Tonic afferent sympathetic activity from the heart

Inhibition of peripheral vascular compensation after the induction of myocardial ischemia in dogs

The fact that peripheral vascular resistance fails to increase, or does so to only a small degree, thus contributing to hypotension after the onset of myocardial ischemia, has been noted in both clinical and experimental studies. We examined this phenomenon in canine kidney, mesentery, spleen, and skeletal muscle after occlusion via ligation of the circumflex coronary artery. Animals anesthetized with pentobarbital sodium (30 mg/kg, IV) and with intact vagus nerves exhibited reduced cardiac output (CO), maintenance of mean arterial blood pressure (MAP) at or near control levels, and increased total peripheral resistance (TPR). Blood flows to the kidney (RBF) and mesentery (MBF) were decreased resulting in substantial elevations of renal vascular resistance (RVR) and mesenteric vascular resistance (MVR). Blood flow supplied to the hindlimb skeletal muscle (FBF) by the femoral artery did not vary substantially from control levels and, therefore, neither did femoral vascular resistance (FVR). Splenic arterial blood flow (SpBF) was somewhat decreased but splenic vascular resistance (Sp VR) remained un changed. Heart rate exhibited no change from control levels.

When coronary artery ligation was preceded by bilateral cervical vagot omy, CO was decreased as before but MAP exhibited sustained decreases. No change from control values was noted in RBF, FBF, MBF, or SpBF. RVR, MVR, and Sp VR were significantly decreased while FVR remained unchanged. TPR in all cases was increased but not to the same degree as was noted in nonvagotomized animals. Again, no change in heart rate occurred.

Unlike the hemodynamic pattern observed in vagotomized dogs subjectedto the induction of myocardial ischemia, treatment of animals with a cholinergic blocking dose of atropine sulfate (1 mg/kg, IV) before coronary artery occlusion produced maintenance of MAP at or near control levels, lesser and slower reductions of CO and significantly increased TPR. No change was observed in RBF, FBF, MBF, SpBF or in RVR, FVR, MVR, and SpVR. Although there were no changes in heart rate from control levels, these values in atropinized dogs were usually higher than corresponding figures in other series.

When the induction of mild pericardial tamponade was substituted for coronary artery ligation, both vagotomized and non-vagotomized dogs exhibited normal peripheral compensations to the resulting hypotension and CO reduction, i.e., RBF decreased markedly and RVR was substantially increased as was TPR. No change in heart rate occurred.

These results indicate that (1) myocardial ischemia is necessary to trigger the abnormal loss of peripheral vascular compensatory responses; (2) vagal afferent nerves serve a protective function by mediating normal compensatory maneuvers, and (3) an additional mechanism, activated by ischemia but masked by the vagi, may cause inappropriate vasodilation when vagal afferent nerve activity is lost. A model has been constructed that presents a possible mechanism mediating the described phenomena.

Afferent fibres from pulmonary arterial baroreceptors in the left cardiac sympathetic nerve of the cat

1. Afferent discharges were recorded from the left cardiac sympathetic nerve or the third sympathetic ramus communicans of anaesthetized cats. Twenty-one single units with baroreceptor activity were obtained.2. The receptors of each unit were localized to the extrapulmonary part of the pulmonary artery, determined by direct mechanical probing of the wall of the pulmonary artery after death of the animals. Conduction velocity of the fibres ranged from 2.5 to 15.7 m/sec.3. Afferent discharges occurred irregularly under artificial ventilation. The impulse activity was increased when pulmonary arterial pressure was raised by an intravenous infusion of Locke solution, or by occlusion of lung roots, and decreased by bleeding the animal from the femoral artery.4. Above a threshold pressure, discharges occurred synchronously with the systolic pressure pulse in the pulmonary artery. A progressive further rise in pressure did not produce an increase in the number of impulses per heart beat. Occlusion of lung roots initially elicited a burst of discharges but the number of impulses for each cardiac cycle gradually decreased.5. The receptors responded to repetitive mechanical stimuli up to a frequency of 10/sec, but failed to respond to stimuli delivered at 20/sec.6. The results provide further evidence for the presence of afferent fibres in the cardiac sympathetic nerve. These afferent fibres are likely to provide the spinal cord with specific information only on transient changes in pulmonary arterial pressure.

Nervous activity of afferent cardiac sympathetic fibres with atrial and ventricular endings

1. We recorded the electrical activity of single afferent cardiac fibres isolated from the third and fourth left thoracic sympathetic rami communicantes of anaesthetized cats. Their conduction velocities ranged from 12 to 32 m/sec.2. The endings of each fibre were localized to one cardiac chamber by mechanical probing of the opened heart performed at the end of the experiment.3. The impulse activity was spontaneous and, in fibres with atrial or ventricular endings, it was in phase with a particular atrial or ventricular event.4. This nervous activity increased during increases in pressure occurring in the chamber where the endings were located. Conversely, decreases in pressure were accompanied by decreased nervous discharge.5. In some experiments the left coronary artery was perfused at different flows and pressures. Brief decreases or increases in coronary flow and pressure decreased or increased, respectively, the discharge of fibres with atrial or ventricular endings. Fibres were excited by intracoronary injections of veratridine.6. Cessation of coronary pump flow increased the discharge of fibres with atrial or ventricular endings only when myocardial ischaemia was accompanied by signs of heart failure.7. These afferent cardiac sympathetic fibres which provide the spinal cord with continuous specific information on cardiac events are likely to contribute to the neural control of circulation.

A cardiocardiac sympathovagal reflex in the cat

The reflex changes in single cardiac vagal efferent fibers elicited by excitation of afferent cardiac sympathetic fibers were studied in cats anesthetized with chloralose and urethane. Efferent vagal and sympathetic units were dissected from the end of a cardiac nerve cut at its junction with the right atrium. In some cases, efferent vagal units were dissected from the cervical vagus. Excitation always evoked a clear reduction in the discharge of cardiac vagal units and a clear increase in the discharge of cardiac sympathetic fibers. The effects on cervical vagal efferents were variable. Hence, excitation of afferent cardiac sympathetic fibers could simultaneously elicit inhibition of the vagal outflow to the heart and excitation of the sympathetic outflow. By contrast, stimulation of the cut central end of the contralateral vagus produced the opposite effects. Convergence of afferent fibers in the vagi and the cardiac sympathetic nerves on the same cardiac vagal and cardiac sympathetic postganglionic neurons was demonstrated. Convergence from carotid sinus baroreceptors was also observed.