Evidence for increased beta-adrenoreceptor responsiveness induced by 14 days of simulated microgravity in humans
Convertino, V.A.; Polet, J.L.; Engelke, K.A.; Hoffler, G.W.; Lane, L.D.; Blomqvist, C.G.; Eckberg, D.L.; Convertino, V.A.
American Journal of Physiology 273(1 Pt 2): R93-R99
1997
ISSN/ISBN: 0002-9513 PMID: 9249537 Document Number: 478691
We studied hemodynamic responses to alpha- and beta-receptor agonists in eight healthy men before and after 14 days of 6 degree head-down tilt (HDT) to test the hypothesis that increased adrenoreceptor responsiveness is induced by prolonged exposure to simulated microgravity. Steady-state infusions of isoproterenol (Iso) at rates of 0.005, 0.01, and 0.02 mu-g cntdot kg-1 cntdot min-1 were used to assess beta-1- and beta-2-adrenoreceptor responsiveness. Infusions of phenylephrine (PE) at rates of 0.25, 0.50, and 1.00 mu-g cntdot kg-1 cntdot min-1 were used to assess responsiveness of alpha-1-vascular adrenoreceptors. Slopes calculated from linear regressions between Iso and PE doses and changes in beat-to-beat heart rate, blood pressure, and leg vascular resistance (occlusion plethysmography) for each subject were used as an index of alpha- and beta-adrenoreceptor responsiveness. HDT increased the slopes of heart rate (1,056 +- 107 to 1,553 +- 83 beats mu-g-1 cntdot kg-1 cntdot min-1; P = 0.014) and vasodilation (-469 +- 111 to -1,446 +- 309 peripheral resistance units cntdot mu-g-1 cntdot kg-1 cntdot min-1; P = 0.0224) to Iso infusion. There was no alteration in blood pressure or vascular resistance responses to PE infusion after HDT. Our results provide evidence that simulated microgravity causes selective increases in beta-1- and beta-2-adrenoreceptor responsiveness without affecting alpha-1-vascular adrenoreceptor responses.