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The diuretic and natriuretic responses to stimulation of atrial receptors were compared in two groups of anaesthetized dogs, one group with a high blood volume and another group with a low blood volume. A diuretic response to stimulation of the atrial receptors was obtained in the two groups of animals. Both the diuretic and the natriuretic responses to stimulation of the atrial receptors were greater in the dogs with high blood volume than in the dogs with low blood volume. In a second series of experiments, a diuretic response to stimulation of the atrial receptors was demonstrated in both groups of animals following surgical denervation of the kidney. In the denervated kidney, a large increase in heart rate, associated with stimulation of the atrial receptors, significantly contributed to the diuretic response in the dogs with high blood volume but not to the response in dogs with low blood volume. In the denervated kidney, a natriuretic response was only obtained in the dogs with high blood volume in which a large increase in heart rate was associated with stimulation of the atrial receptors. The present results show that the diuretic and natriuretic responses to stimulation of atrial receptors are greater in dogs with high blood volume than in dogs with low blood volume, and that increases in heart rate, associated with stimulation of atrial receptors, significantly contribute to the diuretic and natriuretic responses in acutely denervated kidneys in the dogs with high blood volume.

SUMMARY. The purpose of this study was to determine the behavior of left ventricular mechanoreceptors with myelinated vagal afferents and to compare them with endings with nonmyelinated vagal afferents. Single unit activity was recorded from 13 endings with nonmyelinated vagal afferents (conduction velocity 2.1 ± 0.3 m/sec) and from 16 endings with myelinated vagal afferents (conduction velocity 7.3 ± 1.3 m/sec). Resting discharge frequencies of nonmyelinated afferents and of myelinated vagal afferents were 1.7 ± 0.3 and 2.7 ± 0.5 imp/sec (P < 0.1), respectively (at left ventricular end diastolic pressure of 6 mm Hg for both groups). Ten of 16 myelinated vagal afferents had pulse synchronous discharge under basal condition, whereas only 3 of 13 nonmyelinated vagal afferents had such activity. During aortic occlusion, the discharge of myelinated vagal afferents increased 1.7 ± 0.3 imp/sec per mm Hg, whereas nonmyelinated vagal afferents increased significantly (P < 0.05) less (0.5 ± 0.1 imp/sec per mm Hg). Discharge for both groups was linearly related to left ventricular end-diastolic pressure but not to left ventricular systolic pressure. Increases in left ventricular systolic pressure alone did not increase firing for either group. During aortic occlusion, the maximum discharge rates of myelinated vagal afferents (43 ± 7 imp/sec) were significantly higher than those of nonmyelinated vagal afferents (14 ± 3 imp/sec) at left ventricular end-diastolic pressure of 30 ± 2 and 24 ± 2 mm Hg, respectively. Both groups increased their discharge during volume expansion with myelinated vagal afferents showing greater sensitivity than nonmyelinated vagal afferents. All endings studied were in the inferoposterior wall of the left ventricle. All nonmyelinated vagal afferents were in or near the epicardium. In contrast, myelinated vagal afferents were equally distributed between the endocardium and the epicardium. Myelinated vagal afferents had discrete receptive fields (1-2 mm 2 ) whereas those of nonmyelinated vagal afferents were much larger (1 cm 2 ). In conclusion, the discharge of left ventricular endings with nonmyelinated vagal afferents and myelinated vagal afferents both appear to be determined mainly by changes in left ventricular end-diastolic pressure. They may be located at different depths in the left ventricular wall. Myelinated vagal afferents have greater sensitivity and maximum firing frequencies than nonmyelinated vagal afferents. (Circ Res 52: [291][292][293][294][295][296][297][298][299][300][301] 1983)

Plasma volume and blood volume were measured using T‐1824 in two groups of dogs which were kept on a diet differing only in its content of sodium chloride for periods exceeding six weeks. The group on low sodium intake received 0·55 mmol Na+.kg‐1.day‐1 and the group on high sodium intake received 12·4 mmol Na+.kg‐1.day‐1. Both plasma and blood volumes were distinctly greater in the group on high sodium intake than those in the group on low sodium intake, and there was no difference in the haematocrit between the two groups of dogs. It is concluded that a greater plasma volume and red cell volume resulted from high sodium intake than from low sodium intake.