Calcium concentration-dependent mechanisms through which ketamine relaxes canine airway smooth muscle

Background: Ketamine is a potent bronchodilator that, in clinically used concentrations, relaxes airway smooth muscle in part by a direct effect. This study explored the role of calcium concentration (Ca²⁺) in this relaxation. Methods: Canine trachea smooth muscle strips were loaded with the fluorescent probe fura-2 and mounted in a spectrophotometric system to measure force and intracellular calcium concentration ([Ca²⁺](i)) simultaneously. Calcium influx was estimated using a manganese quenching technique. Cyclic nucleotides in the airway smooth muscle strips were measured by radioimmunoassay. Results: In smooth muscle strips stimulated with submaximal (0.1 μM) and maximal (10 μM) concentrations of acetylcholine, ketamine caused a concentration-dependent decrease in force and [Ca²⁺](i). The sensitivity of the force response to ketamine significantly decreased as the intensity of muscarinic receptor stimulation increased; the median effective concentration for relaxation induced by ketamine was 59 μM and 850 μM for tissue contracted by 0.1 μM or 10 μM acetylcholine, respectively (P < 0.05). In contrast, the sensitivity of the [Ca²⁺](i) response did not depend on the intensity of muscarinic receptor stimulation. Ketamine at 1 mM significantly inhibited calcium influx. Ketamine did not significantly increase cyclic nucleotide concentrations. Conclusions: Ketamine-induced relaxation of canine airway smooth muscle is associated with a decrease in [Ca²⁺](i) and calcium influx, effects that are not mediated by an increase in cyclic nucleotides; and the sensitivity of the force response to ketamine decreases as the level of preexisting muscle tone increases, an effect that is not explained by differential effects on [Ca²⁺](i).