Membranes were prepared from the submandibular salivary glands of male young (3-months-old) and aged (24-months-old), Fisher 344 rats, and assayed for adenylate cyclase activity and for the ribosylation of G-proteins in the presence of [P-32]-NAD+ and cholera toxin. Adenylate cyclase activity (basal, fluoride-, forskolin-, and isoproterenol-stimulated) was significantly lower (p < 0.01) in the glands of aged rats. The pattern of ribosylation was different between the 2 groups. Two cholera toxin-specific bands (M(r) 42,000 and 44,000) were ribosylated in the glands of young rats whereas in aged rats, only an M(r) 42,000 band was clearly observed. The incorporation of P-32 from NAD+ into the M(r) 42,000 and 44,000 bands increased with the time of ribosylation (7.5-90 min). In aged rats, however, the incorporation of P-32 into the M(r) 42,000 band was much slower and did not increase much after 60 min of ribosylation. At each time point, the extent of ribosylation was lower in the membranes from the aged rats. These findings suggest that the decrease in adenylate cyclase activity in the submandibular salivary glands of aged rats may be due partly to the changes in their levels of G(s) as a result of ageing.
