Influence of Mg2+ on the in vitro responsiveness of adenylate cyclase from hearts of aging rats.

The influence of [Mg2+] on the basal or stimulated activity of adenylate cyclase from the hearts of young (1 month old) and aged (24 months old) rats has been investigated in vitro. The basal activity of cardiac adenylate cyclase, and its responsiveness to stimulatory or inhibitory effectors, declined with age. This is probably due to alterations at the catalytic moiety of the signal transduction system, such as an impairment in the affinity of the catalytic moiety for ATP and a lower capacity of the catalytic moiety to bind activated stimulatory (Gs) or inhibitory (Gi) guanine nucleotide binding proteins. Compared to the enzyme from the heart of aged rats, unstimulated adenylate cyclase from the heart of young rats was more sensitive to an increase in [Mg2+] in the incubation mixture, as shown by a greater increase in basal activity and in the affinity of the enzyme for ATP. An increase in [Mg2+] counteracted the inhibitory effect of spermine on adenylate cyclase more effectively in young rats than in aged rats. On the other hand, an increase in [Mg2+] facilitated the stimulation of adenylate cyclase by Gpp(NH)p, isoproterenol and forskolin more in aged rats than in young rats. GDP beta S prevented the positive effect of high [Mg2+] on the stimulation of adenylate cyclase by forskolin, suggesting that an increased [Mg2+] favors the activation of Gs or the formation of functional complexes between the catalytic moiety and Gs. We suggest that aging leads to a higher requirement for Mg2+ at the allosteric site on the catalytic moiety whose occupancy is essential for the full expression of stimulated activity.