Relaxant effect of ketamine and its isomers on histamine-induced contraction of tracheal smooth muscle.

The mechanism by which racemic (R(+/-)) ketamine relaxes airway smooth muscle is unclear and there is no information on the differential effects of ketamine and its isomers. In this study, we have examined the spasmolytic effect of R(+/-) ketamine and its isomers S(+) and R(-) ketamine and the role of intracellular calcium and opioid receptors in R(+/-) ketamine-induced relaxation. The tension of isolated guinea pig tracheal strips was measured isometrically with a force displacement transducer and contraction elicited with histamine 10(-5) mol litre-1. In histamine-preconstricted strips, the two ketamine isomers (4.5-18.0 x 10(-4) mol litre-1) produced equipotent relaxation. A subthreshold dose of each isomer of ketamine (10(-4) mol litre-1) which alone did not relax histamine-induced contraction (S(+), P < 0.01; R(+/-), P < 0.01; R(-), P < 0.05) significantly potentiated adrenaline 1.25-5.0 x 10(-9) mol litre-1-induced relaxation (potency: S(+) > R(+/-) > R(-)). Increase in extracellular Ca2+ (1.8-14.4 x 10(-3) mol litre-1) significantly reduced R(+/-) ketamine-induced relaxation. S(-) Bay K 8644, at concentrations up to 2.0 x 10(-6) mol litre-1, partially antagonized R(+/-) ketamine-induced relaxation whereas at 10(-5) mol litre-1 or higher it potentiated the response. Naloxone 1.5-6.0 x 10(-6) mol litre-1 did not affect the relaxation caused by R(+/-) ketamine. We conclude that although both ketamine isomers produced equipotent spasmolytic effects on airway smooth muscle precontracted with histamine, they differed in their ability to potentiate the relaxing effect of adrenaline. S(+) ketamine produced the greatest potentiation. Changes in intracellular Ca2+ level secondary to a reduction in the L-type Ca2+ current may partially mediate the spasmolytic effect of R(+/-) ketamine.