Caveolae and propofol effects on airway smooth muscle

The i.v. anaesthetic propofol produces bronchodilatation. Airway relaxation involves reduced intracellular Ca(2+) ([Ca(2+)](i)) in airway smooth muscle (ASM) and lipid rafts (caveolae), and constitutional caveolin proteins regulate [Ca(2+)](i). We postulated that propofol-induced bronchodilatation involves caveolar disruption.

Caveolar fractions of human ASM cells were tested for propofol content. [Ca(2+)](i) responses of ASM cells loaded with fura-2 were performed in the presence of 10 µM histamine with and without clinically relevant concentrations of propofol (10 and 30 μM and intralipid control). Effects on sarcoplasmic reticulum (SR) Ca(2+) release were evaluated in zero extracellular Ca(2+) using the blockers Xestospongin C and ryanodine. Store-operated Ca(2+) entry (SOCE) after SR depletion was evaluated using established techniques. The role of caveolin-1 in the effect of propofol was tested using small interference RNA (siRNA) suppression. Changes in intracellular signalling cascades relevant to [Ca(2+)](i) and force regulation were also evaluated.

Propofol was present in ASM caveolar fractions in substantial concentrations. Exposure to 10 or 30 µM propofol form decreased [Ca(2+)](i) peak (but not plateau) responses to histamine by ~40%, an effect persistent in zero extracellular Ca(2+). Propofol effects were absent in caveolin-1 siRNA-transfected cells. Inhibition of ryanodine receptors prevented propofol effects on [Ca(2+)](i), while propofol blunted [Ca(2+)](i) responses to caffeine. Propofol reduced SOCE, an effect also prevented by caveolin-1 siRNA. Propofol effects were associated with decreased caveolin-1 expression and extracellular signal-regulated kinase phosphorylation.

These novel data suggest a role for caveolae (specifically caveolin-1) in propofol-induced bronchodilatation. Due to its lipid nature, propofol may transiently disrupt caveolar regulation, thus altering ASM [Ca(2+)](i).