Which of the following terms best describes the antagonism of leukotrienes’ bronchoconstrictor effect (mediated at the leukotriene receptors) by terbutaline (acting at the adrenoceptors) in a patient with asthma ?
**Core Concept**
Leukotrienes are a group of eicosanoid inflammatory mediators that cause bronchoconstriction by binding to specific receptors on airway smooth muscle cells. Terbutaline is a short-acting beta-2 adrenergic receptor agonist (SABA) that induces bronchodilation by stimulating beta-2 adrenoceptors. The interaction between leukotrienes and terbutaline in asthma is a classic example of pharmacological antagonism.
**Why the Correct Answer is Right**
Terbutaline's bronchodilatory effect is mediated through the stimulation of beta-2 adrenoceptors, which are Gs-protein coupled receptors that increase intracellular cAMP levels, leading to smooth muscle relaxation. In contrast, leukotrienes exert their bronchoconstrictor effect by binding to cysteinyl leukotriene receptors (CysLT1R), which are Gq-protein coupled receptors that increase intracellular IP3 levels, leading to smooth muscle contraction. When terbutaline and leukotrienes act on different receptors, their effects are additive, but when terbutaline is present, it can mask or antagonize the bronchoconstrictor effect of leukotrienes, resulting in a net bronchodilatory effect.
**Why Each Wrong Option is Incorrect**
**Option A:** Non-competitive antagonism is incorrect because terbutaline and leukotrienes act on different receptors, and their effects are additive, not inhibitory.
**Option B:** Competitive antagonism is incorrect because terbutaline does not compete with leukotrienes for binding to the same receptor site.
**Option C:** Partial agonism is incorrect because terbutaline does not have a partial agonist effect on leukotriene receptors.
**Option D:** Inverse agonism is incorrect because terbutaline does not reduce the activity of leukotriene receptors.
**Clinical Pearl / High-Yield Fact**
Terbutaline's effect on leukotriene-induced bronchoconstriction is an example of a pharmacological antagonism, where two drugs with different mechanisms of action can interact to produce a net effect. This concept is essential in understanding the complex pathophysiology of asthma and developing effective treatment strategies.
**Correct Answer: D. Inverse agonism is incorrect because terbutaline does not reduce the activity of leukotriene receptors.**