¹Division of Cardiology, Department of Medicine, Wakayama Medical University, Wakayama, Japan

Correspondence: Dr. Kazushi Tsuda, Division of Cardiology, Department of Medicine, Wakayama Medical University, Kimiidera 811-1, Wakayama 641-8509, Japan E-mail: tsudak@mail.wakayama-med.ac.jp

^*Supported in part by grants-in-aid for scientific research from the Ministry of Education, Science, Sports, Culture and Technology of Japan (05670631, 08670819, 10670674), the Japan Clinical Pharmacology Foundation (1992 and1999), the Naito Foundation (1993), the Uehara Memorial Foundation (1994 and1999), the Kimura Foundation for Cardiovascular Diseases (1996), the Mitsui Foundation (1999), the Daiwa Health Foundation (2001), and the Takeda Science Foundation (2002).

Received 29 August 2002; Revised 11 September 2002; Accepted 1 October 2002.

Abstract

In the present study, to determine a possible role of nitric oxide (NO) in the regulation of membrane functions, we examined the relationship between plasma NO level and membrane fluidity of erythrocytes in postmenopausal women. We evaluated the membrane fluidity of erythrocytes obtained from hypertensive and normotensive postmenopausal women by means of an electron paramagnetic resonance (EPR) and spin labeling method. The EPR study revealed that the order parameter (S) for 5-nitroxide stearate in erythrocyte membranes was significantly greater in hypertensive postmenopausal women than in normotensive postmenopausal women. The finding indicated that the membrane fluidity of erythrocytes was decreased in hypertensive postmenopausal women compared with normotensive postmenopausal women. The plasma level of the NO metabolites (nitrite and nitrate) while fasting was significantly lower in hypertensive postmenopausal women than in normotensive postmenopausal women. In addition, the order parameter (S) in the EPR spectra of erythrocyte membranes was inversely correlated with the plasma NO metabolite level, which indicated that the lower membrane fluidity of erythrocytes was associated with the lower plasma NO level in postmenopausal women. These results are consistent with the hypothesis that NO may have a crucial role in the regulation of membrane fluidity of erythrocytes in postmenopausal women.