Vasodilators direct action

Calcium channel blockers cause more pronounced lowering of blood pressure in hypertensive patients than in normotensive individuals. Generally, all calcium channel blockers cause an immediate increase in PRA during acute treatment in patients having hypertension but PRA is normalized during chronic treatment despite the sustained decrease in blood pressure. These agents also do not generally produce sodium and water retention, unlike the conventional vasodilators. This is because they produce diuretic effects by direct actions on the kidney. 

Direct actions include vasodilation of systemic arterioles and coronary arteries, leading to a reduction of arterial pressure and coronary vascular resistance as well as depression of myocardial contractility and the conduc-... 

The effects of phenytoin on the cardiovascular system vary with the dose, the mode and rate of administration, and any cardiovascular pathology. Rapid administration can produce transient hypotension that is the combined result of peripheral vasodilation and depression of myocardial contractility. These effects are due to direct actions of phenytoin on the vascular bed and ventricular myocardium. If large doses are given slowly, dose-related decreases in left ventricular force, rate of force development, and cardiac output can be observed, along with an increase in left ventricular end-diastolic pressure. 

The hemodynamic effects of diazoxide are similar to those of hydralazine and minoxidil. It produces direct relaxation of arteriolar smooth muscle with little effect on capacitance beds. Since it does not impair cardiovascular reflexes, orthostasis is not a problem. Its administration is, however, associated with a reflex increase in cardiac output that partially counters its antihypertensive effects. Propranolol and other -blockers potentiate the vasodilating properties of the drug. Diazoxide has no direct action on the heart. Although renal blood flow and glomerular filtration may fall transiently, they generally return to predrug levels within an hour. 

Nicotinic acid is also a potent vasodilator, probably by a direct action on smooth muscle cells. It produces cutaneous vasodilatation, itching of the skin, facial flushing, a sensation of feeling hot, pounding in the head, gastric irritation, diarrhea, raised transaminases, hyperglycemia, and hyperuricemia. These unpleasant adverse effects limit its acceptability for many patients. Nicotinic acid as such is not used in the treatment of vascular disorders, but some of its derivatives are, albeit with poor evidence of clinical efficacy. 

The direct or central effects of organic nitrates relieve ischemia by direct action on coronary vessels, improving myocardial oxygen supply. Direct vasodilator effects are exerted mostly on large conductance, rather than small resistance coronary vessels including intercoronary collateral vessels. Coronary stenosis dilatation is potentially a major component of the beneficial response. These mecha-... 

The inhalational anesthetics halothane, isoflurane, and enflu-rane all have been reported to have a positive effect in children and adults with severe asthma that is unresponsive to standard medical therapy. The proposed mechanisms for inhalational anesthetics include direct action on bronchial smooth muscle, inhibition of airway reflexes, attenuation of histamine-induced bronchospasm, and interaction with /32-adrenergic receptors. Well-controlled trials with these agents have not been completed. Potential adverse effects include myocardial depression, vasodilation, arrhythmias, and depression of mucociliary function. In addition, the practical problem of delivery and scavenging these agents in the intensive care environment as opposed to the operating room is a concern. The use of volatile anesthetics cannot be recommended based on insufficient evidence of efficacy. 

When infused intrarenally in animals, PAF decreases renal blood flow, glomerular filtration rate, urine volume, and excretion of Na without changes in systemic hemodynamics. These effects are the result of a direct action on the renal circulation. PAF exerts a receptor-mediated biphasic effect on afferent arterioles, dilating them at low concentrations and constricting them at higher concentrations. The vasoconstrictor effect appears to be mediated, at least in part, by COX products, whereas vasodilation is a consequence of the stimulation of NO production by endothelium. 

Nevertheless, it is assumed that treatment with minoxidil lengthens and thickens the small vellus hairs and decreases shedding. Minoxidil is a potassium channel opener that causes vasorelaxation [132] and stimulates cutaneous blood flow to the scalp [133]. Minoxidil sulfate, a metabolite of minoxidil, is a potent vasodilator. Uptake and conversion of minoxidil to minoxidil sulfate occurs within the HE, suggesting a direct action on the follicle [134]. The most probable site of action of minoxidil is the DP [135], and the mechanism of action has been linked to its effects on the Kir6.1/SUR2B potassium channel expressed by the derma papilla [136-138]. 

Intake of a large amount of sodium chloride negates the antihypertensive effects of diuretics. Other mechanisms, such as direct vasodilating action, decreased responsiveness to vasopressor agents, stimulation of prostacyclin [35121 -78-9] production, and reduction in the intracellular calcium... 

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