Active Stress Generation Research Articles

Electrical and mechanical responses of rat saphenous vein to short-term pressure load.

The magnitude and mechanism of myogenic response of vascular smooth muscle (SM) in rat distal saphenous vein was assessed from SM membrane potential (Em) measured in situ and in vitro with glass microelectrodes and from active and passive stress and strain calculated from changes in vessel diameter measured in vitro via videomicroscopy. Elevation of intraluminal pressure from 2.2 +/- 0.2 (SE) mmHg (control) to 15 +/- 0.8 mmHg for 1 h in a series of in vitro vessel segments perfused with physiological salt solution at 0.2 ml/min induced a maintained and reversible depolarization of 18 +/- 0.9 mV. A 7.6 +/- 0.4-mmHg pressure increase induced a 12.9 +/- 1.2-mV depolarization in a second series. In a third series, 5-mmHg pressure increments induced significant increments in active isometric stress and isobaric strain. Opening an acute, reversible in situ femoral artery to saphenous vein shunt caused a 4- to 5-mmHg venous pressure elevation, a 10-fold increase in venous blood flow, and a 12.1 +/- 0.9-mV venous SM depolarization. Thus a short-term pressure load causes sustained, reversible venous SM cell depolarization both in vitro and in situ, coupled with active strain and stress generation in the vein wall. These results support our hypothesis that SM of peripheral veins can contribute to an intrinsic capacity autoregulation.

Serotonin response increases with age in rat coronary resistance arteries.

The effect of aging on morphological and mechanical characteristics and the serotonin reactivity of ring preparations of coronary resistance arteries isolated from a specific part of the left coronary artery were studied in male Wistar rats at 6 weeks, 3 months, and 2 years of age. The media to lumen ratio of the coronary vessels was unaffected by aging, although the effective lumen diameter increased from 176 micron at 6 weeks to 212 micron at 2 years of age. The maximal active tension development, active pressure, and active media stress generation of the vessels were also unaffected by aging. At all ages the spontaneous calcium dependent intrinsic tone was similar, equal to 13% of maximal agonist induced tension development. Serotonin induced a concentration dependent contraction at all ages. Maximal vessel response to serotonin increased with age from 0.38 N.m-1 to 1.45 N.m-1 at 6 weeks and 2 years of age respectively. Serotonin sensitivity was higher in the older rats than in the younger rats. Tachyphylaxis to serotonin developed only in vessels from 6 week old rats. The results confirm that the morphology and mechanics of resistance arteries remain stable over a large age range but that the vascular reactivity to specific agents, such as serotonin, may change over that period. The results also indicate that the coronary blood flow may be seriously affected by serotonin induced vasoconstriction with increasing age in rats.

Temporal changes in mechanical properties of rat jejunal smooth muscle after myenteric plexus ablation.

We determined length-stress properties and active tension development of rat jejunal longitudinal and circular muscle at various times after myenteric neuron ablation. Myenteric neurons were destroyed by serosal application of benzalkonium chloride. Active tension generation in response to both carbachol and barium was depressed in both muscle layers up to 7 days after treatment, at which times the responses were normal. At 15 days, circular muscle responses were still comparable to control, but longitudinal muscle responses were significantly increased. Length-stress parameters of circular muscle were minimally affected, while those of longitudinal muscle were significantly altered 15 and 30 days after treatment. These alterations include changes in resting stress and increased active stress generation at both times. Our results suggest that 1) normal myenteric innervation is necessary for normal contractile activity in rat jejunal smooth muscle and 2) changes in cellular or tissue morphology or alterations in intracellular calcium homeostasis, as seen in other tissue, may occur after myenteric plexus ablation.