Superficial buffer barrier and preferentially directed release of Ca2+ in canine airway smooth muscle

We examined cytosolic concentration of Ca²⁺ ([Ca²⁺](i)) in canine airway smooth muscle using fura 2 fluorimetry (global changes in [Ca²⁺](i)), membrane currents (subsarcolemmal [Ca²⁺](i)), and contractions (deep cytosolic [Ca²⁺](i)). Acetylcholine (10^-4 M) elicited fluorimetric, electrophysiological, and mechanical responses. Caffeine (5 mM), ryanodine (0.1-30 μM), and 4-chloro-3-ethylphenol (0.1-0.3 mM), all of which trigger Ca²⁺-induced Ca²⁺ release, evoked Ca²⁺ transients and membrane currents but not contractions. The sarcoplasmic reticulum (SR) Ca²⁺-pump inhibitor cyclopiazonic acid (CPA; 10 μM) evoked Ca²⁺ transients and contractions but not membrane currents. Caffeine occluded the response to CPA, whereas CPA occluded the response to acetylcholine. Finally, KCl contractions were augmented by CPA, ryanodine, or saturation of the SR and reduced when SR filling state was decreased before exposure to KCl. We conclude that 1) the SR forms a superficial buffer barrier dividing the cytosol into functionally distinct compartments in which [Ca²⁺](i) is regulated independently; 2) Ca²⁺-induced Ca²⁺ release is preferentially directed toward the sarcolemma; and 3) there is no evidence for multiple, pharmacologically distinct Ca²⁺ pools.