Responses of the sino-atrial node to change in pressure in the sinus node artery

The sino-atrial node area was perfused selectively through the right atrial artery in 76 dogs. The relationship between changes in the blood pressure in this artery and sinus node rhythmicity was studied in 56 hearts in which the major blood supply to the sinus node artery came from the right atrial artery. A reciprocal relation was observed between changes of blood pressure in the sinus node artery and the sinus rate. Some characteristics were: (1) The negative chronotropic effect induced by raising the blood pressure was linearly related to the Degrees of pressure change and more prominent than the positive chronotropic effect induced by reducing the pressure. (2) This relationship was most prominent in a limited range between 20 and 100 mm Hg. (3) Dichloroisoproterenol, atropine, hexamethonium, reserpine and vagotomy did not modify these responses. It is concluded that this localized mechanosensitivity of the sinus node to changes in arterial pressure may be responsible for the regulation of the heart rate in the lower range of blood pressure and that complete regulation of the heart rate is achieved physiologically in combination with the reflexes through the carotid and aortic nerves.