Ca2+ ion sequestration by guinea-pig tracheal cartilage: its influence on trachealis reactivity to KCl

Caveolae from canine airway smooth muscle contain the necessary components for a role in Ca(2+) handling

To explain that bronchial smooth muscle undergoes sustained agonist-induced contractions in a Ca(2+)-free medium, we hypothesized that caveolae in the plasma membrane (PM) contain protected Ca(2+). We isolated caveolae from canine tracheal smooth muscle by detergent treatment of PM-derived microsomes. Detergent-resistant membranes were enriched in caveolin-1, a specific marker for caveolae as well as for L-type Ca(2+) channels and Ca(2+) binding proteins (calsequestrin and calreticulin) as determined by Western blotting. Also, the PM Ca(2+) pump was present but not connexin 43 (a noncaveolae PM protein), the sarcoplasmic reticulum (SR) Ca(2+) pump, or the type 1 inositol 1,4, 5-trisphosphate receptor, supporting the idea that SR-derived membranes were not present. Antibodies to caveolin coimmunoprecipitated caveolin with calsequestrin or calreticulin. Thus some of the cellular calsequestrin and calreticulin associated with caveolin on the cytoplasmic face of each caveola. Immunohistochemistry of tracheal smooth muscle crysosections confirmed the localization of caveolin and the PM Ca(2+) pump to the cell periphery, whereas the SR Ca(2+) pump was located deeper in the cell. The presence of L-type Ca(2+) channels, the PM Ca(2+) pump, and the Ca(2+) bindng proteins calsequestrin and calreticulin in caveolin-enriched membranes supports caveola involvement in airway smooth muscle Ca(2+) handling.

The effect of EDTA on contractile responses of guinea-pig trachea in calcium-free medium and on the recovery of the contractile responses in calcium-containing solution

1. Acetylcholine (0.1 mmol/l) and KCl (80 mmol/l) induce contractile responses in guinea-pig trachea. 2. In the absence of extracellular Ca, contractile responses to ACh or KCl decrease as the preincubation time or the EDTA concentration increases. 3. Exposure of the tissue to Ca-free medium decreases the basal tone. 4. Prolonged incubation in the presence of EDTA (1 mmol/l) promotes a delay in the recovery of the tonic component of the contraction elicited subsequently by acetylcholine and KCl in physiological salt solution, and abolishes the phasic one.