Does Chronic Experimental Head-Down Tilt Alter Intramural Innervation Density of the Limb Blood Vessels?

抄録

Earlier, substantial increases in intramural sympathetic innervation density of rat hind limb blood vessels were found after 2 weeks of experimental orthostasis using tubular 45° head-up tilt cages. In the present study, we presumed that chronic head-down tilting induces opposite changes in the innervation density. Tilted rats were kept 45° head-down in long tubular cages for either 2 or 4 weeks (HDT2, HDT4), control animals were maintained in horizontal tilt cages for 2 or 4 weeks (HOR2, HOR4). Saphenous vein, artery as well as brachial vein and artery segments were used for quantitative electron microscopic examinations. Intramural innervation density was defined by both nerve terminal density (NTD) and synaptic microvesicle count (SVC) within the vascular adventitia. Neither HDT2 nor HDT4 resulted in a decrease of NTD and SVC of the saphenous or brachial vein and artery, instead a tendency to increase was observed in some cases. Thus, in contrast to the large increases we found earlier in hind limb vascular innervation density after 2 week head-up tilting, head-down tilting of the same duration—or even twice as long—did not decrease the adventitial innervation density in our model. We assume that the quasi-free locomotor exercise allowed to the tilted animals in the long tubular cages may counteract a possible suppressive effect of chronic head-down tilt on hind limb vascular innervation density.