Calcium antagonists (Ch. 7) act selectively on calcium channels in the cellular membrane of cardiac and vascular smooth muscle cells. Free calcium within the vascular smooth muscle enhances vascular tone and contributes to vasoconstriction. Calcium antagonists reduce the transmembrane calcium influx in these cells and via this mechanism are responsible for relaxation of the vascular smooth muscle and subsequent vasodilatation.
Apart from the vasodilatation, these drugs have a direct diuretic effect that contributes to their long-term antihypertensive action. Nifedipine increases the urine volume and sodium excretion, and may inhibit aldosterone release. This diuretic action is independent of any change in renal blood flow or GFR.
Because of the vasodilator effect on the vasculature, these drugs have been found to be useful in the management of conditions in which pathological vasoconstriction occurs, such as hypertension, Raynaud's disease, migraine and Prinzmetal's angina.
Calcium antagonists are often used as renovascular vasodilators in selected situations, e.g. renal transplantation. In kidney recipients, diltiazem has been shown to cause vasodilatation within the kidneys and improve intrarenal circulation. They also decrease calcium influx and production of oxygen free radicals on reperfusion after the ischaemic insult. In these patients, calcium antagonists significantly improve renal function and decrease the incidence of post-transplant acute tubular necrosis. Furthermore, they may reduce the vasoconstrictive action of cyclosporin. In spite of all the apparent beneficial effects, calcium antagonists have failed to improve graft survival.
Calcium antagonists may cause hypotension and thereby decrease renal perfusion. Therefore their use is not really justified in most cases of post-ischaemic acute renal failure. The effects of the depolarizing and non-depolarizing neuromuscular blocking agents might be enhanced by the calcium antagonists. Caution should always be exercised when this combination is used in patients with renal dysfunction.
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Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...