Nitric oxide synthase inhibition during treadmill exercise reveals fiber-type specific vascular control in the rat hindlimb

Abstract

The control of vascular tone during exercise is highly complex and integrated. Specifically, in regards to the contribution of nitric oxide (NO), the observed magnitude and muscle fiber-type dependency of the NO contribution to exercise hyperemia may differ depending on the timing of NO synthase (NOS) inhibition with respect to the exercise bout (i.e., administration prior to vs. during exercise). We tested the hypothesis that, in the presence of prior cyclooxygenase inhibition (indomethacin, 5 mg/kg(-1)), NOS inhibition (N(G)-nitro-L-arginine methyl ester, L-NAME; 10 mg/kg) administered during submaximal treadmill exercise would blunt blood flow and vascular conductance (VC) in the hindlimb muscle(s) of the rat with the greatest reductions in blood flow and VC occurring in the predominantly oxidative muscles. Adult female Wistar rats (n = 10, age: 3-4 mo) ran on a motor-driven treadmill (20 m/min, 10% grade). Total and regional hindlimb muscle blood flow and VC were determined via radiolabeled microspheres before (control) and after L-NAME administration during exercise. L-NAME reduced (P < 0.05) total hindlimb muscle blood flow (control: 123 + or - 10, L-NAME: 103 + or - 7 ml x min(-1) x 100g(-1)) and VC (control: 0.95 + or - 0.09, L-NAME: 0.63 + or - 0.05 ml x min(-1) x 100g(-1) x mmHg(-1)). There was a significant correlation (r = 0.51, P < 0.05) between the absolute reductions in VC after L-NAME and the percent sum of type I and IIa fibers in the individual muscles and muscle parts; however, there was no correlation (P = 0.62) when expressed as blood flow. Surprisingly, the highly oxidative muscles demonstrated a marked ability to maintain oxygen delivery, which differs substantially from previous reports of L-NAME infusion prior to exercise in these muscles. The demonstration that NO is an important regulator of blood flow and VC in the rat hindlimb during treadmill exercise, but that the fiber-type dependency of NO is altered markedly when NOS inhibition is performed during, vs. prior to, exercise, lends important insights into the integrated nature of vascular control during exercise.

Fig. 1.

Individual responses and means (•) showing the effects of l-NAME administration during exercise on total hindlimb muscle blood flow and vascular conductance (VC). *P < 0.05 vs. control.

Fig. 2.

Relationships between the percent sum of type I and IIa fibers in the individual muscles or muscle parts (n = 28) of the rat hindlimb and the absolute (top) and relative (bottom) reductions in blood flow (Δ blood flow) in the present investigation after l-NAME administration during exercise.

Fig. 3.

Relationships between the percent sum of type I and IIa fibers in the individual muscles or muscle parts (n = 28) of the rat hindlimb and the absolute (top) and relative (bottom) reductions in vascular conductance (Δ VC) in the present investigation after l-NAME administration during exercise.

Fig. 4.

Mean arterial pressure (MAP) at rest (time 0) and during exercise for the first (•) and second (○) exercise bouts in the experimental group (n = 10). During the second run l-NAME was infused (see arrow) over an ∼10-s period immediately prior to the 3-min MAP measurement. Radioactive microspheres were injected (see arrow) at ∼5.5 min of total exercise time. *P < 0.05 vs. rest and minutes 1 and 2 of exercise.

Fig. 5.

Hypotensive responses to acetylcholine injection expressed as absolute (left) and relative (right) changes in MAP before (control) and after l-NAME administration during exercise. *P < 0.05 vs. control.

Fig. 6.

Comparison of the relative changes in blood flow (Δ blood flow) and vascular conductance (Δ VC) between first and second bouts of exercise for time controls (present study, n = 7), and when l-NAME was administered prior to (ref. , n = 6) and during (present study, n = 10) the second exercise bout. *Significantly different from zero.

Fig. 7.

Correlations between the absolute changes in blood flow (Δ blood flow; top) and VC (Δ VC, bottom) and the percent sum of type I and IIa fibers found in the individual muscles and muscle parts of the rat hindlimb after l-NAME administration during (•, solid line; present investigation) vs. prior to (○, dashed line; ref. 17) exercise. Note the consistently lower (or absent) attenuations in blood flow and VC after l-NAME administration during vs. prior to exercise in the muscles comprising greater than ∼80% type I and IIa fibers (i.e., circled data in top panel; see text for discussion). *Significantly different relationship with fiber-type (i.e., less robust fiber-type dependency) when l-NAME is administered during as opposed to before exercise.