Interaction of extracellular albumin and intravenous anaesthetics, etomidate and propofol, on calcium signalling in rat airway smooth muscle cells.

It has been shown in vitro that general anaesthetics modify airway responsiveness via, at least partially, a direct inhibitory effect on calcium signalling in airway smooth muscle cells. However, in vivo, these anaesthetic compounds bind serum proteins. We have investigated the effect of exposure to extracellular albumin of freshly isolated airway smooth muscle cells on the propofol- and etomidate-induced inhibitory effect on calcium signalling. [Ca2+]i was measured by microspectrofluorimetry in rat isolated tracheal smooth muscle cells using the fluorescent dye indo-1. Propofol (3 x 10(-4) M) and etomidate (10(-4) M) were the lowest 'effective' concentrations that altered the [Ca2+]i response. This alteration consisted of a decrease in both the amplitude of the [Ca2+]i peak (from 358 +/- 13 nM to 65 +/- 15 and 108 +/- 27 nM for propofol and etomidate, respectively) and the percentage of responding cells (from 80% to 37 and 25% respectively) in response to the low concentration of ACh and a decrease in the Ca2+ oscillation frequency (from 9.9 +/- 0.3 min(-1) to 4.7 +/- 0.4 and 6.9 +/- 0.4 min(-1), respectively) in response to the high concentration of ACh. Increasing the concentration of albumin reduced the inhibitory effect of etomidate and propofol on the [Ca2+]i response to ACh. When extracellular albumin concentration was kept constant (20 g/L), increasing the concentration of etomidate by one log restored its inhibitory effect on the calcium signal. This study indicates that increasing the concentration of extracellular albumin reduces the inhibitory effect of intravenous anaesthetics on calcium signalling in airway smooth muscle cells. This report suggests that, in extrapolating in vitro dose-response relationships to those from in vivo conditions, the effect of the concentration of extracellular protein can be estimated.