Vascular-system active agents

In contrast, most of recently developed pesticides are systemicaUy active and therefore they penetrate the plant cuticle and move through the plant vascular system. Examples of systemic fungicides are benomyl and hexacona-zole. These systemic agents can not only protect a plant from attack but also inhibit or cure established infections. They are not affected by weathering and also confer immunity to all new plant growth. 

Noradrenaline and adrenaline are the classic catecholamines and neurotransmitters in the sympathetic nervous system. Noradrenaline stimulates the following subtypes of adrenoceptors P, a, U2. It has positive inotropic and chronotropic activities as a result of /3i-receptor stimulation. In addition, it is a potent vasoconstrictor agent as a result of the stimulation of both subtypes (ai,a2) of a-adrenoceptors. After intravenous infusion, its effects develop within a few minutes, and these actions disappear within 1-2 minutes after stopping the infusion. It may be used in conditions of acute hypotension and shock, especially in patients with very low vascular resistance. It is also frequently used as a vasoconstrictor, added to local anaesthetics. Adrenaline stimulates the following subtypes of adrenoceptors /3i, P2, oil, 0L2. Its pharmacological profile greatly resembles that of noradrenaline (see above), as well as its potential applications in shock and hypotension. Like noradrenaline, its onset and duration of action are very short, as a result of rapid inactivation in vivo. Both noradrenaline and adrenaline may be used for cardiac stimulation. Their vasoconstrictor activity should be kept in mind. A problem associated with the use of /3-adrenoceptor stimulants is the tachyphylaxis of their effects, explained by the /3-adrenoceptor downregulation, which is characteristic for heart failure. 

Physiological sites proposed for nitric oxide action include the immune system, where nitric oxide acts as a cytostatic agent, is tumoricidal, and can inhibit viral replication. In the cardiovascular system, nitric oxide is the biological mediator of vasodilator responses to agents such as acetylcholine and bradykinin, which act as receptors on endothelial cells to activate NOS and stimulate nitric oxide production. Diffusible nitric oxide then activates guanylate cyclase in vascular smooth muscle cells, leading to the production of cyclic guano-sine monophosphate (GMP) and vasodilation. In the brain, stimulation of A-methyl-o-aspartate receptors on... 

The vasodilators decrease total peripheral resistance and thus correct the hemodynamic abnormality that is responsible for the elevated blood pressure in primary hypertension. In addition, because they act directly on vascular smooth muscle, the vasodilators are effective in lowering blood pressure, regardless of the etiology of the hypertension. Unlike many other antihypertensive agents, the vasodilators do not inhibit the activity of the sympathetic nervous system therefore, orthostatic hypotension and impotence are not problems. Additionally, most vasodilators relax arterial smooth muscle to a greater extent than venous smooth muscle, thereby further minimizing postural hypotension. 

Sevoflurane, in common with all volatile agents, reduces cardiac output and systemic blood pressure. It does so mainly through a reduction in peripheral vascular resistance. Although it is a systemic vasodilator it does not appear to produce significant dilatation of small coronary vessels and there is no possibility of coronary steal as hypothesised for isoflurane. A small increase in heart rate may be observed. This is less pronounced than with isoflurane and desflurane and is almost certainly the result of reflex activity secondary to the reduction in peripheral vascular resistance. Sevoflurane is associated with a stable heart rhythm and does not predispose the heart to sensitisation by catecholamines. In children, halothane causes a greater decrease in heart rate, myocardial contractility and cardiac output than sevoflurane at all concentrations. For these reasons sevoflurane is advocated for use in outpatient dental anaesthesia, especially in children. 

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