Figure 1. Effect of four concentrations of the optical isomers and the racemate of ketamine on spontaneous heart rate in untreated (UT) (n = 10), opioid receptor-blocked (OB) (n = 5), and catecholamine-depleted (CD) autonomically blocked (n = 8) isolated guinea pig hearts. UT hearts constituted the control group; OB hearts were perfused continuously with solution containing naloxone to block opioid receptors; and CD hearts taken from reserpine-pretreated animals were perfused continuously with atropine, propranolol, and phenoxybenzamine to block autonomic receptors. Absolute values at the initial control and during exposure to individual concentrations of ketamine are shown. Control values between the individual concentrations are not displayed. All values are significantly different from the preceding control values except for 25 micro Meter S(+)-ketamine in the UT group. Values for the CD group are significantly less than those for the UT and OB groups (significance not shown). There is no difference between the UT group and the OB group at equal concentrations. *P less or equal to 0.05 versus S(+)-ketamine at equal concentrations for the same group.