Electro-acupuncture stimulation to muscle afferents in anesthetized rats modulates the blood flow to the knee joint through autonomic reflexes and nitric oxide

Abstract

Recent reports have focused on the mechanisms of the action of electro-acupuncture stimulation (EAS) in the regulation of blood flow to different tissues. In the knee joint, blood flow is known to be modulated mainly by sympathetic postganglionic fibers, but recently the release or induction of nitric oxide (NO) synthesis in response to electrical stimulation has also been suggested. Therefore, a direct observation of the microcirculation is needed to further understand the mechanism by which blood flow is regulated by somatic afferent stimulation. In the present study, the effects of EAS to the vastus medialis muscle on systemic hemodynamics and the knee joint microcirculation were observed in vivo using a real-time confocal laser-scanning microscope system (CLMS). Electrical stimulation (5 mA, 0.5 ms, 5 Hz) was applied for 30 min using a pair of acupuncture needles introduced into the vastus medialis muscle. To clarify a plausible involvement of NO in the responses to EAS, the stimulus was applied either in the presence or absence of N_ω-nitro-L-arginine methyl ester (L-NAME). Stimulation to either the muscle or the skin of the thigh after blockade of neuromuscular transmission was performed to determine the involvement of muscle contraction during EAS treatment. The changes in mean arterial pressure (MAP) and diameter of the arterioles supplying the knee joint were monitored continuously until 60 min poststimulus.

Significant and persistent increases in arteriolar diameter by 26±6% and MAP by 17±2%, respectively, were observed after EAS to the muscle. Electro-acupuncture to the vastus medialis in the presence of L-NAME produced a strong decrease in diameter of the knee joint arterioles by −38±14% under the baseline with a simultaneous increase of 35±5% in MAP. EAS to the skin did not produce changes in arteriolar diameter while a slight increase in MAP by 12±6% over the baseline occurred after the stimulus. EAS to the muscle after neuromuscular blockade did not produce significant changes in diameter, while an increase in MAP by 24±8% was still observed, which facts suggest that the muscle contraction is required to produce vasodilatation. These responses suggest that a dynamic balance between the autonomic nervous system and the release of NO is the primary mechanism mediating the EAS effects on knee joint microcirculation.

Introduction

Somatosensory stimuli, including electro-acupuncture stimulation (EAS), evoke reflex autonomic responses that directly affect the blood flow to different tissues Sato et al., 1997, Kimura and Sato, 1997. Several studies have shown that the blood flow to the brain (Sato and Sato, 1992), peripheral nerves (Sato et al., 1994), skeletal muscles (Noguchi et al., 1999), skin (Jansen et al., 1988), spleen (Kimura and Sato, 1997) and kidneys (Noguchi et al., 1999) can be modulated via sympathetic reflexes elicited by somatic afferent activation.

The cardiovascular functions are also subject to change by somatic afferent stimulation. Blood pressure and heart rate tend to increase or decrease when mechanical, electrical and noxious stimuli are imposed. These changes in systemic hemodynamics are the consequence of complex somato-cardiovascular reflexes integrated at the central nervous system Kimura et al., 1995, Sato et al., 1997.

Osteoarthritis (OA) is the most common form of arthritis causing pain and disabilities particularly in the old-aged people. Pain is the most important symptom, which originates from the synovial membrane, joint capsule, periosteum and other periarticular structures such as ligaments and muscles. Current treatment of OA is mainly symptomatic and includes the use of pharmacological agents (analgesics and anti-inflammatory agents) and physiotherapy McAlidon and Dieppe, 1990, Creamer and Hochberg, 1997, Felson et al., 2000, while in Asian countries acupuncture has been widely used to treat OA (O'Connor and Bensky, 1981). Nowadays, this therapy is also accepted as an alternative care for patients with OA in the United States (Eisenberg et al., 1993) and Great Britain Wadlow and Peringer, 1996, Silvert, 2000. Thus, the World Health Organization has recently approved it for the complementary management of OA (World Health Assembly Resolution 44.34, 1991). Since the most frequently damaged site in OA is the knee joint (McAlidon and Dieppe, 1990), previous clinical trials have proposed EAS as an effective and safe adjunctive therapy to conventional care for patients with OA of the knee (Berman et al., 1999). Electro-acupuncture has a better effect on medial knee joint pain than it does on generalized articular pain, which suggests the action of local rather than central control mechanisms (Creamer et al., 1999). Thus, changes in local blood flow supplied to the joints, periarticular structures and muscles must, therefore, be taken into account when discussing the action of EAS in the treatment of OA. It has been reported that knee joint blood flow decreases after electrical stimulation of sympathetic postganglionic fibers Sato and Schaible, 1987, Koshbaten and Ferrel, 1990. Such a decrease in joint blood flow can be enhanced by administering nitric oxide synthase (NOS) inhibitors (McDougall and Ferrel, 1996), suggesting a release or rapid induction of nitric oxide (NO) synthesis in response to electrical stimulation. The participation of cholinergic sympathetic fibers in the responses of the knee joint seems improbable, because the innervation of muscle and extramuscular feed arteries of the hindlimb as well as the neuron cell bodies in the sympathetic ganglia innervating such vessel in the rat, have been found devoid of cholinergic markers (Guidry and Landis, 2000). Furthermore, several in vivo studies have demonstrated that the blood flow in the limb vasculature and hindquarters of rats is not affected by muscarinic antagonists Folkow and Rubinstein, 1966, Bolme et al., 1970, Linderoth et al., 1994.

The mechanisms mediating acupuncture effects are not yet completely understood, although most previous investigations have demonstrated the release of endogenous opioids and monoamines in the central nervous system Pomeranz and Chiu, 1976, Mayer et al., 1977, Stux and Pomeranz, 1998. Nevertheless, as stated before, somatosensory stimulation and EAS have important implications in the regulation of the autonomic nervous system and consequently in the systemic and microvascular hemodynamics (Yamaguchi et al., 2001). A direct observation of the effects of EAS and other related physical therapies on the hemodynamics of the microcirculation of the diverse vascular beds is, therefore, needed to understand its physiological mechanisms.

In the present study, the effects of EAS on the knee joint microcirculation were investigated using an intravital confocal laser-scanning microscope system. In addition, the role of NO as a mediator of the microvascular responses to EAS was analyzed.