1. To investigate the possibility that arterial hypertension is associated with changes in the physicochemical properties of cell membranes, we have studied the effects of dietary salt loading on platelet membrane microviscosity in hypertension-prone and -resistant Sabra rats.

2. Sixteen hypertension-prone and 14 hypertension-resistant Sabra rats were submitted to either a low-salt (0.25% NaCl) or a high-salt (4% NaCl) diet for 3–4 weeks. Platelet membrane anisotropy was determined, in the presence and absence of extracellular Na+, using two fluorescent probes, diphenylhexatriene and trimethylamino-diphenylhexatriene, inserted in different areas of the cell membranes.

3. A decrease in diphenylhexatriene anisotropy was demonstrated when platelets of hypertension-prone (but not hypertension-resistant) Sabra rats were suspended in a Na+-free medium. This alteration in membrane dynamic properties is localized within the hydrophobic core of the platelet membranes and is independent of salt intake. It reflects an abnormal fluidizing effect of extracellular Na+ removal.

4. Platelets of hypertension-prone and hypertension-resistant Sabra rats did not differ significantly in trimethylamino-diphenylhexatriene fluorescence anisotropy, irrespective of the incubation media used. Extracellular Na+ removal caused an increase in trimethylamino-diphenylhexatriene fluorescence anisotropy in all groups, the change being greatest in salt-loaded rats.

5. This study indicates that platelet membrane microviscosity is specifically altered in the hypertension-prone Sabra rat irrespective of salt intake. This raises the question of the relation of this inherited defect with the susceptibility of this strain to dietary salt loading.

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