Evidence for increased sympatho-adrenomedullary activity in young subjects with borderline hypertension.

Three different studies were performed to estimate the sympatho-adrenomedullary activity in young subjects with borderline hypertension (BHT, n = 40), compared with age-matched normotensive subjects (NT, n = 24). In the first study, 23 BHT and 9 NT were subjected to isometric stress by maintaining handgrip at the 30% level of maximal voluntary contraction for 3 min. The response of plasma total catecholamines at the second and third min during the isometric exercise were greater in BHT than in NT (98.9 ± 24.3 vs. 18.0 ± 30.7 and 93.0 ± 12.6 vs. 47.1 ± 15.4 pg/ml, respectively, p<0.05). In the second study, the effects of intravenous glucagon injection (1 USP unit) were studied in 12 BHT and 9 NT. The increments of plasm epinephrine (E) at 2 and 3 min after injection were significantly greater in BHT than those in NT: 44.1 ± 12.3 vs. 5.1 ± 4.4 pg/ml, and 68.9 ± 13.2 vs. 32.1 ± 8.9 pg/ml, respectively, p<0.05. In the last study, the pressor effects of intravenous norepinephrine (NE) infusion (100 and 200 ng/kg/min for 15 min) were examined in 17 BHT and 15 NT under three different sodium balances: regular customary diet, treatment with diuretics and high-sodium diet. Treatment with diuretics decreased and high-sodium diet increased the pressor response to NE in both groups, but there were no significant differences in NE reactivity between 2 groups throughout the study. The increments in mean blood pressure after NE infusion (200 ng/kg/min) on the three experimental periods correlated significantly with their preinfusion plasma NE concentrations in both BHT and NT: r = -0.58 (p<0.01) and r = -0.54 (p<0.01), respectively. Neither the slopes of these regression lines nor their intercepts differed between the 2 groups. Thus, BHT had augmented responses of plasma catecholamine to both isometric exercise and glucagon stimulation without vascular hyperreactivity to NE. Evidence presented suggests that the responses of both sympathetic nervous system and adrenal medulla to stress are increased in BHT.