9.8
Antihypertensive Drugs: Action of Calcium Channel Blockers
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Q1: How do L-type calcium channels function in vascular smooth muscle cells?
L-type voltage-gated calcium channels in vascular smooth muscle cell membranes open upon depolarization, allowing calcium ions to enter the cell. This influx triggers repolarization and channel closure. The calcium then stimulates the ryanodine receptor, causing larger calcium release from the sarcoplasmic reticulum, which activates calmodulin and initiates the contraction cascade.
Q2: What is the mechanism of smooth muscle contraction in blood vessels?
Calcium activates calmodulin, which binds and activates myosin light-chain kinase. This enzyme phosphorylates the myosin light chain, enabling interaction between actin and myosin filaments. Cross-bridge cycling then occurs, causing smooth muscle contraction in arterial walls and increasing vascular tone and peripheral resistance.
Q3: Why does overactive calcium channel activity lead to high blood pressure?
Overactive L-type calcium channels cause excessive calcium entry into vascular smooth muscle cells, prolonging vasoconstriction. This sustained muscle contraction increases peripheral resistance and elevates blood pressure. The extended contraction state prevents normal vascular relaxation needed to maintain healthy regulation of blood pressure.
Q4: How do calcium channel blockers like amlodipine lower blood pressure?
Calcium channel blockers attach to the α1 subunit of L-type calcium channels, blocking calcium ion entry into vascular smooth muscle cells. This reduces intracellular calcium concentration, weakening muscle contractions and relaxing arterial smooth muscle. The resulting decrease in peripheral resistance lowers blood pressure.
Q5: What role does calmodulin play in smooth muscle contraction?
Calmodulin is a calcium-binding protein activated by increased intracellular calcium concentration. Once activated, calmodulin binds to and activates myosin light-chain kinase, which phosphorylates the myosin light chain. This phosphorylation is essential for initiating the actin-myosin interaction that drives smooth muscle contraction.
Q6: What are examples of calcium channel blockers used as antihypertensive drugs?
Dihydropyridine calcium channel blockers, including amlodipine, nicardipine, and felodipine, are commonly used antihypertensive agents. These drugs selectively block L-type calcium channels in vascular smooth muscle, reducing calcium influx and promoting vasodilation. They effectively decrease peripheral resistance and lower blood pressure in hypertensive patients.
Q7: How does membrane depolarization trigger calcium channel opening in smooth muscle?
When vascular smooth muscle cells receive a stimulus, their membrane potential changes, causing depolarization. This alteration in membrane potential directly instigates the opening of L-type calcium channels, allowing calcium ions to enter from the external environment. The resulting increase in intracellular calcium concentration initiates excitation-contraction coupling.