Vasomotor tone

Sympathetic nerves going to the arterioles are tonically active. In other words, these nerves discharge continuously, causing vasomotor tone. As a result, under resting conditions, arterioles are partially constricted. This vasomotor tone is important because it helps to maintain MAP in the range... 

Collister, J.P and Osborn, J.W., The chronic infusion of hexamethonium and phenylephrine to effectively clamp sympathetic vasomotor tone a novel approach, J. Pharmacol. Toxicol. Methods, 42, 135-147, 1999. 

Since autonomic postganglionic fibers lack a myelin sheath, they are particularly susceptible to blockade by local anesthetics. As a result, vasodilation ensues in the anesthetized region, because sympathetically driven vasomotor tone decreases. This local vasodilation is undesirable (see below). 

Heusser, K. et al., Influence of sibutramine treatment on sympathetic vasomotor tone in obese subjects, Clin. Pharmacol. Ther., 79(5), 500-508, 2006. 

Several drugs from various pharmacologic classes, including antithrombotics, vasomotor tone modulators, antiproliferatives, anti-inflammatories, and lipid modulators, were examined via systemic approach preceding and during the era of BMS (Table I) (17). Unfortunately, the systemic therapies investigated to prevent in-stent restenosis (ISR) have shown... 

Rajagopalan, S., Kurz, S., Munzel, T., et al. 1996. Angiotensin II-mediated hypertension in the rat increases vascular superoxide production via membrane NADH/NADPH oxidase activation. Contribution to alterations of vasomotor tone. J Clin Invest 97 1916-1923. 

A low level of tonic activity of the sympathetic nerves to vascular smooth muscle adrenergic receptors exists so that withdrawal of sympathetic vasomotor tone results in vasodilatation and reduced pressure. Conversely, enhancement of sympathetic vasomotor tone augments the level of vasoconstriction leading to elevated pressure. While the parasympathetic branch of the autonomic nervous system innervates some blood vessels, it does not generally play a role in regulating peripheral resistance. 

Both prerenal factors (dehydration, blood loss, altered vasomotor tone, age-related decreases in renal blood flow in rats) and postrenal factors (obstruction or extravasation of urine to the peritoneal cavity) may cause elevations of the commonly measured analytes that do not reflect primary kidney injury. Plasma analytes also cannot be used to determine the location of renal injury (glomerular versus tubular, or tubular segment affected) (Baum et al. 1975 Corman and Michel 1987 Finco 1997 Newman and Price 1999). 

Similar cardiotoxicity has been reported with 5-fluoro-uracU. The reported events were generally consistent with a drug- or metabohte-induced increase in coronary vasomotor tone and spasm, leading to myocardial ischemia. The authors concluded that more attention should be given to the conversion of flucjdosine to fluorouracil however, it is not clear whether flucjdosine should be contraindicated in patients with vasospastic or exertional angina. 

Cardiovascular reactions include bradycardia and/or conduction disturbances, myocardial depression and peripheral vasodilatation resulting in hypotension, as a result of direct effects on the myocardium and blood vessels. In extreme cases, cardiovascular collapse (cardiac arrest) may result from decreased ventricular contractility, decreased myocardial conduction and loss of peripheral vasomotor tone. 

Tousoulis D, Davies GJ, Tentolouris C, Crake T, Katsimaglis G, et al. 1998. Effects of changing the availability of the substrate for NO synthase by L-arginine administration on coronary vasomotor tone in angina patients with angiograph-ically narrowed and in patients with normal coronary arteries. Am. J. Cardiol. 82 1110-13... 

Mebutamate - The mechanism of the hypotensive action of mebutamate was investigated in cross circulation experiments in anesthetized dogs. The results suggested that the initial and transient hypotension may be largely due to the direct vasodilator action and that the sustained effect may be accounted for by an inhibition of the sympathetic vasomotor tone at the spinal and ganglionic levels. ... 

Using the stereotaxic method, the direct injection of various catecholamine and imidazoline dnxgs into the rostroventrolateral part of the brainstem revealed that only imidazolines could reduce blood pressure in a dose-dependent manner, like clonidine did. Those receptors appeared to be involved in the vasomotor tone regulation and in the mechanism of the hypotensive effect of clonidine-like drugs [1,6-9]. 

All benzodiazepines can impair consciousness and interfere with neurologic assessment. Although rare, brief (< 1 minute) cardiorespiratory depression may occur and can necessitate assisted ventilation or require intubation. This is especially true if a benzodiazepine is used concomitantly with a barbiturate. Hypotension secondary to a reduction in vasomotor tone may occur following large doses of a benzodiazepine. ... 

An action on medullary imidazoline receptors alone is sufficient to inhibit the vasomotor tone and therefore to reduce blood pressure. Actually, the same holds true for the exclusive stimulation of a 2-adrenoceptors within the central nervous system. 

Baroreceptors are stretch receptors in the arterial portion of the cardiovascular system, specifically in the carotid region and in the aortic arch. They monitor (sense) hydrostatic pressure through specialized nerve endings that are sensitive to mechanical deformation of the blood vessel wall. Nerve impulses over these afferent fibers to the central nervous system increase whenever the arterial pressure in the vicinity of the baroreceptors increases. This inhibits outflow of sympathetic impulses, reducing vasomotor tone and decreasing the heart rate. Thus vasoconstrictor tone and cardiac output are reduced as a result of the initial pressure increase that caused these series of events to occur. Both factors tend to return the pressure to previous, lower levels. 

Endocrine hypertension actually includes several diseases characterized by elevated blood pressure with causes that can be traced to an endocrine abnormality or actual injury. Two of the best known are pheochromocytoma and Cushing s syndrome. The former is characterized by a tumor that secretes excessive amounts of catecholamines, primarily epinephrine. This in turn increases vasomotor tone, resulting in a considerable pressure increase. Surgical removal of the tumor is curative. The latter is a disease of the adrenal cortex where tumors may also develop. The cortex is the site of corticosteroid production. Thus excessive production of aldosterone will act on the kidneys, causing retention of sodium and water. This increased blood volume is usually manifested as mild, or sometimes moderate, hypertension. 

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