Evaluation of vascular tone in portacaval shunts comparing the index of contractility and resistance in cats.

A model of prehepatic chronic portal hypertension in cats was used to determine portacaval shunt responses to infused norepinephrine and to possible transmitter overflow into portal blood from nerves supplying the gut. Responses are compared using a new index of contractility. Four weeks after application of a slowly constricting occluder, the portal vein was completely occluded and acute experiments were carried out under pentobarbital anesthesia. Portal pressure was elevated to 15.0 +/- 0.9 mmHg and all portal flow passed through the shunts. In response to intraportal norepinephrine (0.25, 0.5 and 1.25 micrograms.min-1.kg-1) shunt resistance rose by 6% +/- 3%, 19% +/- 4% and 26% +/- 5%, respectively, whereas the index of contractility rose (by 22% +/- 8%, 46% +/- 10% and 89% +/- 20%, respectively), the distending blood pressure also rose (5% +/- 1%, 7% +/- 1% and 14% +/- 3%, respectively). The difference in percentage increase of resistance and the index of contractility is a result of the passive dilator effect of the elevated distending pressure acting on the distensible shunt vessels. Stimulation of mesenteric nerves caused the mesenteric artery to constrict, but the shunt vessels showed no effect. In conclusion, the shunt vessels respond actively to norepinephrine and passively to altered distending pressure. However, transmitter overflow from nerves supplying the intestines is unlikely to play a role in determining resistance in the shunts. Vascular resistance is affected by both active and passive effects, so that the active contractile responses are best evaluated using the index of contractility, which is not altered passively.