Vagus, cardiac effects

The histamine H2-receptor (359 amino acids) is best known for its effect on gastric acid secretion. Histamine H2-receptor activation, in conjunction with gastrin and acetylcholine from the vagus, potently stimulate acid secretion from parietal cells. High concentrations of histamine are also present in cardiac tissues and can stimulate positive chronotropic and inotropic effects via H2-receptor stimulation and activation of adenylyl... 

Heart rate In CHF patients, the heart rate is decreased. Digitalis produce a decrease in heart rate by stimulation of vagus. The vagal effect is probably evoked by sensitization of carotid baroreceptors, and by direct stimulation of vagal centre. The vagal action can be blocked by atropine but after full digitalising dose the effect can not be blocked by atropine and it is due to its direct cardiac action. In CHF patients, the sympathetic activity is increased as a compensatory phenomenon which leads to tachycardia. Digitalis decreases the... 

Cardiovascular System. Atropine is sometimes used to block the effects of the vagus nerve (cranial nerve X) on the myocardium. Release of acetylcholine from vagal efferent fibers slows heart rate and the conduction of the cardiac action potential throughout the myocardium. Atropine reverses the effects of excessive vagal discharge and is used to treat the symptomatic bradycardia that may accompany myocardial infarction.4 Atropine may also be useful in treating other cardiac arrhythmias such as atrioventricular nodal block and ventricular asystole. 

Cryptopine was isolated by Smiles (579) from opium in 1867. Its effect (580) is in many respects similar to that of papaverine and berberine. In dogs it produces hypotension, inhibition of cardiac activity, stimulation of respiration, inhibition of faradization of the vagus, and decrease in the oculocardiac reflex in rabbits, it produces relaxation of the skeletal muscle (581). For guinea pigs the LD100 s.c. was found to be 160 mg/kg (582). Mercier el al. (583, 584) reported the effect of cryptopine on the intestine, the nervous system, blood pressure, cardiac activity, and respiration to range between that of papaverine and berberine. 

The sinoatrial (SA) node is innervated by both the sympathetic (beta and parasympathetic (vagus) nervous systems. Sympathetic activation increases the discharge rate of the SA pacemaker cells, and thereby increases heart rate (a positive chronotropic effect). Sympathetic nerves also innervate adrenergic receptors (betaj) on cardiac ventricular cells leading to an increase in stroke volume (a positive inotropic effect). Vagal activation, on the other hand, has the opposite effect and decreases heart rate and conduction velocity. In normal adults, cardiac vagal innervation is functionally predominant, so abolition of vagal activity results in a pronounced tachycardia (increased heart rate). 

Atropine may be used to treat some types of arrltythmias. It increases the heart rate hy blocking the effects of ACh on the vagus. In this context, it is used to treat certain reversible hiadyairhythmias that may aceompany aeute myocardial in-raiction. It is also used us an adjunct to anesthesia to protect against bradycardia, hypotension, and even cardiac arrest ittduccd by the skeletal muscle relaxant sueeinylcholine chloride. 

Information is fairly sparse, but these interactions appear normally to be of relatively minor importance. Be aware that changes in neuromuscular blockade (increases or decreases) can occur if beta blockers are used, but they seem to be unpredictable, and then often only modest in extent. The possible combined cardiac depressant effects of beta blockade and anaesthesia are well known (see Anaesthetics, general + Beta blockers, p.97). These effects may not be prevented when a neuromuscular blocker is used that has little or no effect on the vagus (such as atracurium or vecuronium). 

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