beta-Adrenergic agonists regulate cell membrane fluctuations of human erythrocytes

1. Mechanical fluctuations of the cell membrane (CMFs) in human erythrocyte s reflect the bending deformability of the membrane-skeleton complex. These fluctuations were monitored by time-dependent light scattering from a smal l area (similar to 0.25 mu m(2)) of the cell surface by a method based on p oint dark field microscopy. 2. Exposure of red blood cells (RBCs) to adrenaline (epinephrine) and isopr oterenol (isoprenaline) resulted in up to a 45% increase in the maximal flu ctuation amplitude and up to a 35% increase in the half-width of the amplit ude distribution. The power spectra of membrane fluctuations of control and treated cells revealed that adrenaline stimulated only the low frequency c omponent (0.3-3 Hz). Analysis of the dose-response curves of beta-adrenergi c agonists yielded an EC50 of 5 x 10(-9) and 1 x 10(-11) ar for adrenaline and isoproterenol, respectively. Propranolol had an inhibitory effect on th e 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 ma ximal level after 20-30 min but being abolished after 60 min. However, in t he presence of 3-isobutyl-1-methylxanthine, a partial stimulatory effect wa s maintained even after 60 min. Pentoxifylline and 8-bromo-cAMP elevated CM F's. However, exposure of ATP-depleted erythrocytes to adrenaline or 8-brom o-cAMP did not yield any elevation in CMFs. These findings suggest that the beta-agonist effect on CMFs is transduced via a cAMP-dependent pathway. 4. Deoxygenation decreased CMFs and filterability of erythrocytes by simila r to 30%. The stimulatory effect of isoproterenol on CMFs was 2.2-fold high er in deoxygenated RBCs than in oxygenated cells. 5. Exposure of RBCs to adrenaline resulted in a concentration-dependent inc rease in RBC filterability, demonstrating a linear relationship between CMF s and filterability, under the same exposure conditions to adrenaline. Thes e findings suggest that P-adrenergic agonists may improve passage of erythr ocytes through microvasculature, enhancing oxygen delivery to tissues, espe cially under situations of reduced oxygen tension for periods longer than 2 0 min.