change with age. Nuclear runoff assay was performed to determine the transcription rate of G_s 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 G_sα 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 G_sα mRNA was specific and probably was due to a reduction in the transcription activity. Thus alteration in G_s transcription may contribute, at least in part, to the impaired renal adenylate cyclase activation in aged rats.