1. Atrial natriuretic peptide (ANP) levels were measured in cardiac tissues and in plasma from adult rats exposed to chronic alveolar hypoxia for periods of 2 h, 24 h and 7 days. Levels were also measured in rats that were maintained in hypoxia for 7 days and then returned to air for 24 h.
2. Plasma ANP was not altered at 2 h but was significantly increased at both 24 h and at 7 days. Plasma ANP in animals exposed to hypoxia for 7 days was normal 24 h after returning to air breathing, despite the persistence of indices of pulmonary hypertension.
3. No significant right atrial hypertrophy was observed under these conditions of chronic hypoxia. A reduction in right atrial ANP content was found at 24 h and was accompanied by a decrease in the number of electrondense granules per right atrial muscle cell. After exposure to hypoxia for 7 days, right atrial ANP and granule number was not different from control, and no alteration was found in right atrial ANP level after removal from the hypoxic environment.
4. No significant right ventricular hypertrophy was produced by exposure to hypoxia for 2 or 24 h. In the former group ventricular ANP had decreased significantly compared with control. Right ventricular hypertrophy was found in both the hypoxic groups after exposure for 7 days, when selective increases in right ventricular ANP content were found.
5. These findings are consistent with the hypothesis that ANP release occurs on exposure to chronic hypoxia and is independent of the associated cardiac hypertrophy and pulmonary vascular remodelling. The findings may have relevance to the natriuresis and reported changes in the renin-angiotensin-aldosterone axis under hypoxic conditions.