Effects of head-up tilt on mean arterial pressure, heart rate, and regional cardiac output distribution in aging rats

摘要

Many senescent individuals demonstrate an inability to regulate mean arterialpressure (MAP) in response to standing or head-up ti however, whether this agingeffect is the result of depressed cardiac function or an inability to reduce peripheralvascular conductance remains unknown. Therefore, the purpose of this research was toinvestigate the effects of aging on MAP, heart rate (HR), regional blood flow (viaradioactive-microspheres), and vascular conductance during head-up tilt in consciousyoung (4 mo; n=12) and old (24 mo; n=10) male Fischer-344 rats. Heart rate and MAPwere measured continuously during normal posture and during 10 minutes of head-uptilt. Blood flow was determined during normal posture and at the end of 10 minutes ofhead-up tilt. Young rats increased MAP significantly at the onset of head-up tilt andgenerally maintained the increase in MAP for the duration of head-up tilt, while agedrats showed a significant reduction in MAP after 10 minutes of head-up tilt. In thenormal posture, aged rats demonstrated lower blood flow to splanchnic, bone, renal, andskin tissues versus young rats. With tilt there were decreases in blood flow to skin, bone, and hind-limb in both age groups and in fat, splanchnic, reproductive, and renaltissues in the young. Bone blood flow was attenuated with age across both conditions inhind foot, distal femur, femur marrow, and proximal and distal tibia. Head-up tilt causeda decrease in blood flow across both age groups in all bones sampled with the exceptionof the hind foot. These results provide evidence that the initial maintenance of MAP inaged rats during head-up tilt occurs through decreased regional blood flow and vascularconductance, and that the fall in pressure is not attributable to an increase in tissue bloodflow and vascular conductance. Therefore, reductions in arterial pressure during headuptilt are likely a result of an old age-induced reduction in cardiac performance. Inaddition, this is the first study to demonstrate a decreased bone vascular conductance inboth young and old rats during head-up tilt.