Effect of propranolol on the relationship between atrial systolic pressure and type A atrial receptor discharge in cats.

The responses of type A atrial receptors to graded infusions of saline and large doses of propranolol were examined in anesthetized cats. Infusion of saline raised the mean atrial pressure, but usually the amplitude of the atrial a wave was reduced. In general the receptor discharge was unaffected. Propranolol reduced the discharge from the control level when it was injected in doses no less than 4 mg/kg. Infusions of saline after propranolol resulted in an increase in the discharge and the increase was related to the amplitude and/or initial pressure of the a wave. In one case the discharge after propranolol was less than that when the atrium was widely slit open. It is concluded that at least part of the effect of the drug is due to a direct depression of the receptor rendering it less sensitive to the stretch provided by atrial contraction. The demonstration of a stimulus-response relationship between the a wave and the receptor discharge at low levels of activity suggests that under normal conditiions the receptor operates on a plateau of maximum activity, thus making a response to small changes in stimulus strength obscure.