Vascular tone and contraction are determined largely by the availability of calcium from extracellular sources (influx via calcium channels) or intracellular stores. Drug-induced inhibition of calcium influx via voltage-gated channels results in widespread dilation and a decrease in contractile responses to stimulatory agents. In general, arteries and arterioles are more sensitive to the relaxant actions of these drugs than are the veins, and some arterial beds (e.g., coronary and cerebral vessels) show greater sensitivity than others. Peripheral vasodilation and the consequent fall in blood pressure are commonly accompanied by reflex tachycardia when nifedipine and its analogues are used; this is in contrast to verapamil and diltiazem, whose effects on peripheral vessels are accompanied by cardiodepres-sant effects.
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