RAT AORTIC VASOREACTIVITY IS ALTERED BY OLD-AGE AND HINDLIMB UNLOADING

Prolonged bed rest in young adults leads to a number of cardiovascular alterations, including orthostatic intolerance and decreased exercise capacity. Similar changes occur with advanced age. These modification s of cardiovascular function have been suggested to be causally relate d to changes in peripheral vascular reactivity. Using rat hindlimb unl oading as an animal model of physical inactivity, this study was desig ned to determine whether prolonged decreases in weight-bearing activit y induce changes in vascular reactivity that are similar to those occu rring in senescent rats and whether the imposition of inactivity on ol d rats further modifies any age-related alterations in vasomotor respo nsiveness. Responses to vasoactive compounds were examined in vitro by using isolated abdominal aortic rings. Maximal isometric contractile force evoked by the vaso-constrictors KCl, norepinephrine (NE), and ar ginine vasopressin was lower in aortic segments from young hindlimb-un loaded (YHU), old control (OC), and old hindlimb-unloaded (OHU) rats c ompared with that from young control (YC) rats. Sensitivity [mean effe ctive concentration (EC(50))] to KCl was enhanced in segments from bot h old and unloaded animals compared with YC rats, but EC(50) values fo r the other constrictors were not different among groups. Vasorelaxati on responses induced by acetylcholine (10(-7) M NE preconstriction) we re lower in vessel rings from OC (1 x 10(-7) to 3 x 10(-6) M), YHU (10 (-7) to 10(-5) M), and OHU (10(-7) to 10(-5) M) rats than those from Y C animals. In addition, vessel rings from OC, YHU, and OHU rats were l ess sensitive to sodium nitroprusside-induced relaxation. This study d emonstrates that both physical inactivity via hindlimb unloading and o ld age result in similar alterations in vascular responsiveness. In ad dition, the imposition of reduced activity on old rats is of little fu rther consequence. These results indicate that 2 wk of hindlimb unload ing-induced inactivity in young animals is associated with alterations in vascular reactivity that are equivalent to those in vessels from o ld animals.