Increased airway irritability (bronchial hyperreactivity) is a characteristic feature of patients suffering from asthma. The mechanism(s) causing this disorder are unknown, and consequently, learning about them is important for our understanding of the pathogenesis and course of reversible, obstructive airway disease. My hypothesis is that bronchial hyperreactivity results from airway smooth muscle (ASM) which is inherently, or which becomes hyperresponsive to stimulation. To date, all published work done in humans and animals with hyperreactive airways has been in vivo, where ASM hyperresponsiveness could not be directly measured or distinguished from other possible pathologic features, including mucosal damage, airway edema or secretions. Therefore, I propose to study in vitro the propeties of ASM from subjects with bronchial hyperreactivity that I have characterized in vivo. My preliminary studies in vitro indicate that both the sensitivity and maximal response of ASM to acetylcholine (ACh) are increased in both humans and ozone-exposed guinea pigs who are hyperreactive to inhaled methacholine (Meth). These findings suggest both receptor and nonreceptor-mediated mechanisms. I first plan to correlate the degree of hyperreactivity to Meth in these subjects with the (degree of the) pharmacologic and electromulation. Intracellular electrical activity from single human or guinea pig ASM cells will be recorded using glass microelectrodes. The sensitivity and maximal response of stimulated ASM will be determined using force transducers to measure isometric tension. I will also examine whether the degree of ASM hyperresponsiveness in the hyperreactive guinea pigs correlates with airway wall thickness or inflammation using light microscopy. Thereafter, I plan to pursue mechanisms possibly responsible for this ASM hyperresponsiveness. To study possible receptor-mediated hyperresponsiveness, I will use radioligand binding assays to determine whether the binding affinity or concentration of muscarinic receptors is increased. If normal, I will use light microscopic autoradiography, and Dr. Poo will use microiontophoresis to see whether there is nonuniform receptor distribution. To study possible nonreceptor-mediated hyperresponsiveness, I will simultaneously record mechanical and electrical activity to see if the ASM is electromechanically abnormal.
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