1. To study the importance of cardiac innervation in the regulation of atrial natriuretic peptide, plasma atrial natriuretic peptide levels were measured during symptom-limited, graded exercise on a cycle ergometer in seven male orthotopic cardiac transplant recipients.

2. Resting plasma atrial natriuretic peptide was significantly higher in the transplant recipients than in two control groups, one matched to the transplant recipients (group 1) and the other to the age of the donor heart (group II).

3. The response to exercise of the cardiac transplant recipients was compared with the response of control group II. Mean maximal work load achieved with exercise was around 40% lower in the cardiac transplant recipients. During exercise, plasma atrial natriuretic peptide levels increased in both the cardiac transplant recipients and the control subjects. The increase in plasma atrial natriuretic peptide with exercise was greater in absolute, but less in percentage, terms in transplant recipients than in the control subjects.

4. The increase in mean arterial pressure with exercise was similar in patients and in control subjects; however, heart rate increased in the patients by only 33% compared with a rise of 151% in the control group.

5. These results provide insight into the control of the sodium regulatory hormone atrial natriuretic peptide. First, factors other than a change in heart rate appear of importance in the regulation of atrial natriuretic peptide. Secondly, these findings suggest that cardiac innervation is not of dominant importance in the modulation of atrial natriuretic peptide secretion.

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