β‐Adrenergic agonists regulate cell membrane fluctuations of human erythrocytes

1 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. 2 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 β‐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. 3 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 β‐agonist effect on CMFs is transduced via a cAMP‐dependent pathway. 4 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. 5 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 β‐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.