In vitro effects of etomidate on intrinsic myocardial contractility in the rat

Etomidate is available in two different solvents: propylene glycol for induction of anesthesia and ethanol for maintenance of anesthesia. The direct effect of etomidate (1 and 5 micrograms/ml) and of its solvents on cardiac muscle was studied using rat left ventricular papillary muscle. Etomidate induced a slight positive inotropic effect in both solvents, as shown by an increase in maximum unloaded shortening velocity (Vmax) but not in force. At 0.5 mM Ca++ 5 micrograms/ml etomidate increased Vmax (128 +/- 18%, P less than 0.05) but not force (103 +/- 16%, NS). Using various afterloaded twitches, the peak power output (Emax) was calculated: 1 and 5 micrograms/ml etomidate increased Emax (107 +/- 8%, P less than 0.05, and 108 +/- 10%, P less than 0.05, respectively). This increase was related to the increase in Vmax and not in isometric force. Etomidate did not modify the elastic components of papillary muscle, isometric relaxation, and contraction-relaxation coupling under high load. Several findings suggest that etomidate in propylene glycol impaired the sarcoplasmic reticulum (SR) function: 1) it impaired the isotonic relaxation, the contraction-relaxation coupling under low load, and the load sensitivity of relaxation; and 2) it decreased postrest potentiated contraction, which is highly dependent on the SR. Nevertheless, alteration of SR function was only significant at high [Ca++]o and the beat-to-beat postrest recovery was not modified, indicating that the deleterious effects on SR function were moderate. The isotonic relaxation (max Vr) was more impaired by etomidate in propylene glycol (78 +/- 9%, P less than 0.001) and by propylene glycol alone (69 +/- 9%, P less than 0.001) than by etomidate in ethanol (97 +/- 12%, NS) and by ethanol alone (92 +/- 8%, P less than 0.05). This suggests that propylene glycol was responsible for the decrease in SR function. Etomidate in propylene glycol thus has a dual action on rat myocardium: 1) a slight positive inotropic effect due to etomidate per se, and 2) a slight decrease in SR function probably related to propylene glycol. However, because etomidate in propylene glycol induced a slight decrease in isometric force under certain experimental conditions (i.e., after isometric stabilization), etomidate in propylene glycol may induce a slight negative inotropic effect in some clinical conditions as a result of its dual action on the myocardium.