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Astragalus has not been significantly investigated in clinical studies, so in vitro and animal tests provide the evidence for its pharmacological activities. IMMUNE ENHANCING

Several in vitro and in vivo studies confirm immune-enhancing activity. Astragalus stimulates macrophage activity and enhances antibody responses (Chu et al 1998, Jin et al 1994, Sugiura et al 1993). Astragalus enhances lymphocyte histogenesis in vitro (Sun et al 1983). Immunostimulant effects have also been observed in the presence of immunosuppressive therapy in vivo (Jin et al 1999).

Although usually administered in the oral form, research has also been undertaken with injectable forms. A study conducted with both normal and immunosuppressed mice found that astragalus administration increased antibody responses and T helper cell activity (Zhao et al 1990). A flavonoid identified in the stem and leaves of astragalus is believed to be one of the main constituents responsible for immune modulation (Jiao et al 1999) and more recently, several studies have identified that astragalus polysaccharide (APS) also exerts significant biological effects.

One study investigating the effects of APS identified that it stimulates mouse B cells and macrophage activity, but not T cells (Shao et al 2004). It has been demonstrated that APS stimulates macrophages to produce nitric oxide (NO) through induction of NO synthase transcription (Lee & Jeon 2005) leading to increased cytolytic function of macrophages.

Guo et al found that the polysaccharide increases cellular and humoral immune responses in Eimeria fene//a-infected chickens when administered as 1 g in every kg of food. When given for 1 week, it increased systemic antibodies, intestinal antibodies, antigen-specific splenocytes and erythrocyte-antibody-complement cells as compared with controls (Guo et al 2004a). The same research team used both in vitro and animal models to further investigate the effects of APS and found increased beneficial gut flora and decreased harmful gut bacteria (Guo et al 2003, 2004b).

Clinical note - Polysaccharides and immunity

One of the most promising recent alternatives to antibiotic treatment is the use of immunomodulators for enhancing host defence responses. Several types of immunomodulators have been identified, most recently botanically sourced polysaccharides isolated from mushrooms, algae, lichens and higher plants. These polysaccharides tend to have a broad spectrum of therapeutic properties and relatively low toxicity. One of the primary mechanisms responsible for immunomodulation involves non-specific induction of the immune system, which is thought to occurvia macrophage stimulation and modulation of the complement system. According to one report, polysaccharides isolated from 35 plant species among 20 different families have been shown to increase macrophage cytotoxic activity against tumour cells and microorganisms, activate phagocytic activity, increase reactive oxygen species (ROS) and nitric oxide (NO) production, and enhance secretion of cytokines and chemokines, such as TNF-alpha, IL-1 beta, IL-6, IL-8, IL-12, IFN-gamma and IFN-beta2 (Schepetkin & Quinn 2005). These effects have a major influence on the body's ability to respond rapidly and potently to a diverse array of pathogens, giving the polysaccharides wide clinical application.

CARDIOVASCULAR EFFECTS — POSITIVE INOTROPIC AND HYPOTENSIVE ACTIVITY

The effect of astragalus on heart function has been the subject of several Investigations and, most recently, a Cochrane systematic review, which analysed studies of Chinese herbs used in viral myocarditis and concluded that astragalus significantly improves cardiac function, arrhythmia and creatinine kinase levels (Liu et al 2004a). The review assessed data from 10 randomised clinical trials that used a single preparation of astragalus and one that used a combination containing mainly astragalus; however, the authors stated that the trials had poor quality in terms of design, reporting and methodology.

Several constituents from Astragalus spp. have demonstrated effects on heart contractility, heart rate and blood pressure. In particular, 3-nitropropionic acid (NPA) has been shown to decrease blood pressure and induce bradycardia when administered as an IV preparation in normotensive rats or renal hypertensive dogs (Castillo et al 1993). Another compound, astragaloside IV, demonstrated positive inotropic activity in patients with congestive heart failure (Luo et al 1995).

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