Figure 3. The concentration-response curve of the effect of carbachol alone ([square, open]) and with the addition of 1.3 MAC halothane ([round bullet, filled]) on the isometric force of contraction of rat papillary muscles. The effect of carbachol was investigated after prestimulation of the muscles with 0.3 micro Meter isoproterenol. Halothane did not modify the potency or efficacy of the negative inotropic effect of carbachol. All values are given with 95% confidence intervals. The forces of contraction before the exposure to carbachol (predrug value) were 15.4 mN/mm2(12.2–18.6 mN/mm2) under control conditions (n = 10) and 8.4 mN/mm2(6.2–10.6 mN/mm2) in the presence of 1.3 MAC halothane (n = 10; P < 0.05). Ordinate = the force of contraction in the percentage of predrug value; abscissa = the molar concentration of carbachol.