The effect of oxytocin on the contractile force of human atrial trabeculae

We performed an in vitro examination of the inotropic effect of oxytocin, chlorobutanol, and their combination to assess the effect of these drugs on the contractile force of human atrial trabeculae. Right atrial tissue samples were obtained during cardiac surgery with cardiopulmonary bypass. Trabeculae of the atrial appendage were dissected and mounted on muscle stands in a modified Krebs-Henseleit buffer bath. This isometric atrial trabecula preparation was subjected to a cumulative pharmacological protocol of either pure oxytocin, pure chlorobutanol, or a combination of the two drugs until no further change occurred in either developed force or resting force of the atrial trabeculae. A "no drug" buffer solution was used to assess the effect of time on the natural decay of the atrial preparation. The relative developed force of oxytocin plus chlorobutanol solution and pure chlorobutanol were similar in magnitude and lower than that in control experiments (P < 0.001) Pure oxytocin did not change the contractile force of atrial tissue. We conclude that pure oxytocin does not have a cardiodepressive effect in this human atrial preparation. Chlorobutanol has a negative inotropic effect, which is of a magnitude similar to a combined solution of chlorobutanol and oxytocin. Therefore, chlorobutanol added as a preservative to the commercial synthetic oxytocin solution may contribute to hypotension observed in patients after an intravenous bolus injection.

We obtained specimens of heart tissue from patients undergoing cardiac surgery and conducted a laboratory study of the effects of oxytocin and its preservative (chlorobutanol) on these tissue samples. Chlorobutanol decreased the ability of the heart to contract, while as pure oxytocin had no effect. This explains why maternal blood pressure may decrease and provides impetus to produce oxytocin with another, safer preservative.