Effect of age on blood pressure and membrane-dependent vascular responses in the rat.

Alterations in endothelium-dependent, sodium pump-mediated, and calcium-dependent responses of vascular smooth muscle were investigated in 5-7-, 24-26-, and 50-52-week-old male Sprague-Dawley rats. Age-dependent changes in systolic blood pressure were also determined. Although systolic blood pressure increased significantly with age, rats in all 3 age groups were considered normotensive. Initial studies on the passive force-response characteristics of strips of aortic and femoral arterial smooth muscle revealed that the level of passive force required for maximum active tension generation increased with increasing age. Subsequent studies were carried out using optimum passive force requirements. Endothelium-dependent relaxations of aortic smooth muscle induced by acetylcholine and the calcium ionophore A23187 decreased significantly with increasing age. An age-dependent decrease in the contractile response of aortic smooth muscle to ouabain and potassium-free physiological salt solution (PSS) was observed. Potassium relaxation of femoral smooth muscle following contraction to norepinephrine (NE) in a potassium-free PSS was also significantly attenuated with increasing age. No age-related alterations in calcium sensitivity (in the presence of 10(-7) M NE) or calcium relaxation (membrane stabilization) of femoral arterial smooth muscle was seen. These results show that endothelium-dependent and sodium pump-mediated responses are reduced in vascular smooth muscle of the rat with increasing age. However, no changes in calcium-dependent responses are apparent. These observations are discussed in relation to the vascular changes observed in hypertension.