Role of TNF-α in Regulating the Exercise Pressor Reflex in Rats With Femoral Artery Occlusion

Abstract

Responses of sympathetic nerve activity and arterial blood pressure are augmented during activation of the exercise pressor reflex in rats with femoral artery occlusion. The present study examined the role played by proinflammatory tumor necrosis factor-α (TNF-α) in regulating augmented sympathetic responsiveness induced by stimulation of muscle metabolic receptors and static muscle contraction following 72 h of femoral artery occlusion. We first observed that the levels of TNF-α and protein expression of TNF-α receptor type 1 (TNFR1) were increased in the dorsal root ganglion (DRG) of hindlimbs with femoral artery occlusion. Note that TNF-α was observed within DRG neurons of C-fiber afferent nerves. Capsaicin (TRPV1 agonist) and AITC (TRPA1 agonist) were injected into arterial blood supply of the hindlimbs to stimulate metabolically sensitive thin-fiber muscle afferents. The effects of these injections on the sympathetic and pressor responses were further examined in control rats and rats with femoral artery occlusion. As TNF-α synthesis suppressor pentoxifylline (PTX) was previously administered into the hindlimb with femoral artery occlusion, sympathetic, and pressor responses induced by capsaicin and AITC were attenuated. In occluded rats, PTX also attenuated the exaggeration of blood pressure response induced by muscle contraction, but not by passive tendon stretch. Overall, the results suggest that TNF-α plays a role in modulating exaggerated sympathetic nervous activity via the metabolic component of the exercise pressor reflex when the hindlimb muscles are ischemic in peripheral arterial disease.

Keywords: TNF-α; blood pressure; exercise; hindlimb ischemia; muscle afferent nerve; peripheral arterial disease.

FIGURE 1

Effects of femoral artery occlusion on TNF-α signal in sensory nerves. (A) Seventy-two hours of femoral artery occlusion increased the levels of TNF-α in the DRG tissues as compared with them in DRG of control limbs (n = 6 in each group); and PTX given into the hindlimb muscles attenuated amplification of TNF-α in the DRG tissues of limbs with femoral artery occlusion (n = 8). ^∗P < 0.05, occlusion group vs. control and occlusion with prior PTX. (B) Immunofluorescence was used to examine double-labeling for TNF-α and peripherin/NF-200 (n = 3). Peripherin was used to label DRG neurons that project thin C-fibers. NF200 was used to identify A-fibers of DRG neurons. Representative photomicrographs show co-existence of TNF-α and peripherin staining in DRG neurons (top panel), whereas few TNF-α and NF-200 staining were observed in DRG neurons (bottom panel). Arrows indicate representative positive cells for both TNF-α and peripherin after they were merged. Scale bar = 50 μm. (C) Representative bands (left panel) and averaged data (right panel), demonstrating that femoral artery occlusion upregulated protein expression TNF-α receptor subtype TNFR1, but not TNFR2. A significant difference in TNFR1 was seen between control and occluded groups. ^∗P < 0.05 vs. control. n = 6 in each group.

FIGURE 2

Responses of RSNA, MAP, and HR evoked by stimulation of TRPV1 and TRPA1 receptors with arterial injection of capsaicin and AITC. (A) Three dosages of capsaicin evoked increases in RSNA and MAP in a dose-dependent manner in control rats (n = 10) and occluded rats (n = 8). The effects were greater in occluded rats. As PTX was given previously in occluded rats (n = 8), amplified RSNA and MAP responses were attenuated. ^∗P < 0.05, compared with control rats and ^†P < 0.05, compared with occluded rats without PTX. (B) In the similar way, the RSNA and MAP were increased after injection of three dosages of AITC in control rats (n = 8), occluded rats (n = 6), and occluded rats with PTX treatment (n = 8). The responses were enhanced in occluded rats and PTX can attenuate increases of RSNA and MAP responses. ^∗P < 0.05, compared with control rats and ^†P < 0.05, compared with occluded rats without PTX.

FIGURE 3

Responses of MAP and HR evoked by static muscle contraction and passive tendon stretch. (A) Muscle contraction was evoked by electrical stimulation of the L4 and L5 ventral roots. Femoral artery occlusion enhanced MAP and HR responses as compared with control rats. In occluded rats (n = 10), PTX significantly inhibited amplification of MAP responses induced by muscle contraction. ^∗P < 0.05, compared with control rats and ^†P < 0.05, compared with occluded rats without PTX (n = 8 in each group). No significant differences were seen in muscle developed tension among three groups. For example, 535 ± 30 g in control, 547 ± 28 g in occluded rats, and 550 ± 35 g in occluded rats with PTX. (B) Showing that femoral artery occlusion amplified MAP response induced by the passive tendon stretch, but PTX failed to attenuate amplification of MAP response (P > 0.05, occlusion vs. occlusion plus PTX; n = 6 in each group). Muscle tension was ∼500 g. ^∗P < 0.05, compared with control rats (n = 8).