Response of Adrenergic Receptors to 10 Days Head-Down Tilt Bedrest

Abstract

Adrenergic receptor responses to 10 days head-down tilt (HDT) bed-rest were measured in six healthy young males. The densities of alpha2-receptors on platelets, beta2-receptors on lymphocytes, and the responsiveness of beta2-receptors to isoproterenol stimulation were assessed, as were the urinary catecholamine excretion rates. The densities of alpha2- and beta2-receptors were low before HDT, and were high during HDT. While the density of alpha2-receptors decreased after HDT, that of beta2-receptors remained high. No changes in the responsiveness of beta2-receptors were observed. The urinary catecholamine levels were high before HDT, decreased during the bedrest period. After HDT urinary norepinephrine excretion increased sgnificantly. 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.