Cardiopulmonary responses to chronic hypoxia in transgenic mice that overexpress ANP

Elevated plasma atrial natriuretic peptide (ANP) levels have been shown to blunt pulmonary hemodynamic responses to chronic hypoxia, but whether elevated circulating ANP levels negatively feedback on cardiac expression of the ANP gene is unknown. Using a recently developed strain of transgenic mouse (TTR-ANF) that expresses a transthyretin promoter-ANP fusion gene in the liver, we studied the effect of chronically elevated plasma ANP levels on cardiac hypertrophic and pulmonary hemodynamic responses and expression of the endogenous cardiac ANP gene during chronic hypoxia. Plasma ANP levels were 10-fold higher in TTR-ANF mice than in their non-transgenic littermates. After 3 wk of hypobaric hypoxia (0.5 atm), right ventricular hypertrophy and pulmonary hypertension had developed in both groups of mice, but TTR-ANF mice had lower right ventricle-to-left ventricle plus septum weight ratios (0.39 +/- 0.01 vs. 0.45 +/- 0.02), right ventricular systolic pressures (25 +/- 2 vs. 29 +/- 2 mmHg), and lung dry weight-to-body weight ratios (0.48 +/- 0.03 vs. 0.57 +/- 0.01 mg/g) and less muscularization of peripheral pulmonary vessels (8.3 +/- 1.4 vs. 17.4 +/- 2.5%) than nontransgenic controls. Right atrial and ventricular steady-state ANP mRNA levels were the same in both groups of mice under normoxic and hypoxic conditions despite much higher plasma ANP levels and less pulmonary hypertension in TTR-ANF mice. We conclude that chronically elevated plasma ANP levels attenuate the development of hypoxic pulmonary hypertension in mice but do not suppress cardiac expression of the endogenous ANP gene under normoxic conditions nor blunt the upregulation of right ventricular ANP expression during chronic hypoxia.