Studies of the mechanisms underlying impairment of beta-adrenoceptor-mediated effects in human hypertension

To investigate the impairment of beta-adrenoceptor responsiveness in human hypertension, we evaluated the effect of an oral salt load (400 mEq/day of NaCl for 7 days) on plasma catecholamine concentrations and beta-adrenoceptor-mediated effects in 11 young patients with mild essential hypertension. Responses of heart rate and plasma cAMP to isoproterenol administration were used as indices of beta-adrenoceptor responsiveness. Salt loading induced a significant reduction in the dose of isoproterenol required to raise the heart rate by 25 bpm (CD₂₅) (from 7.6 ± 1.5 to 5.3 ± 0.9 μg, p <0.05) and an increase in the slopes of the regression lines for heart rate changes and isoproterenol doses (ΔHR/IS) (from 3.3 ± 0.6 to 4.7 ± 0.7, p <0.05) and for plasma cyclic AMP (cAMP) level changes and isoproterenol doses (ΔcAMP/IS) (from 0.3 ± 0.06 to 1.4 ± 0.3, p <0.05). After salt loading there was a significant reduction in plasma catecholamine concentrations with a significant relationship between changes in upright plasma epinephrine levels and changes in CD₂₅ (r = 0.904, p <0.01) and in the slopes for ΔHR/IS (r = 0.983, p <0.001) and ΔcAMP/IS (r = 0.922, p <0.001). These results support the hypothesis that the impairment of beta-adrenoceptor sensitivity observed in human hypertension is associated with a beta-adrenoceptor overstimulation due to chronically elevated adrenergic tone.