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-Adrenergic agonists regulate cell membrane fluctuations of human erythrocytes
Mechanical fluctuations of the cell membrane (CMFs) in human erythrocytes reflect the bending deformability of the membrane-skeleton complex. These fluctuations were monitored by time-dependent light scattering from a small area (~0·25 µm2) of the cell surface by a method based on point dark field microscopy.
Exposure of red blood cells (RBCs) to adrenaline (epinephrine) and isoproterenol (isoprenaline) resulted in up to a 45 % increase in the maximal fluctuation amplitude and up to a 35 % increase in the half-width of the amplitude distribution. The power spectra of membrane fluctuations of control and treated cells revealed that adrenaline stimulated only the low frequency component (0·3-3 Hz). Analysis of the dose-response curves of -adrenergic agonists yielded an EC50 of 5 × 10-9 and 1 × 10-11 M for adrenaline and isoproterenol, respectively. Propranolol had an inhibitory effect on the stimulatory effect of isoproterenol. These findings show a potency order of propranolol > isoproterenol > adrenaline.
The stimulatory effect of adrenaline was a temporal one, reaching its maximal level after 20-30 min but being abolished after 60 min. However, in the presence of 3-isobutyl-1-methylxanthine, a partial stimulatory effect was maintained even after 60 min. Pentoxifylline and 8-bromo-cAMP elevated CMFs. However, exposure of ATP-depleted erythrocytes to adrenaline or 8-bromo-cAMP did not yield any elevation in CMFs. These findings suggest that the -agonist effect on CMFs is transduced via a cAMP-dependent pathway.
Deoxygenation decreased CMFs and filterability of erythrocytes by ~30 %. The stimulatory effect of isoproterenol on CMFs was 2·2-fold higher in deoxygenated RBCs than in oxygenated cells.
Exposure of RBCs to adrenaline resulted in a concentration-dependent increase in RBC filterability, demonstrating a linear relationship between CMFs and filterability, under the same exposure conditions to adrenaline. These findings suggest that -adrenergic agonists may improve passage of erythrocytes through microvasculature, enhancing oxygen delivery to tissues, especially under situations of reduced oxygen tension for periods longer than 20 min.
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