Circulatory system vasodilators

VIP has a large number of ill-defined physiological actions, some of which are shared with other similar polypeptide hormones (secretin and GIF). It acts as a neurotransmitter in the central and autonomic nervous systems and causes vasodilation and relaxation of the smooth muscles of the circulatory and genitourinary systems and the gut. Other actions of VIP include an increase of water and electrolyte secretion from the pancreas and gut release of hormones from the pancreas, gut, and hypothalamus stimulation of lipolysis, glycolysis, and bile flow and inhibition of gastrin and gastric acid secretion. Most of the actions of VIP tend to be of short duration because of its rapid degradation. 

Carbon dioxide is a rapid, potent stimulus to ventilation. Inhalation of 10% CO can produce minute volumes of 75 L/min in normal individuals. Carbon dioxide acts at multiple sites to stimulate ventilation. Elevated Pco causes bronchodilation, whereas hypocarbia causes constriction of airway smooth muscle these responses may play a role in matching pulmonary ventilation and perfusion. Circulatory effects of CO result from the combination of direct local effects and centrally mediated effects on the autonomic nervous system. The direct effects are diminished contractility of the heart and vascular smooth muscle (vasodilation). The indirect effects result from the capacity of CO to activate the sympathetic nervous system these indirect effects generally oppose the local effects ofCO. Thus, the balance of opposing local and sympathetic effects determines the total circulatory response to CO. The net effect of CO inhalation is an increase in cardiac output, heart rate, and blood pressure. In blood vessels, however, the direct vasodilating actions of carbon dioxide appear more important, and total peripheral resistance decreases when the Pco is increased CO also is a potent coronary vasodilator. Cardiac arrhythmias associated with increased Pco are due to the release of catecholamines. 

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