| To examine the effect of aging and exercise mode on serum immunoreactive growth hormone (IGH) concentration vs. bioactive growth hormone (BGH) concentration during and after exercise, nine younger and eight older active women (ages, mean $pm$ 1 SEM: 23.7 $pm$ 1.0 and 61.6 $pm$ 1.3 yrs, respectively) performed acute bouts of both aerobic exercise (AE) and heavy resistance exercise (HRE). Exercise lasted 45 min and consisted of cycling at 70% of peak aerobic capacity or total-body weight training at 10-repetition maximum resistances. Venous serum samples obtained every 5 min during and for 30 min after exercise were pooled for analysis. Determinations of IGH (immunoradiometric assay) and BGH (rat tibial assay) concentrations were performed on the pooled serum before (neat) and after Sephadex fractionation into ranges of low (3-30kD), mid (30-55kD), and high ($>$55kD) molecular weight (MW). Neat serum IGH but not BGH concentration was significantly (p $leq$ 0.05) attenuated with age in both AE and HRE. Regardless of exercise mode, serum fractionation revealed an age-related shift of IGH activity from the low into the mid MW range and a shift of BGH activity from the low into both the mid and high MW ranges. In the younger women only, neat serum BGH concentration was higher during and after HRE vs. AE despite an equal IGH response between exercise modes. Although there were no differences in the fraction distribution of IGH between exercise modes in either age group, the proportion of BGH activity occurring in the mid MW range was significantly higher in HRE than in AE in the younger women. These data indicate that, during and after both AE and HRE in women, there may be an age-related shift from lower to higher MW serum GH variants resulting in a reduced serum GH immunoreactivity without a concomitant reduction in serum GH bioactivity. Furthermore, younger women may exhibit a higher serum BGH concentration during and after HRE vs. AE due to either (1) the presence in serum of a 30-55kD potentiating factor in response to HRE or (2) the presence in serum of a 30-55kD inhibitory factor in response to AE. |
