Response of adrenergic receptors to 10 days head-down tilt bedrest.

Adrenergic receptor responses to 10 days head-down tilt (HDT) bed-rest were measured in six healthy young males. The densities of alpha 2-receptors on platelets, beta 2-receptors on lymphocytes, and the responsiveness of beta 2-receptors to isoproterenol stimulation were assessed, as were the urinary catecholamine excretion rates. The densities of alpha 2- and beta 2-receptors were low before HDT, and were high during HDT. While the density of alpha 2-receptors decreased after HDT, that of beta 2-receptors remained high. No changes in the responsiveness of beta 2-receptors were observed. The urinary catecholamine levels were high before HDT, decreased during the bedrest period. After HDT urinary norepinephrine excretion increased significantly. It is likely that the receptors were down-regulated before HDT in response to a situation that was perceived as being stressful. There were no changes in receptor characteristics specifically attributable to HDT.