Sympathetic and respiratory responses to hypoxia in essential hypertension.

An increased sympathetic nervous response to hypoxia and reduced beta-adrenergic receptor function have been reported in hypertension. This study examines the relationship between hypoxia and beta-adrenergic receptor function in sixteen normotensive and eight hypertensive subjects. We measured the average arterial oxygen saturation and the end tidal carbon dioxide partial pressure in hypertensive and normotensive groups under normoxia and mild isocapnic hypoxia (15% O2, 85% N2). The ratio of isoproterenol-stimulated cAMP to basal cAMP on lymphocytes was measured in the two groups under normoxia. We also measured plasma norepinephrine levels and calculated the "Chronotropic 25 Dose" of isoproterenol in the two groups under normoxia and hypoxia. Hypertensives had higher plasma norepinephrine levels for either breathing condition (F=7.16, p=0.015). Under hypoxia, hypertensives showed a significant decrease in the average arterial oxygen saturation (F=4.92, p=0.038) and higher "Chronotropic 25 Dose" implying decreased beta-adrenergic receptor sensitivity (F=6.30, p=0.011). These results suggest that hypertensives have a diminished ventilatory response and impaired beta-adrenergic response under hypoxia.