Smooth Muscle Receptor Analysis

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Rat ileum consists of longitudinal and circular smooth muscle. The rat ileal smooth muscles express five isoforms of muscarinic acetylcholine receptor MAchR (M1-M5)(Yuan et al., 2011), and seven classes of 5-hydroxytryptamine (5-HT, serotonin) receptors (5-HT1-5-HT7)(Briejer et al., 1997). These receptors are major drug targets and belong to the family of G protein-coupled receptor (GPCR) consisting of seven transmembrane spanning domains (Hannon & Hoyer, 2008; Yuan et al., 2011). Both MAchR and 5-HT receptors are essential in the regulations of smooth muscle contraction in gastrointestinal tracts (Yuan et al., 2011). M2 receptors are more abundantly found in the ileal smooth muscle compared to M3 receptor (Tanahashi et al., 2013). Both M2 and M3 …show more content…

Atropine is a non-selective muscarinic cholinergic antagonist and it functions by directly blocking the M3 receptor (Bhattacharjee et al., 2013). This is an example of pharmacological antagonism whereby atropine competitively inhibits Ach, antagonizing the action of Ach at postganglionic muscarinic sites (Bhattacharjee et al., 2013). The inhibition effect of atropine is concentration-dependent (Wali et al., 1987). In high concentrations, atropine reduces Ach-induced contractions by blocking ion channels present at the postsynaptic membrane of the smooth muscle cells (Wali et al., 1987).
    
    Another antagonist, adrenaline, promotes longitudinal smooth muscle relaxation by interacting with B2-adrenergic receptor coupled to Gs protein (Kosterlitz et al., 1970). This is known as the physiological antagonism. Consequently, the binding of adrenaline depresses the Ach-induced contractions in the muscle (Kosterlitz et al., 1970) besides activating enzyme adenylyl cyclase which then leads to the production of cyclic adenosine monophosphate (cAMP), thus decreasing motility in smooth

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