Skeletal muscle blood flow during exercise is reduced in a rat model of pulmonary hypertension

Abstract

Pulmonary arterial hypertension (PAH) is characterized by exercise intolerance. Muscle blood flow may be reduced during exercise in PAH; however, this has not been directly measured. Therefore, we investigated blood flow during exercise in a rat model of monocrotaline (MCT)-induced pulmonary hypertension (PH). Male Sprague-Dawley rats (∼200 g) were injected with 60 mg/kg MCT (MCT, n = 23) and vehicle control (saline; CON, n = 16). Maximal rate of oxygen consumption (V̇o_2max) and voluntary running were measured before PH induction. Right ventricle (RV) morphology and function were assessed via echocardiography and invasive hemodynamic measures. Treadmill running at 50% V̇o_2max was performed by a subgroup of rats (MCT, n = 8; CON, n = 7). Injection of fluorescent microspheres determined muscle blood flow via photo spectroscopy. MCT demonstrated a severe phenotype via RV hypertrophy (Fulton index, 0.61 vs. 0.31; P < 0.001), high RV systolic pressure (51.5 vs. 22.4 mmHg; P < 0.001), and lower V̇o_2max (53.2 vs. 71.8 mL·min^-1·kg^-1; P < 0.0001) compared with CON. Two-way ANOVA revealed exercising skeletal muscle blood flow relative to power output was reduced in MCT compared with CON (P < 0.001), and plasma lactate was increased in MCT (10.8 vs. 4.5 mmol/L; P = 0.002). Significant relationships between skeletal blood flow and blood lactate during exercise were observed for individual muscles (r = -0.58 to -0.74; P < 0.05). No differences in capillarization were identified. Skeletal muscle blood flow is significantly reduced in experimental PH. Reduced blood flow during exercise may be, at least in part, consequent to reduced exercise intensity in PH. This adds further evidence of peripheral muscle dysfunction and exercise intolerance in PAH.

Keywords: PAH; blood flow; exercise; pulmonary hypertension; skeletal muscle.

Figure 1.

Study protocol. MCT, monocrotaline; RVSP, right ventricular systolic pressure.

Figure 2.

A–C: PH phenotype as evidenced by increased RV weight (0.22 ± 0.01 vs. 0.41 ± 0.03), increased Fulton index (0.62 ± 0.05 vs. 0.31 ± 0.02), and increased RVSP (51.5 ± 5 vs. 22.4 ± 2 mmHg) in MCT (n = 12–13) compared with CON (n = 11). **P ≤ 0.001. CON, control; MCT, monocrotaline; RV, right ventricle; RVSP, right ventricular systolic pressure.

Figure 3.

Resting (A) and exercising (B) blood flow in MCT (n = 7) vs. CON (n = 8). At rest, no significant difference is seen between groups (P = 0.35). When expressed relative to power output during exercise, blood flow is significantly lower in MCT vs. CON (P = <0.001). BF, biceps femoris; CON, control; EDL, extensor digitorum longus; GA, gastrocnemius; MCT, monocrotaline; RF, rectus femoris, SL, soleus; ST, semitendinosus; TA, tibialis anterior; VL, vastus lateralis.

Figure 4.

Evidence of altered metabolism during exercise via increased blood lactate in MCT (n = 23) vs. CON (n = 16). *P ≤ 0.05. CON, control; MCT, monocrotaline.

Figure 5.

Exercising blood lactate was inversely correlated to exercising blood flow in the EDL (A), tibialis anterior (B), and soleus (C), and a significant inverse association for blood flow with blood lactate during exercise was also observed when blood flow is expressed as a compiled value for all skeletal muscles (D) (n-12). EDL, extensor digitorum longus; SL, soleus; TA, Tibialis anterior.

Figure 6.

Representative images of immunofluorescent staining of muscle tissue imaged at ×10 magnification. Capillaries (yellow, as overlay of green and red), myocyte membrane (glycoproteins, green), vasculature (lectin, red), nuclei (DAPI, blue) in soleus muscle sections. CON, control; MCT, monocrotaline.