The purpose of the present study was to examine changes in membrane fluidity in hypertension by means of an electron spin resonance (ESR) and a spin labelling methods. 2. Erythrocytes from spontaneously hypertensive rats (SHR) and from patients with essential hypertension were examined and compared with those from age-matched normotensive controls. ESR spectra were obtained for a fatty acid spin label agent (5-nitroxide stearate) in the membranes. The values of outer hyperfine splitting (2T' parallel) and of order parameter (S) of the ESR spectra were significantly higher in erythrocytes from SHR and patients with essential hypertension than in those from normotensive controls. Similar results were obtained in cultured vascular smooth muscle cells of SHR. This finding shows that the membrane fluidity might be lower in SHR and in essential hypertension. 3. When Ca was loaded to erythrocytes with a Ca-ionophore (A23187), the parameters of the ESR spectra showed a greater increase (membrane fluidity was decreased) in SHR and in patients with essential hypertension than that in the normotensive controls. The Ca-induced alterations in membrane fluidity were not definitely observed in secondary hypertension. 4. These results suggest that the lower membrane fluidity might be a genetically determined abnormality of hypertension. The marked reduction of the membrane fluidity by Ca-loading in SHR and in essential hypertension might support the hypothesis that an abnormality of the Ca-handling at cellular levels could affect physical properties of the biomembranes in genetic hypertension.
