The effect of atenolol on the spontaneous and reflex activity of the sympathetic nerves in the cat: influence of cardiopulmonary receptors.

1 Atenolol reduces sympathetic efferent discharge and attenuates the responses of the sympathetic nerves to changes in blood pressure. The present experiments were carried out to determine whether these changes were mediated by cardiopulmonary receptors whose afferents lie in the vagal nerves.2 Cats were anaesthetized with alpha-chloralose and artificially ventilated. In one group of cats recordings were made of sympathetic efferent discharge from few-fibre preparations from the lumbar trunk, splanchnic or renal nerves over a range of blood pressures. In a second group of cats changes in heart rate and blood pressure in response to bilateral occlusion of the common carotid arteries were investigated. In all cats the influence of vagal afferent fibres was removed by cooling both vagal nerves in the neck, both before and after administration of atenolol (3 mg kg(-1) i.v.).3 Cooling both vagal nerves produced significant increases in blood pressure, heart rate and spontaneous sympathetic efferent discharge but did not affect the relationship between sympathetic efferent discharge and mean blood pressure or the responses to carotid occlusion. Atenolol significantly reduced blood pressure, heart rate and sympathetic efferent discharge but the change in sympathetic efferent discharge on vagal cooling was less than before giving the drug. Atenolol also attenuated the reflex responses of the sympathetic nerves to changes in blood pressure and reduced responses to carotid occlusion. This attenuation was not removed by vagal cooling.4 Thus, neither the reduction in spontaneous sympathetic efferent discharge nor the attenuation of the baroreceptor reflex seen after atenolol, are due to an increased input to the brain from vagal afferent fibres. Other possible mechanisms whereby atenolol might exert its effects on the sympathetic nerves are discussed.