Contraction of airway smooth muscle cause airway obstruction, and relaxation of this muscle is the principal action of B-adrenergic agonists used to treat asthma. Calcium, both extracellular and that released from intracellular stores, is an important second messenger that contributes to the determination of tension in smooth muscle. However, a paradox underscores our limited understanding of how cytosolic calcium concentrations ([Ca2+])i are regulated in airway smooth muscle cells. First, studies have shown that B-adrenergic agonists and muscarinic agonists both stimulated calcium influx and caused large increases in basal [Ca2+]i, yet paradoxically, had different effects on tension. To explain this, it is hypothesized that the cytosol is not homogeneous with respect to calcium concentrations. Independent of regulating the net flux of calcium that determines tension, muscarinic and B-adrenergic agonists stimulate calcium influx pathways that are either functionally or anatomically segregated at the periphery of the cell. These segregated influx pathways maintain peripheral sarcoplasmic reticulum (SR) calcium stores but are not major determinants of tension. Progress supports this general hypothesis and, further, it appears that SR refilling may have unique features in airway smooth muscle. Since SR calcium stores are important for understanding smooth muscle responses to multiple stimulations by contractile agonists as probably occurs in asthma, th aims of continuing this study are to better understand SR refilling.
The aims are: (1) in the absence of agonist, determine how the sarcoplasmic reticulum (SR) refills after being depleted of calcium; (2) determine whether protein tyrosine kinases and the cytoskeleton support SR refilling: (3) characterize S refilling during sustained muscarinic stimulation; (4) characterize SR refilling in the presence of B-adrenergic agonists. To address these aims, the principal method is radiometric imaging of [Ca2+]i in single, bovine airway smooth muscle cells loaded with the calcium dye, fura-2.
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