Influence of long-term experimental orthostatic body position on innervation density in extremity vessels

The aim of this study was to quantitate the density of nerve terminals as w ell as their synaptic vesicle population in the adventitia of saphenous (SV and SA) and brachial veins and arteries (BV and BA) obtained from rats mai ntained in a horizontal control or a tilted position. Adult animals were ke pt individually in tubelike cages in a 45 degrees head-up position. After 2 wk, both tilted and control animals were anesthetized, and the whole body was perfused with fixative solution at physiological pressure. Vessels segm ents were then excised for electron microscopy and immunohistochemistry. Th e nerve terminal density (NTD) of SA was 8.20 +/- 1.46 nerve terminals/100 mum(2) cross section of adventitia and that of SV was 4.53 +/- 0.61 nerve t erminals/100 mum(2) cross section of adventitia in control rats' Tilting ca used a significant increase in NTD of both SA (70%) and SV (52%). The synap tic microvesicle density (SyVD) was larger in SA than SV in control rats (3 0.48 +/- 4.41 vs. 13.38 +/- 2.61 synaptic vesicles/10 terminal sections), b ut tilting resulted in more pronounced changes in SyVD of SV (95%) than SA (54%). No significant changes in NTD and SyVD of BA were found after tilt ( -3.6% relative to 4.99 +/- 0.33 compared with 0.4% relative to 24.89 +/- 3. 7, respectively). Whereas NTD of BV exhibited a tendency to increase (3.73 +/- 0.86 vs. 2.31 +/- 0.29 nerve terminals/100 mum(2) cross section of adve ntitia), SyVD did not change significantly (18.96 +/- 2.74 vs. 22.85 +/- 3. 17 synaptic vesicles/10 terminal sections). A large number of nerve termina ls of all vessels were tyrosine hydroxylase immunoreactive (containing nore pinephrine). These findings support the hypothesis that long-term gravitati onal load causes adaptive morphological and functional remodeling of sympat hetic innervation in blood vessels of the extremities.