Figure 7. Differential modulation of ROV in GM branch orders by αARs in young versus old mice .

Peak ROV (% max) in FA, 1A, 2A and 3A following 100, 250, 500 and 1000 ms contraction at 100 Hz in the GM of young and old mice. Peak ROV increased as contraction duration increased in both age groups. Stimulation of αARs with NA (10⁻⁹  m) attenuated peak ROV in young mice but not in old mice. In contrast, inhibition of αARs with phentolamine (Phentol, 10⁻⁶  m) enhanced peak ROV in old mice but not in young mice. The effect of αAR modulation on peak ROV tended to be greater in FA and 1A than in 2A and 3A. ^Δ P < 0.05, main effect of branch order. ^# P < 0.05, main effect of Phentol or NA. ^ψ P < 0.05, NA vs. control in designated branch order. ^Φ P < 0.05, Phentol vs. control in designated branch order. Summary data are means ± SEM, n = 6 per age group.