Different hepatic vascular response to noradrenaline and histamine between guinea-pig and rat

Ethanol predominantly constricts pre-sinusoids of isolated perfused livers of rat, guinea pig and mouse

AIMS

Ethanol constricts hepatic vessels of isolated perfused livers of rats, but not dogs. However, it is not known whether ethanol constricts or dilates the hepatic vessels in other species such as guinea pigs and mice. In addition, the sites of hepatic venoconstriction induced by ethanol were not known in rat livers. We therefore studied the effects of ethanol on the segmental hepatic vascular resistance and liver weight of mice, rats and guinea pigs.

METHODS

The isolated livers were portally perfused with diluted blood at constant flow. The sinusoidal pressure was measured by the double occlusion method and was used to determine the pre- and post-sinusoidal resistance. The change of liver weight was also measured. Ethanol was administered cumulatively into the perfusate to gain clinically relevant concentrations of 1-300 mM.

RESULTS

Ethanol dose dependently caused predominant pre-sinusoidal constriction in livers of all three species. When compared with the livers of the guinea pigs and rats, the mouse livers were the weakest in response. Dose-dependent decreases in liver weight and bile flow accompanied predominant pre-sinusoidal constriction in guinea pigs and rats.

CONCLUSION

Ethanol predominantly constricts pre-sinusoids in rat, guinea pig and mouse livers, although the mouse liver response was much weaker. Ethanol-induced pre-sinusoidal constriction is accompanied by reduction of liver blood volume in guinea pigs and rats.

Effects of l-NAME on thromboxane A2-induced venoconstriction in isolated perfused livers from rat, guinea pig and mouse

Effects of L-NAME on U-46619 (a thromboxane A(2), analogue) -induced hepatic segmental venoconstriction were examined in mouse, rat and guinea pig isolated perfused livers. All livers were perfused portally and recirculatingly at a constant flow with diluted blood. U-46619 was administrated into the reservoir in a cumulative manner to gain the concentrations of 0.001-3 microM at 10 min after L-NAME or D-NAME (100 microM). The portal venous pressure, hepatic venous pressure and perfusate flow were monitored. In addition, the sinusoidal pressure was measured by the double occlusion pressure, and was used to determine the pre- (Rpre) and post-sinusoidal (Rpost) resistances. U-46619 concentration-dependently caused predominant presinusoidal constriction in all three species. The rat livers were the strongest while the mouse livers were the weakest in responsiveness and sensitivity to U-46619. L-NAME mainly augmented the U-46619-induced increases in Rpre, but not in Rpost, in rat and guinea pig. This augmentation was stronger in rat. However, L-NAME did not augment the response to U-46619 in mouse. In conclusion, in rat and guinea pig, NO may be released selectively from the presinusoids in response to U-46619, and then attenuate the U-46619-induced presinusoidal constriction. In mouse, U-46619-induced venoconstriction is weak and not modulated by NO.

Leukotrienes and cyclooxygenase products mediate anaphylactic venoconstriction in ovalbumin sensitized rat livers

Hepatic anaphylactic venoconstriction is partly involved in anaphylactic hypotension. We determined the chemical mediators responsible for anaphylaxis-induced segmental venoconstriction in perfused livers isolated from ovalbumin-sensitized rats. Livers were perfused portally and recirculatingly at constant flow with diluted blood. The portal venous pressure (Ppv), hepatic venous pressure (Phv), liver weight and hepatic oxygen consumption were continuously measured. The sinusoidal pressure was measured by the double occlusion pressure (Pdo), and was used to determine the pre-sinusoidal (Rpre) and post-sinusoidal (Rpost) resistances. After antigen injection, both Ppv and Pdo increased, resulting in 5.6- and 1.6-fold increases in Rpre and Rpost, respectively. Liver weight showed a biphasic change of an initial decrease followed by an increase. Hepatic oxygen consumption significantly decreased after antigen. Anaphylaxis-induced increase in Rpre was most extensively inhibited by 38.6% by pretreatment with ONO-1078 (100 microM, a cysteinyl leukotriene receptor-1 antagonist), among all antagonists or inhibitors administrated individually including TCV-309 (20 microM), AA-2414 (10 microM), ketanserin (10 microM) and indomethacin (10 microM). Combined pretreatment with indomethacin and ONO-1078 exerted additive inhibitory effects and attenuated Rpre by 65.8%. However, TCV-309, a platelet activating factor (PAF) receptor antagonist, did not affect the anaphylactic response. In contrast, anaphylaxis-induced increase in Rpost was attenuated only by ONO-1078 combined pretreatment. The antigen-induced changes in liver weight and hepatic oxygen consumption were attenuated significantly when hepatic venoconstriction was attenuated. It is concluded that cysteinyl leukotrienes and cyclooxygenase products, but not PAF, are mainly involved in anaphylaxis-induced pre-sinusoidal constriction in isolated perfused rat livers.

L-NAME augments PAF-induced venoconstriction in isolated perfused livers of rat and guinea pig, but not mouse

Platelet-activating factor (PAF), one of vasoconstrictive lipid mediators, is involved in systemic anaphylaxis. On the other hand, nitric oxide (NO) is known to attenuate anaphylactic venoconstriction of the pre-sinusoids in isolated guinea pig and rat livers. However, it is not known whether NO attenuates PAF-induced hepatic venoconstriction. We therefore determined the effects of L-NAME, a NO synthase inhibitor, on PAF-induced venoconstriction in blood- and constant flow-perfused isolated livers of mice, rats and guinea pigs. The sinusoidal pressure was measured by the double occlusion pressure (Pdo), and was used to determine the pre- (Rpre) and post-sinusoidal (Rpost) resistances. PAF (0.01-1 microM) concentration-dependently caused predominant pre-sinusoidal constriction in all livers of three species studied. The guinea pig livers were the most sensitive to PAF, while the mouse livers were the weakest in responsiveness. L-NAME pretreatment selectively increased the basal Rpre in all of three species. L-NAME also significantly augmented the PAF-induced increases in Rpre, but not in Rpost, in rat and guinea pig livers. This augmentation was stronger in rat livers than in guinea pig livers at the high concentration of 0.1 microM PAF. However, L-NAME did not augment PAF-induced venoconstriction in mouse livers. In conclusion, in rat and guinea pig livers, NO may be released selectively from the pre-sinusoids in response to PAF, and then attenuate the PAF-induced pre-sinusoidal constriction. In mouse liver, PAF-induced venoconstriction is weak and not modulated by NO.

OXYGEN CONSUMPTION, ASSESSED WITH THE OXYGEN ABSORPTION SPECTROPHOTOMETER, DECREASES INDEPENDENTLY OF VENOCONSTRICTION DURING HEPATIC ANAPHYLAXIS IN PERFUSED RAT LIVER

Anaphylactic shock is accompanied by a decrease in oxygen consumption. However, it is not well known whether oxygen consumption decreases during local anaphylactic reaction in liver. We determined the effects of anaphylaxis and norepinephrine on oxygen consumption in isolated rat livers perfused portally and recirculatingly at constant flow with blood (hematocrit, 12%). Oxygen consumption was continuously measured by monitoring the portal-hepatic venous oxygen saturation differences using the absorption spectrophotometer, the probes of which were built in perfusion lines. Hepatic anaphylaxis was induced by an injection of ovalbumin (0.01 or 0.1 mg) into the perfusate of the isolated liver of the rat sensitized with subcutaneous ovalbumin (1 mg). Hepatic venoconstriction and liver weight loss were similarly observed in response to norepinephrine (0.01-10 micromol L(-1)) and anaphylaxis. However, hepatic anaphylaxis reduced oxygen consumption, whereas norepinephrine increased it. There was a possibility that anaphylactic venoconstriction could reduce the perfused surface area, resulting in decreased oxygen consumption. However, pretreatment with a vasodilator of sodium nitroprusside substantially attenuated venoconstriction but not the decrease in oxygen consumption during anaphylaxis. Thus, we conclude that local hepatic anaphylaxis decreases oxygen consumption independently of venoconstriction in isolated blood-perfused rat livers.

Effects of platelet-activating factor, thromboxane A2 and leukotriene D4 on isolated perfused rat liver

Vasoconstrictive lipid mediators, thromboxane A(2) (TxA(2)), platelet-activating factor (PAF) and leukotriene D(4) (LTD(4)) have been implicated as mediators of liver diseases. There are species differences in the primary site of hepatic vasoconstriction in response to these mediators. We determined the effects of a TxA(2) analogue (U-46619), PAF and LTD(4) on the vascular resistance distribution, weight and oxygen consumption of isolated rat livers portally perfused with blood. The sinusoidal pressure was measured by the double occlusion pressure (P(do)), and was used to determine the pre- (R(pre)) and post-sinusoidal (R(post)) resistances. All these three mediators increased the hepatic total vascular resistance (R(t)). The responsiveness to PAF was 100 times greater than that to U-46619 or LTD(4). Both of PAF and U-46619 predominantly increased R(pre) over R(post). At the comparable increased R(t) levels, U-46619 more preferentially increased R(pre) than PAF. In contrast, LTD(4) increased both the R(pre) and R(post) to similar extent. U-46619 caused liver weight loss, while high concentrations of either LTD(4) or PAF produced liver weight gain, which was caused by substantial post-sinusoidal constriction and increased P(do). PAF and U-46619 decreased hepatic oxygen consumption while LTD(4) induced biphasic change of an initial transient decrease followed by an increase. In conclusion, PAF is the most potent vasoconstrictor of rat hepatic vessels among these three mediators. Both TxA(2) and PAF constrict the pre-sinusoidal veins predominantly. TxA(2) more preferentially constricts the pre-sinusoids than PAF, resulting in liver weight loss. However LTD(4) constricts both the pre- and post-sinusoidal veins similarly. High concentrations of LTD(4) and PAF cause liver weight gain by substantial post-sinusoidal constriction. PAF and TxA(2) decrease hepatic oxygen consumption, whereas LTD(4) causes a biphasic change of it.

Intrahepatic Flow Disturbance: Possibility of a Hidden Cause of Drug Toxicity

The liver has an intricate microvascular system that allows homogenous perfusion throughout the organ. However, the regulatory mechanisms of intrahepatic circulation are still unclear, and the effects of drugs on this system have rarely been reported. Oxethazaine, a topical anesthetic, was incidentally found to induce a consistent increase in portal pressure in the isolated perfused rat liver, which led us to characterize this phenomenon. For this, a vital staining method was developed to detect microcirculatory alterations in the isolated liver. Using this method, not only vasoconstrictors like endothelin-1, but the drugs oxethazaine and clomipramine, a tricyclic antidepressant, were found to induce flow redistribution to the deeper and hilar portions of the liver with minimal perfusion at the periphery, which was due to a short-circuit flow at the center owing to the constriction of the intrahepatic portal vein branches. Hepatic nerve stimulation also produced a similar flow disturbance. Since the portal pressure increases by these compounds were inhibited by the Rho-kinase inhibitors Y27632 and HA1077, portal vein branches may employ a Rho-kinase-dependent pathway for sustained contraction. However, oxethazaine, clomipramine, and endothelin-1 may activate this pathway differently. The intrahepatic flow disturbance could play a hidden role in drug toxicity of certain drugs.

Involvement of platelet-activating factor and leukotrienes in anaphylactic segmental venoconstriction in ovalbumin sensitized guinea pig livers

The hepatic anaphylactic venoconstriction is partly involved in anaphylactic hypotension, and is characterized by significant post-sinusoidal constriction and liver congestion in guinea pigs. We determined what chemical mediators are involved in anaphylaxis-induced segmental venoconstriction and liver congestion in perfused livers isolated from ovalbumin sensitized guinea pigs. Livers were perfused portally and recirculatingly at constant flow with diluted blood. The sinusoidal pressure was measured by the double occlusion pressure (Pdo), and was used to determine the pre-sinusoidal (Rpre) and post-sinusoidal (Rpost) resistances. An antigen injection increased both the portal vein pressure and Pdo, resulting in 4.1- and 2.3-fold increases in Rpre and Rpost, respectively. Hepatic congestion was observed as reflected by liver weight gain. Pretreatment with TCV-309 (10microM, platelet-activating factor (PAF) receptor antagonist) or ONO-1078 (100microM, human cysteinyl-leukotriene (Cys-LT) receptor 1 antagonist), but not indomethacin (10microM, cyclooxygenase inhibitor), ketanserin (10microM, serotonin receptor antagonist), or diphenhydramine (100microM, histamine H1 antagonist), significantly attenuated this anaphylactic hepatic venoconstriction. Anaphylaxis-induced increases in Rpre and Rpost were significantly inhibited by TCV-309 (by 48%) and ONO-1078 (by 36%), respectively. Combined TCV-309 and ONO-1078 pretreatment exerted additive inhibitory effects on anaphylactic hepatic venoconstriction. Anaphylactic hepatic weight gain was converted to weight loss when post-sinusoidal constriction was attenuated. It is concluded that anaphylaxis-induced pre-sinusoidal constriction is mainly caused by PAF and the post-sinusoidal constriction by Cys-LTs in guinea pig livers.

Hepatic venoconstriction is involved in anaphylactic hypotension in rats

We determined the roles of liver and splanchnic vascular bed in anaphylactic hypotension in anesthetized rats and the effects of anaphylaxis on hepatic vascular resistances and liver weight in isolated perfused rat livers. In anesthetized rats sensitized with ovalbumin (1 mg), an intravenous injection of 0.6 mg ovalbumin caused not only a decrease in systemic arterial pressure from 120 ± 9 to 43 ± 10 mmHg but also an increase in portal venous pressure that persisted for 20 min after the antigen injection (the portal hypertension phase). The elimination of the splanchnic vascular beds, by the occlusions of the celiac and mesenteric arteries, combined with total hepatectomy attenuated anaphylactic hypotension during the portal hypertension phase. For the isolated perfused rat liver experiment, the livers derived from sensitized rats were hemoperfused via the portal vein at a constant flow. Using the double-occlusion technique to estimate the hepatic sinusoidal pressure, presinusoidal ( Rpre) and postsinusoidal ( Rpost) resistances were calculated. An injection of antigen (0.015 mg) caused venoconstriction characterized by an almost selective increase in Rpre rather than Rpost and liver weight loss. Taken together, these results suggest that liver and splanchnic vascular beds are involved in anaphylactic hypotension presumably because of anaphylactic presinusoidal contraction-induced portal hypertension, which induced splanchnic congestion resulting in a decrease in circulating blood volume and thus systemic arterial hypotension.

Effects of platelet-activating factor and thromboxane A2 on isolated perfused guinea pig liver

Lipid mediators, thromboxane A2 (TxA2) and platelet-activating factor (PAF), are potent vasoconstrictors, and have been implicated as mediators of liver diseases, such as ischemic-reperfusion injury. We determined the effects of a TxA2 analogue (U-46619) and PAF on the vascular resistance distribution and liver weight (wt) in isolated guinea pig livers perfused with blood via the portal vein. The sinusoidal pressure was measured by the double occlusion pressure (P(do)), and was used to determine the pre- (R(pre)) and post-sinusoidal (R(post)) resistances. U-46619 and PAF concentration-dependently increased the hepatic total vascular resistance (R(t)). The minimum concentration at which significant vasoconstriction occurs was 0.001 microM for PAF and 0.1 microM for U-46619. Moreover, the concentration of U-46619 required to increase R(t) to the same magnitude is 100 times higher than PAF. Thus, the responsiveness to PAF was greater than that to U-46619. Both agents increased predominantly R(pre) over R(post). U-46619 caused a sustained liver weight loss. In contrast, PAF also caused liver weight loss at lower concentrations, but it produced liver weight gain at higher concentrations (2.5 +/- 0.3 per 10g liver weight at 1 microM PAF), which was caused by substantial post-sinusoidal constriction and increased P(do). In conclusion, both TxA2 and PAF contract predominantly the pre-sinusoidal veins. TxA2 causes liver weight loss, while PAF at high concentrations increases liver weight due to substantial post-sinusoidal constriction in isolated guinea pig livers.