Hypoxemia increases renin secretion rate in anesthetized newborn lambs

The response of renin secretion rate (RSR) to acute systemic hypoxemia (mean arterial p0₂ 34±8 torr) was studied in mechanically ventilated, anesthetized newborn lambs 5–10 days of age (n=6). Ventilation of these lambs with room air (normoxemia) was followed by administration of low oxygen inhaled gas mixture (f_i0₂ 0.11) which was associated with no change in arterial pC0₂, pH, mean arterial pressure (MAP), renal blood flow (RBF, measured by electromagnetic flow probe), and calculated renal vascular resistance (RVR). Arterial plasma renin activity (PRA_A 4.28±1.73 to 6.46±3.00 ng AI/ml · hr), renal vein plasma renin activity (PRA_RV, 6.26±3.79 to 11.44±7.11 ng AI/ml · hr) and renin secretion rate (RSR, 19.86±21.70 to 51.32±48.54 units/min · KgBW) increased significantly (p<0.05) in response to hypoxemia. Restoration of normoxemia (arterial p0₂ 100±18 torr) was associated with significant decline in MAP (to 65±14 mmHg) and RBF (to 9.0±2.1 ml/min · KgBW) and further increases in PRA_A (to 8.98±3.40 ng AI/ml · hr), PRA_RV (to 19.04±10.62 ng AI/ml · hr) and RSR (to 88.6±77.6 units/min · KgBW). PRA_A correlated strongly with PRA_RV (r=0.84) and RSR (r=0.60) in these lambs. These results suggest that PRA_A, PRA_RV and RSR increase in response to hypoxemia in anesthetized lambs by a mechanism other than renal arterial baroreceptor stimulation, although this mechanism may be active during recovery from hypoxemia. Furthermore, PRA_A closely approximates RSR in newborn lambs under these conditions.