The effect of β₁-adrenoceptor blockade on cardiac output and organ blood flow in conscious rats

The acute effects of the β₁-adrenoceptor inhibition by atenolol were investigated on conscious rats. Cardiac output and organ blood flow were measured by 15-um radiolabeled microspheres during control and 20 min after administration of atenolol (1 mg/kg body wt). Arterial blood pressure and heart rate were measured continuously. Arterial blood gases and pH were determined immediately after the two microsphere injections and arterial blood samplings. The mean arterial blood pressure fell significantly shortly after the injection of atenolol and was reduced by 9% (p<0.02) after 20 min. Heart rate fell by 17% (p<0.05). The total peripheral vascular resistance increased by 25% (p<0.05). At the same time the arterial acid-base chemistry remained unaltered from the control. The cardiac output fell by 24% (p<0.05). Blood flow fell to all organs and tissues (0-67% except to the brain, adrenals, liver, ears and diaphragm. The greatest decrease was seen in perfusion of the adipose tissue (67%) and of the spleen (60%) indicating that the blood flow to these organs are normally highly influenced by β₁-adrenoceptor stimulation. The estimated work load of the heart was reduced by 20% which correlated well to a reduction of myocardial blood flow to the same degree.