Isoproterenol (isopropylarterenol, isopropyl NE, isoprenaline; see Table 10-1) is a potent, nonselective b receptor agonist with very low affinity for a receptors. Consequently, isoproterenol has powerful effects on all b receptors and almost no action at a receptors.
PHARMACOLOGICAL ACTIONS The major cardiovascular effects of isoproterenol (compared with Epi and NE) are illustrated in Figure 10-2. Intravenous infusion of isoproterenol lowers peripheral vascular resistance, primarily in skeletal muscle but also in renal and mesenteric vascular beds. Diastolic pressure falls. Systolic blood pressure may remain unchanged or rise; mean arterial pressure typically falls. Cardiac output increases due to the positive inotropic and chronotropic effects of the drug in the face of diminished peripheral vascular resistance. The cardiac effects of isoproterenol may lead to palpitations, sinus tachycardia, and more serious arrhythmias.
Isoproterenol relaxes almost all varieties of smooth muscle when the tone is high, an action that is most pronounced on bronchial and GI smooth muscle. Isoproterenol's effect in asthma may be due in part to inhibition of antigen-induced release of histamine and other mediators of inflammation, an action shared by b2-selective agonists.
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If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.