Epinephrine-and glucagon-sensitive adenylate cyclases of rat liver during aging. Evidence for membrane instability associated with increased enzymatic activity

The influence of post-maturational aging on the activity and stability of rat liver epinephrine- and glucagon-sensitive adenylate cyclases (ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1) was studied in liver homogenates from male and female Wistar rats ages 3, 12, and 24 months. Enzyme activity was measured in the unstimulated (basal) state and with fluoride activation (5 mM) as well as with a range of concentrations of glucagon and epinephrine. Basal activity was the same at 3 and 12 months but at 24 months was 1.4 times higher in both sexes. With fluoride and glucagon (1 nM-10 μM) activities were slightly but significantly greater (1.2 times) at 24 months vs. 3 and 12 months in females but not in males. In contrast, epinephrine-stimulated activity doubled from 3 to 24 months in both males and females, with most of the increase occurring between 12 and 24 months. The concentration of epinephrine necessary for 50% activation was the same (0.25 μM) at all ages, showing that increased epinephrine response during aging is not due to altered affinity of receptors for hormone. Increased epinephrine-sensitive activity was due either to increased numbers of enzyme catalytic units or to an increase of V but not to altered K_m for substrate which was unchanged with age for both hormone- and fluoride-activated enzyme. Stability of the membrane-bound enzyme was examined in homogenates kept at 0°C for 24 h in the presence of protease and esterase inhibitors. Loss of adenylate cyclase activity was significantly correlated with increasing age. At each age epinephrine activity was least stable, glucagon activity was most stable, and fluoride activity was intermediate. Glucagon-sensitive experimental deficiency of essential fatty acids, appears to correlate with decreased hormone responsiveness in vivo [42]. Moreover, during dietary manipulations altered fat cell adenylate cyclase activity is similarly associated with parallel changes of hormone-induced lipolysis [43].