Figure 4. The muscarinic inhibitory action of the racemic S(+)/R(-) ketamine/benzethonium chloride (BCl) mixture and Ketalar. Oocytes expressing the m1 muscarinic receptor were voltage clamped at -70 mV, and inward Cl sup - current was recorded in response to 10 sup -7 M acetyl-beta-methylcholine. At least five oocytes were used to determine each data point. (A) The concentration-inhibition relation of muscarinic inhibitory action of racemic S(+)/R(-) ketamine with BCl in a concentration equivalent to that present in Ketalar. The calculated median inhibitory concentration is 15 +/- 2 micro Meter, and the Hill coefficient is 0.6. (B) Isobolographic analysis of the interaction between equimolar S(+)/R(-) ketamine and BCl. The interaction is synergistic, and R is 2.5. *P = 0.016. (C) The concentration-inhibition relation of the muscarinic inhibitory action of Ketalar. The calculated median inhibitory concentration is 7.2 +/- 3.8 micro Meter, and the Hill coefficient is 0.7.