Inhibition of beta-1 receptor but not vagotomy can abolish the L-NAME evoked bradycardia in anesthetized rat

We reported previously that the nitric oxide synthesis inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) decreases cardiac output. Several studies have shown that inhibition of nitric oxide synthesis decreases the heart rate. In the present study, we investigated the effect of a single bolus administration of L-NAME on blood pressure and heart rate monitored for one hour in anesthetized rats and the influence of vagotomy and beta1-receptor blocker metoprolol on the L-NAME induced bradycardia. After L-NAME treatment, the blood pressure rose immediately after the injection of the drug (peak response in the third minute: +24%, p<0.001) and fell to the control level in the 20th minute. The heart rate decreased immediately after L-NAME administration, the lowest value being reached in the 10th minute (-14%, p<0.001). However, bradycardia was sustained even after the blood pressure had returned to the control level. Bilateral vagotomy failed to influence the negative chronotropic effect of L-NAME, but bradycardia was completely abolished by metoprolol pretreatment. We concluded that the bradycardia evoked by L-NAME is mainly due to the withdrawal of sympathetic tone upon the heart rate. However, the cause of sustained bradycardia after normalization of blood pressure cannot be elucidated.