JNTU B.Pharmacy III PHARMACOLOGY-I Previous Question Paper (September-2021 - R17)

The Volume of Distribution (Vd) is a theoretical volume that reflects the extent to which a drug is distributed in body tissues compared to plasma. Factors affecting Vd include:

• Lipid Solubility: Highly lipid-soluble drugs easily cross cell membranes and accumulate in adipose tissue, resulting in a large Vd.
• Plasma Protein Binding: Drugs that bind extensively to plasma proteins (e.g., albumin) are restricted to the vascular compartment, leading to a small Vd.
• Tissue Binding: Drugs that bind strongly to extravascular components (e.g., muscle or bone) show a higher Vd.
• Molecular Size: Small, water-soluble molecules can distribute into total body water, whereas large molecules are often confined to the plasma.
• Physiological Factors: Age, gender, and body composition (e.g., increased adipose tissue in obesity) significantly alter the distribution space for drugs.
• Disease States: Conditions such as renal failure, hepatic cirrhosis, or congestive heart failure alter protein levels and fluid distribution, thereby changing Vd.

Antagonism occurs when one drug reduces the effect of another. The comparison between competitive and non-competitive antagonism is as follows:

In competitive antagonism, the antagonist competes for the orthosteric binding site. Because the interaction is reversible, the inhibition can be surmounted by increasing the concentration of the agonist. In contrast, non-competitive antagonism involves the antagonist binding to an allosteric site or forming a covalent bond with the receptor, which prevents the agonist from eliciting a full response regardless of the agonist concentration used.