INTRODUCTION

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HEPATIC VENOUS RESISTANCE (R_hv) determines, in part, the amount of blood passively stored in the liver (12, 15). Redistribution of blood from the liver and other organs in the abdomen to the heart changes the filling pressure of the right heart and, therefore, cardiac output. Agents that influence R_hv are thus potentially important mechanisms in the maintenance of cardiovascular homeostasis (9, 12, 27, 28).

The vasculature of the liver is unusual. About 35% of the liver is blood (6, 10, 12, 15), with ~80% in the sinusoids (6, 26). Furthermore, the compliance of the liver is about 10 times that of the body as a whole (4, 14, 19). The splanchnic bed, including the liver, receives ~30% of the total cardiac output and contains ~25% of the total blood volume, making it a primary site for cardiovascular compensation for blood volume changes from normal (5, 9, 12). For example, ~65% of the blood translocated to compensate for a small hemorrhage comes from the canine splanchnic bed (5).

Whereas the total blood volume of mammals is ~75 ml/kg body wt, only the stressed volume (estimated as the product of total body vascular compliance and the mean circulatory filling pressure: 3 ml · mmHg¹ · kg¹ × 7 mmHg = 21 ml/kg) is available for blood redistribution between the periphery and the heart. The remaining ~55 ml/kg (the unstressed volume) is available only via active changes in capacitance vessel smooth muscle or other contractile element stimulation via neural activity, drugs, or hormones (12, 29). Even though the liver comprises only ~3% of the total body weight, it contains ~15% of the total blood volume (15) and about one-half the total systemic stressed blood volume. The unstressed volume of the liver has not been well defined (10).

The vascular volume of the liver may be changed transiently via three mechanisms: 1) active changes in capacitance vessel contractile element activity, 2) passive changes in distending pressure from changes in outflow or inflow pressure, and 3) passive changes in distending pressure from changes in tissue blood flow (5, 27).

1) With an active increase in the tension developed by the smooth muscle in the vessel walls or in sinusoidal pericyte (Ito cells) activity, up to 50% of the hepatic blood volume can be mobilized (12). In that the liver blood volume is ~10 ml/kg body wt (350 ml/kg liver × 0.03 kg liver/kg body wt = 10.5 ml/kg body wt), a 50% decrease in liver blood volume is thus equivalent to a 5 ml/kg transfusion.

2) A passive reduction in liver blood volume of 20 ml/kg liver occurs from a reduction in abdominal vena caval pressure of 1 mmHg, if there is no concomitant change in flow (4, 14). If it is assumed that the liver comprises 3% of the body mass, a 5-mmHg reduction in caval pressure can induce a 3 ml/kg body wt increase in circulating blood volume outside the liver (5 mmHg × 20 ml · mmHg¹ · kg¹ × 0.03 kg liver/kg body wt = 3 ml/kg body wt).

3) A passive reduction in liver volume occurs with a reduction in hepatic blood flow in the amount of 0.07 ml per 1 ml/min of flow if there is no change in outflow pressure (4, 14). With the assumption of a cardiac output of 100 ml · min¹ · kg¹, of which liver venous flow is 30 ml · min¹ · kg¹, a 50% decrease in liver blood flow can induce a 1 ml/kg body wt redistribution of blood volume (15 ml · min¹ · kg¹ × 0.07 ml per ml/min = 1 ml/kg body wt).

These three mechanisms leading to blood volume redistribution with respect to the liver are additive (5, 9). However, during active vascular capacitance vessel stimulation, if the outflow resistance is concomitantly increased, the increase in intrahepatic distending pressures will attenuate the magnitude of the volume reduction (12).

The liver and gastrointestinal (GI) bed comprise most of the splanchnic bed drained by the portal vein. The volume flow sensitivity of the GI bed of 0.06 ml · ml¹ · min¹ is similar to that of the liver, but the compliance of 2 ml · mmHg¹ · kg¹ is much less (32, 33). If something induces an increase in portal venous pressure (P_pv), then this will lead to a passive increase in the GI blood volume. Blood will then be redistributed from the heart and so decrease cardiac output. Even though the mass of the GI bed is similar to that of the liver, its much lower compliance reduces, but does not eliminate, its importance as a site for blood pooling or rapid redistribution.

The passive mechanisms, leading to redistribution of blood, are closely similar in potential magnitude of compensation to that from active mechanisms (5, 31). A reduction in vessel volume or diameter alone cannot be taken as proof of an active process, because the distending pressure may have been decreased by a decrease in flow or upstream or downstream pressure.

Our goal was to describe how vasoactive drugs [norepinephrine (NE), isoproterenol (Iso), histamine (Hist), adenosine (Ado), and acetylcholine (ACh)] participate in mechanisms of blood volume translocation by sampling pressures along the hepatic vascular tree in conjunction with estimates of overall liver volume and flow changes. To help distinguish between local and systemic effects, the responses to intraportal vein and systemic intravenous infusions were compared. We have measured the hepatic venular pressure (P_µhv) with micropipettes and the total hepatic blood flow (F_hv) to compute the R_hv. Our major hypothesis was that the agents act differently along the liver vasculature. Secondary hypotheses were as follows: 1) NE and Hist cause an increase in R_hv, but Iso, Ado, and ACh cause R_hv to decrease. 2) NE causes an active decrease in hepatic volume, but Hist causes a passive increase related to the increased outflow resistance, and Ado and ACh cause a passive engorgement of the liver related to increased blood flow. Not all hypotheses were supported.

	METHODS

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The study was approved by the Indiana University Animal Care and Use Committee (Study MD285). New Zealand White rabbits [3.2 ± 0.2 (SD) kg, n = 11] were sedated with 4 mg/kg chlorpromazine HCl (Schein Pharmaceuticals). Under local anesthesia (0.2-ml infiltration of 0.5% lidocaine), the central ear artery and vein were cannulated to measure systemic arterial pressure (P_sa) and to infuse the general anesthetic, respectively. -Chloralose (C-0128, Sigma Chemical) and urethan (U-2500, Sigma Chemical) were dissolved at 40°C in water with 10% polyethylene glycol 200 (Baker Chemical) for concentrations of 10 and 100 mg/ml, respectively. The anesthetics were administered intravenously at 50 and 500 mg/kg, respectively, over a 30-min period. For maintenance, 1-ml boluses were used if the eyelids responded to light or there were any nonrespiratory movements. Lactated Ringer solution (70%) and 6% Dextran 70 (Abbott) (30%) were infused at a rate of 4-6 ml · h¹ · kg¹ to replace fluid losses.

As in previous experiments (22, 23), the animals were placed on a heated table to maintain body temperature. The trachea was cannulated, and the animals were connected to a piston ventilator set at 30-50 breaths/min and a tidal volume of 25 ml. In some experiments the animals were allowed to breathe spontaneously. A mixture of 95% O₂-5% CO₂ was added to the respiratory inflow path at 600 ml/min.

The liver and abdominal organs were exposed with a 10- to 15-cm-long midline incision. A polyethylene catheter (1 mm OD, 0.6 mm ID) was advanced from a cecal vein into the portal vein to within 1.7 ± 1.8 cm of the liver hilum. A 1.3-mm-OD catheter was advanced through a branch of the right jugular vein and positioned ~2 cm upstream to the diaphragm to measure abdominal caval pressure (P_avc) at the level of the hepatic venous outflow.

To isolate the liver from the respiratory movements, we used a vinyl-coated, 2-mm-thick lead retractor attached to a rack-and-pinion manipulator and shaped similarly to the diaphragm. It had a 1 × 2.5-cm slot to prevent obstruction of the vena cava and aorta. The falciform ligament was cut. Motion of the preparation was typically <10 µm during a respiratory cycle. With the animal lying on its right side, the medial or the left lateral liver lobe was placed on a metal support. The liver surface was bathed with a continuous drip of a cell culture medium equilibrated with 95% N₂-5% CO₂ and heated to body temperature.

Microvascular transmural pressure was measured using a servo-null micropipette pressure-measuring system (model 5A, Instruments for Physiology and Medicine, San Diego, CA). The pipettes were pulled to 2.5-3.5 µm OD, sharpened to a 30° bevel, and filled with a 2 M NaCl solution. A pipette was inserted in a microvessel using a micromanipulator (model MO-203, Narishige) while the field was viewed with a microscope (Labphot, Nikon) coupled to a charge-coupled device camera (model XC-77, Hamamatsu) and video monitor (model PVM 1344Q, Sony), and a videocassette recorder (model HR-6800U, JVC) with a video timer (model VTG-33, FOR-A) provided a permanent record of the visual data. The diameters of the microvessels were measured from the calibrated video image. Each 1 mm on the screen represented 1.55 µm at the tissue. Additional details about the technique are available elsewhere (22).

We attempted to reduce the error in the pressure measurements to <0.2 mmHg. A saline reservoir connected to all pressure transducers via three-way stopcocks provided a common hydrostatic zero and allowed common simultaneous calibrations. The reservoir level and the saline pool at the puncture sites were aligned using a laser pointer mounted on a leveled horizontal table. At the end of the experiment the chest was opened and the height of the liver and reservoir with respect to the middle of the right atrium was measured so that all pressures could be referred to this level. Pressure transducers (model P23Db or De, Statham) were calibrated periodically at 0 and 10 mmHg using a water manometer. They were held in a temperature-controlled block at 35 ± 0.1°C. The servo-null micropressure system was calibrated at 0, 10, and 20 mmHg after the balance and gain were set and just before the pipette was moved to the liver surface.

The arterial, portal venous, and abdominal vena caval catheters were flushed continuously with pressurized (300 mmHg) saline flowing through a high resistance at a rate of ~3 ml/h (Criti-Flo TA4004, Viggo-Spectramed, Oxnard, CA). The connecting lines were clamped periodically for 0.2 min to estimate the pressure offset produced by this flow.

F_hv was measured with a single 8-mm-wide, four-crystal transit-time ultrasonic flowmeter probe (model H8A9, Transonic Systems, Ithaca, NY) placed around the portal vein and the hepatic artery within 2 cm of the hilum of the liver. The design of the flowmeter probe provides a relatively uniform ultrasonic field across the probe orifice and is flow-direction sensitive. Thus the resulting signal output is the algebraic sum of flow in all the blood vessels enclosed within the lumen. An acoustical couplant recommended by Transonic Systems (H-R Lubricating Jelly) was used to ensure an adequate signal quality. The zero offset was determined at the end of the experiment after the animal was dead and the arterial pressure was <15 mmHg.

We estimated changes in liver volume by recording changes in the thickness of the lobe at the site of micropuncture. A variable differential transformer (type 6206, Automatic Timing Controls, King of Prussia, PA) with a resolution of 8 µm was coupled to the microscope barrel to follow changes associated with the focusing of the pipette at the point of insertion. The measurement theory and limitations are similar to those when a pair of ultrasonic crystals attached to the two sides of a lobe is used (13).

The hepatic venules studied were identified as relatively large (25-75 µm) microvessels fed by convergent flow from smaller sinusoids. Restriction of blood flow in the vessel by the ~3-µm pipette was thus minimal (21).

ACh (A-6625, Sigma Chemical), Ado (A-925, Sigma Chemical), Hist (H-7250, Sigma Chemical), Iso HCl (Elkins-Sinn), and NE bitartrate (Abbott) were used. Drug infusions were from 1.25-ml syringes (gastight no. 1001.25, Hamilton) driven by a 2-in. micrometer, which was turned via a motor generator (model E-350, Electro-Craft) and a modified power supply in amplifier mode (model 6823A, Hewlett-Packard). The output of the generator and a 10-turn (1,000 divisions) reference potentiometer were summed to provide the error signal. The calibration factor was 0.245 µl/min per division.

We estimated the blood concentration of the drugs at the level of the hepatic venules by assuming that our flow probe provided an accurate measure of total hepatic blood inflow and, thus, in steady state the outflow via the hepatic veins. From the rate of intraportal vein infusion and the concentration of the drug, we knew the dose in micrograms per minute per kilogram body weight. From this dose rate and the flow, we estimated the portal venous concentration. The concentration in the portal vein was higher than that in the hepatic veins, because the liver extracts the drugs at the sinusoids and because the portal venous blood is diluted by relatively drug-free hepatic arterial blood, which provides about one-third of the total hepatic venous flow in mammals but probably much less in herbivores.

Protocol. The micropipette was immersed in the fluid pool above the vessel to record zero transmural pressure and then inserted in the vessel. After control pressure was recorded for 2 min, one of the five drugs was infused via the portal cannula for 5 min at 12-245 µl/min. The doses of the drugs used had been estimated from pilot runs to elicit a detectable P_sa, F_hv, or heart rate (HR) response. Responses (6-s average) were obtained at the 1st, 3rd, and 5th min during the infusion and then at 3 min after the infusion was stopped.

Data acquisition, filtering, and analysis. The raw signals from all pressure transducers and flows were displayed on a six-channel Beckman type R strip chart recorder. All data were also digitized using a 12-bit data acquisition system (series 500, Keithley) at 0.1-min intervals after prefiltering with four-pole Bessel filters with a 3-dB cutoff frequency of 0.18 Hz. The pressure data resolution was 0.1 mmHg.

Statistics. Comparisons were made using Student's t-tests. For the tests for response to drugs, we used the average of the data at 3 and 5 min after initiation of the infusion compared with the average of the two control periods starting 1 and 2 min before the start of the infusions. To compensate for the repeated measures in the same animal, the degrees of freedom used, when the probability given the t value was computed, represented the number of animals in the group, rather than the total number of observations. Differences were considered significant if P < 0.05.

	RESULTS

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Average control values after the surgery are given in Table 1. In the 11 rabbits sedated with chlorpromazine at 4 mg/kg and with lidocaine deposited subdermally at the site of arterial vessel cannulation, P_sa averaged 80 ± 6 (SD) mmHg when only 17 ± 17% of the anesthetic had been given. In this study, data were not used if the control P_sa was <45 mmHg. The diameter of the hepatic venules averaged 49.2 ± 15.4 µm OD.

NE, an -adrenergic agonist, infused via the portal vein induced responses shown in Fig. 1. The transport time through the catheter in the portal vein was ~0.5 min in this example. P_pv, P_µhv, and R_hv then started to increase, and lobe thickness started to decrease. By 0.6 min the mean P_sa started to increase and F_hv started to decrease. At 0.7 min the HR started to decrease. As P_sa increased and R_hv decreased from an early peak, F_hv started to increase. By ~3 min into the 5-min infusion, variables had reached a plateau. Soon after the infusion was stopped, P_sa decreased and P_avc increased transiently. There was a transient concomitant reflex increase in HR. All variables tended to return to control by ~3 min after the infusion was stopped.

View larger version (37K): [in this window] [in a new window]   Fig. 1.   Splanchnic vascular response to norepinephrine infused via portal vein at 7.2 µg · min1 · kg1 (expt L50718, rabbit anesthetized at 135 min). Ppv, portal venous pressure (mmHg); Pµhv, hepatic venular pressure (mmHg); Pavc, abdominal vena caval pressure (mmHg); Thk, hepatic lobe thickness at site of puncture (%change); HR, heart rate (beats/min); Fhv, total hepatic blood flow (ml · min1 · kg1); Psa, mean arterial blood pressure (mmHg); Rhv, hepatic venous outflow resistance between microveins and abdominal vena cava (mmHg · min · kg · ml1). Drug, rate of drug infusion (µl/min). Infusion was stopped for 0.2 min at 368.8-369.0 and at 370.8-371 min to check correction for flushing pressure offset.

NE infused into the portal vein at 8.2 ± 3.1 (SD) µg · kg body wt¹ · min¹ significantly increased R_hv 71% (using the 3- and 5-min-period data average; Table 1, Fig. 2). P_pv and P_µhv were increased 3.4 and 1.6 mmHg, respectively, at 3-5 min, while the liver lobe thickness was actively decreased 8.1% (Table 1, Figs. 2 and 3). NE, which was not fully extracted by the liver, caused the P_sa to increase 40%, F_hv to increase 18%, and HR to decrease 7.2%. The increased blood flow and outflow resistance, with resulting increase in P_µhv, would be expected to be associated with engorgement, but instead the liver contracted. Clearly, NE caused venous smooth muscle or sinusoidal contraction (active venoconstriction).

View larger version (30K): [in this window] [in a new window]   Fig. 2.   Percent change from control of Rhv (A), Fhv (B), and lobe thickness (C) at site of Pµhv measurement induced by ACh, adenosine (Ado), histamine (Hist), isoproterenol (Iso), and norepinephrine (NE). Drugs were infused for 5 min. Variable changes significantly different from control (P < 0.05) are circled.

View larger version (37K): [in this window] [in a new window]   Fig. 3.   A: influence of NE infused at 8.2 ± 3.1 µg · min1 · kg1. B: influence of Hist infused at 34 ± 16 µg · min1 · kg1. C: influence of ACh infused at 320 ± 164 µg · min1 · kg1. See legends of Figs. 1 and 2 for definition of abbreviations. Variable changes significantly different from control (P < 0.05) are circled.

Iso, a -adrenergic agonist, infused into the portal vein at 8.6 ± 3.7 µg · kg body wt¹ · min¹, did not significantly increase R_hv, F_hv, or liver lobe thickness (Table 1, Fig. 2). Iso, which was not extracted by the liver, caused P_sa to decrease significantly 10 ± 16% and HR to increase 5.1 ± 5.9% (Table 1). There was no significant change in P_µhv or P_pv. The estimated concentrations of NE and Iso in portal venous blood were similar: ~1 µmol/l (Table 1).

Hist, infused via the portal vein, induced the changes shown in Fig. 4. In this example, at ~0.5 min, when Hist started to enter the hepatic vasculature, R_hv, P_pv, and P_µhv started to increase. By 0.8 min P_sa started to decrease, and by 1 min the lobe thickness started to increase. This was mostly a passive response, because it was associated with an increase in distending pressures, P_µhv and P_pv. As with NE infusion, within ~3 min after termination of the Hist infusion, the variables had generally returned to control.

View larger version (39K): [in this window] [in a new window]   Fig. 4.   Splanchnic vascular response to Hist infused via portal vein at 28 µg · min1 · kg1 (expt L50921, anesthetized at 132 min). See Fig. 1 legend for definition of abbreviations.

On average, Hist infused via the portal vein at 34 ± 16 µg · kg body wt¹ · min¹ caused highly significant increases in R_hv (219%) and in P_pv and P_µhv (1.7 and 1.0 mmHg, respectively; Table 1, Figs. 2 and 3). Consequently, the liver was engorged by 3.7%. Even though P_sa was significantly decreased 31% by the Hist not extracted by the liver, the F_hv was not significantly changed. These data suggest that Hist caused venous constriction leading to the increase in R_hv, P_pv, and P_µhv while inducing some hepatic and intestinal arterial vasodilation, as suggested by the unchanged flow during a reduction in P_sa. With no change in flow, the hepatic engorgement was likely a passive response related to the increase in outflow resistance.

Ado, infused into the portal vein at high rates (132 ± 63 µg · kg body wt¹ · min¹), had no significant effect on R_hv but did elicit an 18% increase in F_hv (Table 1, Fig. 2) that was associated with a 0.31 ± 0.48 mmHg increase in P_µhv and a nonsignificant 0.28 ± 0.72 mmHg increase in P_pv. The 1.8% increase in liver lobe thickness was highly significant. Ado, which was not totally extracted by the liver, acted systemically to decrease P_sa by 5.6%. The increase in flow in conjunction with the decrease in P_sa suggests that Ado caused hepatic or intestinal arteriolar dilation. In that R_hv was not changed, whereas F_hv and P_µhv were increased, the engorgement of the liver was likely a passive response to the increased flow.

ACh infused at 320 ± 154 µg · min¹ · kg¹ had no significant influence on R_hv, P_µhv, or lobe thickness, but P_pv was increased 2.2 mmHg, even though F_hv decreased 28% and P_sa decreased 26% (Table 1, Figs. 2 and 3). ACh infused into the portal vein was not completely extracted by the liver and thus induced a 26% decrease in P_sa but did not significantly change HR (Table 1). Because the decreases in P_sa and F_hv were proportionally similar, the hepatic and intestinal arteriolar resistances were not likely changed appreciably. The P_µhv relative to the P_pv-P_avc gradient (gP_µhv) was significantly decreased from 42% to only 27%. These data suggest that ACh acted primarily on the portal venules of the liver.

Systemic intravenous infusion. Infusion of the drugs via an ear vein resulted in similar responses. NE given intravenously at a lower dose than when administered via the portal vein induced an increase in P_sa, P_pv, and P_µhv, whereas the thickness decreased (Table 2). There were no significant changes in F_hv or R_hv, however. Intravenous Iso was generally without effect on the liver vasculature. However, the decrease in P_sa was greater and the increase in HR larger than when a larger dose of Iso was given via the hepatic vein. F_hv was not changed at the doses used with either route of administration. Intravenous Hist caused a marked decrease in P_sa and F_hv. P_µhv was increased, but not enough to cause statistically significant engorgement. Intravenous Ado, by its vasodilatory action, led to an increased F_hv and so P_µhv, P_pv, and thus liver thickness, while P_sa decreased. At the doses used, none of the four drugs caused a significant change in R_hv. ACh, given intravenously in pilot runs, caused such a drastic decrease in HR that we abandoned the use of ACh in our intravenous protocols.

	DISCUSSION

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Venous resistance. The five vasoactive drugs influenced the hepatic microvasculature to different degrees and even directions. NE and Hist, administered into the portal vein, significantly increased, as expected, the R_hv between the 50-µm-OD hepatic venules and the abdominal vena cava. However, NE induced a contraction of the liver and expulsion of blood, whereas Hist led to an engorgement of the liver. The magnitude of increase in R_hv by Iso was similar to that induced by NE, but the change was statistically insignificant because of high variability. [The change in R_hv from control was between 33 and 446%. When the change was computed in terms of hepatic venous conductance (the reciprocal of resistance), the change from control was between 82 and 69% and averaged 13 ± 49%.] The effects of Ado and ACh on R_hv were insignificant.

Partition of hepatic pressure gradient. In our study of the pressures in the portal venules, sinusoids, and hepatic venules of the same microvascular network (23), P_µhv averaged only 22% of gP_µhv. In the current study, P_µhv was 39% of the gradient (Table 1). (The difference between these means was not significant by a group-mean t-test.) From this study and the data reviewed in our microvascular pressure gradient study (23), we conclude that P_µhv just downstream from the sinusoids is ~35% of gP_µhv.

Lobe volume. With the increase in both P_µhv and P_pv it is likely that the distending pressure in the hepatic sinusoids also was increased by both NE and Hist. However, with NE this passive mechanism to increase the liver vascular volume was more than offset by the active contraction, leading to a net reduction in liver volume. Because ~80% of the blood in the liver is in the sinusoids (6, 26), active contraction of the sinusoid walls via perisinusoidal (Ito) cells is likely (25, 37, 40). The presence of the ~20-µm-thick hepatic capsule (7), the movement of the liver during respiration, the difficulty of clearly identifying the sinusoidal walls, and the lack of intrasinusoidal pressure measurements during drug infusion prevented us from obtaining direct data to test this assumption of active sinusoidal contraction by NE. In contrast to the marked influence of NE on lobe thickness, Iso was without effect. Our data confirm our earlier conclusion (30) that the -adrenergic agonist Iso has little influence on the hepatic vascular capacitance. In the cat, histamine does not induce liver engorgement (11).

Blood flow. F_hv was increased by NE and Ado, not changed by Iso and Hist, and decreased by ACh (Table 1). The increased F_hv induced by NE was related to the increase in P_sa, and the decrease in F_hv induced by ACh resulted from a proportionally similar decrease in P_sa (Table 1). The increase in outflow resistance (R_hv) by NE and Hist led to the increase in P_µhv, whereas the increase in P_µhv by Ado (as well as some of the increase by NE) was related to an increased blood flow. Hepatic arterioles are dilated by Ado (18). We did not measure hepatic arterial flow separately and thus cannot distinguish between changes in hepatic arteriolar and GI arteriolar resistance, which together control F_hv. Depending on the conditions or agents, these resistances do not necessarily change proportionally or even in the same direction (16).

ACh. Koo and Liang (17) reported a dose-dependent sinusoidal dilation from ACh infused via the intraportal vein for 5 s. Without a measure of sinusoidal pressure and flow, the dilation may have been passive in response to an increased sinusoidal pressure. Even though the intraportal infusion of ACh induced a marked decrease in P_sa, our data support the suggestion of McCuskey and Reilly (25) that ACh influences in the liver are via -adrenergic receptors at the blood concentrations used. The increase in R_hv induced by ACh was not statistically significant, but because F_hv decreased markedly while lobe thickness and P_µhv did not change (Fig. 2), some venoconstriction was likely. Shibamoto et al. (35) reported that ACh contracts portal and intrahepatic veins but causes no change in liver weight. Using an isolated organ, a constant-flow perfusion system, and a triple-occlusion approach to estimate sinusoidal pressure, they found the sinusoidal and portal venous pressures to be increased. Our data confirm these findings, except neither P_µhv nor R_hv was significantly increased. (A response may have been obscured in noise.) The constancy of liver weight or lobe thickness argues against blood pooling in the liver as a cause of the decrease in cardiac output and so P_sa. From this study and others (34, 35) we conclude that ACh has little direct influence on the hepatic capacitance vessels.

Ado. Ado had little influence on the hepatic vasculature except for likely arterial dilation (18). The increase in lobe thickness was related to the increase in P_µhv that was a consequence of the increase in F_hv, which occurred even though P_sa decreased (Table 1). We found no evidence that Ado directly dilates intrahepatic capacitance vessels or sphincters (24).

Hist. Hist caused a dramatic increase in outflow resistance in the rabbit, as expected from studies of dogs and pigs (3, 21, 35). The volume response was primarily passive, not active, because the increase in liver volume followed the increase in P_µhv and P_pv, which, because flow was not changed, resulted from the increase in R_hv (Table 1).

Iso. Iso had no significant influence on the liver vasculature even at doses administered into the intraportal vein large enough to traverse the liver and elicit an increase in HR and a decrease in P_sa. These data support our earlier conclusions using dogs (30). Our data do not support the contention of Shigemi et al. (36) that the -adrenergic system, when activated by the carotid sinus baroreflex system, causes hepatic vein dilation, nor do our data support the conclusion of Green (8) that Iso dilates the hepatic outflow vessels.

Limitations of the methods. The extensive surgery and manipulation required to place the flow probe and reduce the motion of the liver during respiration lead to appreciable deterioration of the animal. The goal was to provide an indication of the general direction of induced changes by the drugs and not, at this stage, to provide data that were quantitatively representative of intact, conscious, fully reflexic animals.

Venous resistance. Accurate estimation of R_hv [R_hv = (P_µhv  P_avc)/F_hv ] was difficult. The pressure gradient between the hepatic venule micropipette puncture site and the abdominal vena cava (P_avc) averaged only ~2 mmHg at control (Tables 1 and 2). Uncertainty in zero referencing and transducer zeroing and the estimation of flush-related pressure drop combined to result in an uncertainty on the order of 0.2 mmHg. In some runs the P_µhv was close to or even less than P_avc. Data were not discarded unless some technical flaw could be noted. In other cases, P_µhv was close to P_pv.

Were the punctured hepatic venules representative? It has been argued that blood flow through the surface vessels of the liver may not be representative (2, 20). Using laser-Doppler flowmeters, some investigators claim differences in distribution of flow between the surface and deep sinusoids, but more recent studies have seriously questioned the reliability of the laser-Doppler flowmeter (39) and confirmed that flow is relatively homogeneous throughout the liver (38). Although the surface of the liver of the pig has morphological features different from that of the core, the differences are small and may not be functionally significant (7).

Estimation of portal venous concentration of drug. The concentration of drug in the portal vein during the last 2 min of infusion was estimated as the product of drug delivery rate and flow (F_hv) divided by the molecular weight of the drug. The sinusoidal-blood concentration of drug was reduced by the inflow of hepatic arterial blood. Because P_sa and HR changed, significant amounts of each of the drugs clearly traversed the liver. This extraction of drug by the liver also leads to uncertainty as to the concentration in the hepatic venules. Complete mixing across the portal venous flow stream was assumed. However, because the blood flow in the portal vein is probably not turbulent, the infused drug may have streamed along a part of the portal vein cross section and thus was delivered nonuniformly to the various branches and, thus, lobes of the liver (1). This would also explain part of the high variability in lobe thickness response to infused NE, when small ultrasonic crystals were used compared with a simultaneous liver plethysmograph method (13). In that study the variability was high, but the means were similar.