Competitive Antagonism.

Here, ACh is the agonist of muscarinic receptors.

Mechanism of smooth muscle contraction – When ACh stimulates the G-protein coupled muscarinic receptors, a conformational change occurs, leading to the activation of the G-protein. The activated G-protein activates the intracellular enzyme, PLC (phospholipase C). This enzyme in turn hydrolyses PI(4,5)P2 (phosphatidylinositol-4,5-bisphosphate). This hydrolysis results in the production of two compounds – DAG (diacylglycerol) and IP3 (inositol-1,4,5-triphosphate). IP3 is soluble and diffuses through the cytosol. It binds to receptors on the endoplasmic reticulum resulting in the release of calcium ions (Ca2+) from the intracellular stores. The rise in intracellular calcium ions results in contraction of the smooth muscle. This happens when the intracellular calcium ions bind to calmodulin and activate it. Activation of calmodulin results in activation of the myosin light-chain kinase (MLCK) enzyme by the calcium-calmodulin complex, which then catalyzes the transfer of phosphate group from ATP to myosin cross bridges. The phosphorylated myosin cross bridges then interact with actin, causing shortening of the fiber. This results in muscle contraction. The cross-bridge cycle is powered by ATP.

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Competitive Antagonism.
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Second messenger involved – IP3 (inositol-1,4,5-triphosphate) is the second messenger, as it relays the signal from outside the cell to the inside, resulting in the release of calcium ions that cause muscle contraction.

Acetylcholine (ACh) and adrenaline (Ad) are two agonists whose actions are independent of one another and result in opposite effects. Ad brings about the relaxation of pre-contracted smooth muscle by acting via the G-protein coupled β-adrenergic receptors on the cells.

Mechanism of Ad induced relaxation of pre-contracted smooth muscle – Ad binds to the G-protein linked β-adrenergic receptor causing a conformational change in the G-protein. This activates the enzyme adenylate cyclase.

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