Effect of long-term tilt on mechanical and electrical properties of rat saphenous vein.

Abstract

Femoral vein pressure in adult male Sprague-Dawley rats kept in specially designed tubelike cages rose immediately from a control value of 2.9 +/- 0.2 (SE) mmHg to a gravity-induced sustained value of 5.9 +/- 0.2 mmHg on initiation of a 2-wk 45 degrees head-up tilt period. Femoral arterial pressure was not altered by tilting. In 2 wk mean external diameter, but not total wall thickness, of in vitro distal saphenous vein segments from tilted rats was increased approximately 30% above that of segments from nontilted controls at each of four successive 5-mmHg intralumenal pressure (IP) increments applied between 0 and 20 mmHg. Consequently, in tilted rats isobaric stress was increased 38% at low and 24% at high IP, whereas incremental distensibility was decreased at mid IP. Vascular smooth muscle (VSM) in tilted rat vein, but not artery, was hyperpolarized relative to controls both in vitro at normal physiological pressures [membrane potential (Em) = -58.2 +/- 0.8 vs. -52.4 +/- 0.8 mV, respectively] and in situ during local neural blockade (Em = -61.3 +/- 2.3 vs. -53.5 +/- 0.5 mV, respectively). The conclusion is that a moderate chronic elevation of IP in a vein results in hyperpolarization of its VSM and an elevation of its total capacity due to an as yet unexplained mechanism of physiological adaptation.