Evidence for increased beta-adrenoreceptor responsiveness induced by 14 days of simulated microgravity in humans

We studied hemodynamic responses to alpha- and beta-receptor agonists in eight healthy men before and after 14 days of 6 degrees 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 . kg(-1). 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 . kg(-1)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). kg(-1). min(-1); P = 0.014) and vasodilation (-469 +/- 111 to -1,446 +/- 309 peripheral resistance units .mu g(-1). kg(-1). 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.