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Quantitation of the hepatic arterial buffer response to graded changes in portal blood flow
GASTROENTEROLOGY 1990;99:1024-1028
Quantitation of the Hepatic Arterial Buffer Response to Graded Changes in Portal Blood Flow W. WAYNE LAUTT, DALLAS J. LEGARE, and WALEED R. EZZAT Hepatorenal Research Unit, Department of Pharmacology & Therapeutics, Faculty of Medicine, University of Manitoba, Winnipeg, Manitoba, Canada
Hepatic arterial blood flow changes inversely in response to altered portal blood flow. The hepatic arterial capacity to buffer portal flow changes was studied over a wide range of portal flow with arterial pressure held steady (the active buffer response) or uncontrolled. The active component of the buffer response led to nearly full dilation of the hepatic artery at low portal flows as shown by inability to dilate further in response to adenosine infusion; at high portal flows the hepatic artery was nearly fully constricted as shown by lack of further constriction to norepinephrine. With pressure uncontrolled, active and passive effects combined to produce an increased compensation with similar efficiency (44% + 4%) over the full range of portal blood flows. Thus, although the active component of the hepatic arterial buffer response becomes less efficient at very high and low portal flows, the combination of active and passive effects leads to a larger buffer capacity which is equally efficient over a wide range of portal blood flow changes.
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hanges in portal venous blood flow (PVF) produce inverse changes in flow in the hepatic artery [HA) by a recently described mechanism, dependent upon adenosine washout but independent of oxygen demand or supply (1). This HA buffer response (2) tends to maintain total blood flow to the liver at a constant rate which, in turn, tends to maintain portal and intrahepatic pressures, liver blood volume and hepatic clearance of drugs and hormones steady. These relationships and the putative mechanism of the HA buffer response have recently been reviewed (3). The HA buffer response occurs in cats, dogs, rats, and humans (3). The objective of the present study was to quantitate the HA buffer response over a wide range of portal blood flow changes and to determine if the efficiency of the response is altered if arterial blood
pressure is controlled or allowed to alter secondary the portal flow changes.
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Materials and Methods Cats (18-h fasted) of either sex were anesthetized with sodium pentobarbital (32.5 mg/kg) (Somnotol, MTC Pharmaceuticals, Mississauga, Ontario] given intraperitoneally. Supplemental doses (6.5 mg] were given iv. as required. The following preparation has been described and evaluated (41.After laparotomy and splenectomy, electromagnetic flow probes were placed to measure HA blood flow and superior mesenteric arterial blood flow. All blood entering the portal vein was derived from the superior mesenteric artery; this flow is, therefore, referred to as PVF. A micrometer-controlled screw clamp was used to produce graded reductions in blood flow. Portal flow was reduced in random order to 20%, 40%, 6070, 80%‘, and 100% of control flow, returning to control flow between each step. When the portal flow was reduced, HA blood flow and arterial pressure increased. The increase in blood flow was allowed to stabilize (about 1 min), and then the screwclamp on the HA was used to return HA pressure to control level. The HA flow measured at that point is the true or active buffer response. Each flow change required about 3 min for all measurements to be made. The second preparation used was identical to the first, except that a catheter in the femoral artery diverted blood through a rotary pump (Cole-Parmer, Chicago, Ill.] to a silk filter and a small windkessel chamber, constructed of a glass test tube, to trap any bubbles and to buffer pressure oscillations from the pump. The blood was pumped to the superior mesenteric artery via a catheter. Blood pressure downstream from the catheter tip was measured from a catheter in the cecal artery. Blood flow in the portal vein was increased to roughly double basal levels and then decreased __.--___ Abbreviations used in this paper: HA, hepatic artery: portal venous flow. @ 1990 by the American Gastroenterological Association 0016-5065/90/$3.00
PVF,
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HEPATIC ARTERIAL BUFFER RESPONSE
in a series of six roughly equal steps. Systemic blood pressure, HA conductance, and HA flow were measured at each point. Curves for each experiment were drawn, and values were read at PVF changes of 5 ml/min/kg. The mean data shown in Figures 1, 2, and 3 were obtained from points on each individual curve. Body temperature, systemic arterial pressure, central venous pressure, and portal venous pressure were monitored (4). Vascular conductance was calculated from flow per kilogram body weight divided by the pressure gradient between the arterial and venous pressures. Portal venous pressure was used for both the superior mesenteric and HA conductance because portal venous pressure is insignificantly different from hepatic sinusoidal pressure (5). The hepatic arterial buffer response is the inverse change in HA blood flow in response to changes in PVF. The buffer capacity is calculated as the change in HA flow expressed as a percent of the change in PVF; 100% would thus represent complete compensation. In the present context, the buffer capacity is measured with blood pressure controlled to quantitate the active adenosine-mediated dilation of the HA in response to reduced portal flow. The buffer capacity is also determined with blood pressure uncontrolled so that this total buffer capacity represents the effect of the active buffer response and the effects of altered arterial blood flow. Means and standard errors are reported throughout, with statistical comparisons made using blocked ANOVA with multiple comparisons by Duncan’s test or Z-sided paired Student’s t-test.
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Figure 1. Relationship between hepatic arterial flow (HAFJ and PVF with uncontrolled blood pressure and using a superior mesenteric arterial long circuit to control portal blood flow. Linear regression (R’ = 0.99) indicated a slope of -0.442. The negative slope expressed as percent (44.2%) is equivalent to the mean HA buffer capacity over the full range of superior mesenteric arterial flows. The hepatic arterial buffer response (fIABR) calculated for each 5-ml change in SMAF is shown beneath the curve. Buffer capacity is shown as (change in HA flow)/(change in portal flow) as a percentage. Although the buffer capacity at the highest flow range tends to be reduced, the differences are not significant (blocked ANOVA). Each of the 7 curves used for the mean data were highly linear (B” = g.gl-o.gtt), and the buffer capacity calculated from the slope ranged from 54.6%~64.5%.
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The first series, with no arterial long circuit, consisted of 7 cats. Control parameters (mean + SE] were arterial blood pressure (femoral), 143.8 f 11.1 mmHg; portal venous pressure, 7.9 + 0.5 mmHg; superior mesenteric arterial blood flow, 40.1 + 7.1 ml min-’ - kg-r; hepatic arterial flow, 19.7 2 2.9 ml min-l - kg-‘; hepatic arterial conductance, 0.183 + 0.031 ml - min-’ . kg-l . mmHg-‘; superior mesenteric arterial conductance, 0.299 + 0.085 ml - min-’ kg-’ . mmHg-‘. The HA buffer responses to reduced portal blood flow were calculated during uncontrolled arterial
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Figure 2. The HA pressure (HAP) at various PVF obtained by shunting blood from the femoral artery into the superior mesenteric artery. The combination of the passive effect of the altered blood pressure and the active vascular response (Figure 2) leads to the relatively constant buffer capacity seen over a wide range of PVF (Figure 1).
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Figure 3. The active response of the HA as reflected by the HA conductance (HAC) at various PVF. At the highest and lowest PVF, the HAC approaches its maximal range of response. The HAC did not decrease significantly more than the value shown at 55 ml/min PVF when norepinephrine was infused into the HA.
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GASTROENTEROLOGYVol.98.No.4
pressure, which includes the HA flow response to the combined increase in blood pressure and the buffer response. Similar calculations were done during controlled HA pressure, which represents the true active buffer response to the changes in PVF. Figure 4 shows the buffer responses at all PVF reductions, with arterial pressure controlled and uncontrolled. With pressure controlled, the buffer capacity of the HA was highest (24.2% i-2.9%) at a 60% reduction in PVF, and significantly higher than the buffer capacity seen with 20% and 100% reductions in PVF (15.1% 2 2.7Y~, 17.7% + 3.5%, respectively, F [4,24] = 7.8). The PVF at the maximal response was approximately 16 ml/ min - kg-*. In Figure 5, the response of the HA is expressed as percent change of arterial conductance from the control level to the peak dilation of the HA. The change in HA conductance increased sharply from a value of 4.8Y0 * 1.0% (at 20% decrease in PVF) to a value of 28.7Y0+ 3.6% (at 100% decrease in PVF). These data were calculated for the true buffer, with arterial pressure held at control levels. When pressure was uncontrolled, the conductance was lower, indicating some degree of autoregulation (vasoconstriction in response to the increased arterial pressure). The differences were small, with the maximal difference between pressure controlled and uncontrolled being reached at the 80% decrease in PVF (percent change in HA conductance was 26.00/o + 2.3% with pressure
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Figure 4. The calculated buffer capacity (HA flow/PVF x 100%) of the hepatic arterial buffer response (HABR) in response to reductions in PVF with arterial blood pressure held at control level or uncontrolled. The buffer capacity with pressure uncontrolled is similar for all changes in PVP, whereas the active (true) buffer response is maximal at superior mesenteric arterial flow reductions of 60% (in the range of 16 ml . min-’ e kg-l). The buffer capacity at 20% and 100% reduction is significantly greater than that at 60%. Superior mesenteric arterial flow in this preparation is measured and controlled and is equal to total portal blood flow.
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Figure 6. The change in hepatic arterial conductance (HAC) with changes in PVF calculated from the responses shown in Figure 4 with pressure controlled. The changes in HAC represent the active buffer response to change in portal flow, reaching nearmaximal dilation at very low portal flow, as confirmed by inability of the artery to dilate significantly more in response to infused adenosine.
controlled and 20.3% i 2.29’0 with pressure uncontrolled: p < 0.01). To determine if the highest buffer capacity was limited because of full dilation of the HA, a paired comparison was made between the highest conductance reached during the buffer tests (0.254 + 0.043 ml . min-’ - kg-l . mmHg-l] and that reached during an intraportal infusion of adenosine (0.4-l mg . kg-’ . min-‘1. Adenosine caused a similar dilation (to a conductance of 0.299 i 0.045 ml . min? - kg-l . mmHg_-‘; p > 0.05 by Z-sided paired t-test) to that seen during the largest buffer-induced dilation. For the second series, with the superior mesenteric arterial (portal) blood flow controlled by the long circuit, a basal flow was established to provide estimated normal blood flow levels. The basal parameters (mean t SE], measured before obtaining a full pressure-flow curve (n = 71, were: arterial blood pressure, 105 5 8 mm Hg; blood pressure in the superior mesenteric artery (distal to the circuit catheter tip), 82 + 14 mm Hg; portal venous pressure, 10.3+ 1.1mm Hg; portal venous flow (PVF), 15.5f 1.2ml . min-’ . kg body wt-I; superior mesenteric arterial conductance, 0.31 + 0.07 ml . min-’ . kg body wt.-’ . mmHg; HA flow, 19.8 + 1.4ml . min-’ . kg body wt-‘; and HA conductance, 0.25 k 0.02 ml - min-’ . kg-* . mmHg-I. Figure 1 shows the HA flow as PVF was increased and decreased from the mean control flow of 15.5 + 1.2 ml . min-’ - kg-‘. The change in HA flow expressed as a percentage of the change in PVF is the buffer capacity of the HA and ranged from 51.2(7r25.29’; at different points on the curve. Mocked analy-
April 1990
sis of variance indicated that the differences in buffer capacity did not change significantly over the entire flow range (F[5,30] = 1.94). At 95% statistical power, buffer capacities over the full range of portal flow were not significantly different. The highest and lowest buffer capacities became significant at 90% power. The next largest difference would have required one additional experiment, and all subsequent differences would have required 11 or more experiments to achieve statistical significance at 90% power. The average buffer capacity (uncorrected for arterial pressure effect] over the full range is 44.2%, calculated from the slope of Figure 2 (R’ = 0.99; linear regression]. When each of the seven curves of the relationship between PVF and HA flow was analyzed separately by linear regression, R2 varied from 0.91-0.99, with the slopes (negative slope expressed as percent to represent the mean buffer capacity over the full range of changes of PVF) ranging from 34.6%-64.5% (mean, 44.2 -+ 3.8). The buffer capacity thus calculated incorporates two mechanisms of HA flow change: active changes in vascular conductance and passive changes secondary to arterial blood pressure. Figure 2 shows that as blood was diverted from the systemic circulation into the superior mesenteric artery, arterial blood pressure decreased. Figure 3 shows the change in HA conductance, indicating that at the highest and lowest PVF, the active vascular response approached its limits, thus producing a sigmoid curve. To determine if the conductance plateaued because the artery was nearly maximally constricted, norepinephrine (1.25 pg/kg/ min) was infused into the HA. Vascular conductance decreased by 78% + 8% to a minimal level of 0.0618 + 0.0242, which was not significantly lower than the conductance reached at a PVF of 35 ml . min-’ . kg (0.115 +-0.02; p > 0.05; &sided paired t-test). Discussion
The HA buffer response is the adenosinemediated inverse change in HA blood flow in response to changes in PVF (1). For mechanistic studies, it is essential that the HA flow response be caused only by the active conductance changes imposed by the buffer response. The 25% buffer capacity usually found, however, is not a good index of the most important physiological response, i.e., the actual change in HA blood flow that would occur with pressure uncontrolled. If PVF undergoes a large increase, for example, the HA flow will decrease because of the buffer mechanism but also will be affected by the ensuing decrease in blood pressure that occurs with dilation of the splanchnic blood supply. A further complication arises from the active autoregulatory response of the HA to altered blood pressure, a response also mediated by adenosine (6).
HEPATIC ARTERIAL
BUFFER RESPONSE
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The first series of experiments used multiple graded reductions in PVF and showed that as blood flow in the portal vein is reduced, the HA conductance increases rapidly to approach a plateau at the largest reductions in PVF. Despite the observations that the active buffer capacity is dependent on the degree of change in PVF, being most efficient at PVF reductions of about 60%, the buffer capacity remained remarkably stable if blood pressure was uncontrolled. The blood pressure effect increased the net buffer capacity despite the demonstration that the elevated blood pressure also caused the HA to actively constrict, thus showing an interaction between the buffer response, the passive effect of the increased blood pressure, and HA autoregulation in response to the increased blood pressure. A more rigorous vascular model was then used wherein the blood flow in the portal vein could be increased as well as decreased. This model avoided the need for multiple mechanical occlusions of the superior mesenteric artery and multiple manipulations of the HA pressure. The relationship between HA and PVF is linear, with a mean buffer capacity over the entire range of PVF from 5-35 ml - min-’ kg-’ of 44.2% +- 3.8% (range for individual animals, 34.6%-64.5%). Previous studies have suggested both linear (7) and nonlinear (8,9) relationships between HA and PVF. The linear relationship reported by Mathie and Blumgart (7) used a portocaval surgical shunt to control PVF. Linearity was determined by regression fit of 2-4 data points from each of 10 dogs. The mean buffer capacity calculated from the regression equation was 22.7%. Hanson and Johnson (8) showed a curve in which the HA flow plateaued at low PVF. Their buffer capacity was, however, very low; about 8%. Gelman and Ernst (9) reported a greater increase in HA flow at the lowest PVF. These studies also used regression of multiple points from different animals. However, by eliminating only 3 data points (of 45) with unusually high arterial flows at the lowest PVF, their data are rendered clearly linear, with a mean buffer capacity of roughly 3570, similar to the 44.2% + 3.8% capacity reported here. In the current experiments, a full curve was obtained for each animal, allowing for blocked ANOVA at various points as well as calculation of mean buffer capacity over the full range of PVF for each curve. Although the relationship of HA and portal flows was clearly a linear relationship, the buffer response showed a tendency toward a lesser buffer capacity (25.2%) at the highest PVF compared with the highest buffer capacity (51%) seen for portal flow changes in the middle range. Thus, overall, the relationship between PVF and HA flow is highly linear over a wide range of flows when blood pressure is uncontrolled. The active buffer capacity (pressure controlled) be-
1028 LALJTT ET AL.
comes less efficient at the extremes of PVF change. The dilation of the HA is near maximal at fully reduced PVF, as shown by the lack of significantly greater dilation induced by infusion of adenosine. Similarly, the maximal reduction in HA conductance seen at the highest PVF represents near-maximal vasoconstriction because intra-arterial infusion of norepinephrine did not lead to significantly greater contraction. Thus, it seems that the HA buffer response is able to lead to maximal or near-maximal constriction and dilation of the HA at very high and low PVF, respectively. A major question that remains unanswered related to quantitation of this response is the effect of surgery and species. Data on the efficiency of the buffer capacity in conscious animals are not available. Anesthetics, however, may alter the response. Halothane, for example, seems to eliminate the buffer response (lo), which may be the reason for the liver being much more susceptible to damage using that anesthetic. No different buffer capacity is seen using pentobarbital or chloralose with ketamine induction (unpublished observation]: however, in surgery using pentobarbital anesthesia, total hepatic blood flow has been shown to be greatly reduced as determined by very-low-dose ethanol clearance (11) and may thus produce quantitative data that are not representative of those seen in the conscious state. It is important to quantitate the buffer capacity in the conscious state and in humans because of the need to fully assess the physiological role of the buffer response. The presence of the dilation of the HA in response to brief occlusion of the portal vein is said to be the best predictor of successful outcome of a portocaval shunt in humans (12). However, if various anesthetics can modulate the buffer capacity, this tool becomes less reliable. Pharmacological studies are also confounded by the presence and magnitude of the buffer response. For example, iv. glucagon, isoproterenol, and adenosine cause dilation of the superior mesenteric artery, but, at appropriate doses, may actually decrease HA flow, thus leading to the impression that the HA is pharmacologically uniquely insensitive to these agents. However, if the buffer response is mechanically prevented (by holding portal flow steady) or blocked with adenosine-receptor antagonists, the HA shows normal dose-related dilations to these agents (13,14). The great variability in reported responses of the HA to iv. glucagon (15-18) is likely to be explained by variable buffer capacities in the different studies rather than by intrinsic pharmacological differences in response to glucagon. Therefore, pharmacological studies of the splanchnic circulation must avoid or take into account the impact of altered portal blood flow on the HA.
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References 1. Lautt
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3. 4.
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6. 7.
8. 9.
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WW. Mechanism and role of intrinsic regulation of hepatic arterial blood flow: the hepatic arterial buffer response. Am J Physioll985;249:G549-G556, Lautt WW. Role and control of the hepatic artery. In: Lautt WW, ed. Hepatic circulation in health and disease. New York: Raven, 1981, pp 203-220. Lautt WW, Greenway CV. Conceptual review of the hepatic vascular bed. Hepatology 1987;7:952-963. Lautt WW, Legare DJ, d’iilmeida MS. Adenosine as putative regulator of hepatic arterial flow (the buffer response). Am J Physioll985;248:H331-H338, Lautt WW, Greenway CV. Legare DJ, Weisman H. Localization of intrahepatic portal vascular resistance. Am J Physioll986;251: G375-G381. Ezzat WR, Lautt WW. Hepatic arterial pressure-flow autoregulation is adenosine mediated. Am J Physioll987;252:H836-845. Mathie RT, Blumgart LH. The hepatic haemodynamic response to acute portal venous blood flow reductions in the dog. Pflugers Arch 1983;399:223-227. Hanson KM, Johnson PC. Local control of hepatic arterial and portal venous flow in the dog. Am J Physiol 1966;211:712-720. Gelman S, Ernst EA. Role of pH, PCO, and 0, content of portal blood in hepatic circulatory autoregulation. Am J Physiol 1977; 233:E255-E262. Gelman S. General anesthesia and hepatic circulation. Can J Physiol Pharmacol1987;65:1762-1779.
11 Greenway CV, Lautt WW, Sitar DS. Hepatic blood flow: estimation from clearance of very low dose infusions of ethanol in cats. Can J Physiol Pharmacol1988;66:1192-1197. 12 Burchell AR, Moreno AH, Panke WF, Nealon TF. Hepatic artery flow improvement after portacaval shunt: a single hemodynamic clinical correlate. Ann Surg 1976:184:289-300. 13. Lautt WW, d’Almeida MS, McQuaker J. D’Aleo L. Impact of the hepatic arterial buffer response on splanchnic vascular responses to intravenous adenosine, isoproterenol and glucagon. Can J Physiol Pharmacol 1988;66:807-813. MS, McQuaker JE, D’Aleo L, Lautt WW. Competing 14. d’Almeida effects of intravenously infused dilator agents and raised portal blood flow on hepatic arterial conductance. Proc Western Pharmacol Sot 1988;31:113-115. 15. Gelman S, Dillard E, Parks DA. Glucagon increases hepatic oxygen supply-demand ratio in pigs. Am J Physiol1987;252:G648G653. 16. Krarup N, Larsen JA. The effect of glucagon on hepatosplanchnit hemodynamics, functional capacity, and metabolism of the liver in cats. Acta Physiol Stand 1974;91:42-52. 17. Lindberg B, Darle N. Effect of glucagon on hepatic circulation in the pig. Arch Surg 1976;111:1379-1383, 18. Richardson PDI, Withrington PG. Liver blood flow. II. Effects of drugs and hormones on liver blood flow. Gastroenterology 1981;81:356-375.
Received October 14,1988.Accepted September l&1989. Address requests for reprints to Dr. W. W. Lautt, Professor and Head, Department of Pharmacology, Faculty of Medicine, University of Manitoba, Winnipeg, Manitoba R3E 0W3, Canada. Waleed R. Ezzat was supported by a scholarship from the University of Bhagdad, Iraq. This study was financed by a grant from the Manitoba Heart Foundation. Graphics were done by Janet McQuaker; manuscript preparation was done by Karen Sanders.
Quantitation of the Hepatic Arterial Buffer Response to Graded Changes in Portal Blood Flow W. WAYNE LAUTT, DALLAS J. LEGARE, and WALEED R. EZZAT Hepatorenal Research Unit, Department of Pharmacology & Therapeutics, Faculty of Medicine, University of Manitoba, Winnipeg, Manitoba, Canada
Hepatic arterial blood flow changes inversely in response to altered portal blood flow. The hepatic arterial capacity to buffer portal flow changes was studied over a wide range of portal flow with arterial pressure held steady (the active buffer response) or uncontrolled. The active component of the buffer response led to nearly full dilation of the hepatic artery at low portal flows as shown by inability to dilate further in response to adenosine infusion; at high portal flows the hepatic artery was nearly fully constricted as shown by lack of further constriction to norepinephrine. With pressure uncontrolled, active and passive effects combined to produce an increased compensation with similar efficiency (44% + 4%) over the full range of portal blood flows. Thus, although the active component of the hepatic arterial buffer response becomes less efficient at very high and low portal flows, the combination of active and passive effects leads to a larger buffer capacity which is equally efficient over a wide range of portal blood flow changes.
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hanges in portal venous blood flow (PVF) produce inverse changes in flow in the hepatic artery [HA) by a recently described mechanism, dependent upon adenosine washout but independent of oxygen demand or supply (1). This HA buffer response (2) tends to maintain total blood flow to the liver at a constant rate which, in turn, tends to maintain portal and intrahepatic pressures, liver blood volume and hepatic clearance of drugs and hormones steady. These relationships and the putative mechanism of the HA buffer response have recently been reviewed (3). The HA buffer response occurs in cats, dogs, rats, and humans (3). The objective of the present study was to quantitate the HA buffer response over a wide range of portal blood flow changes and to determine if the efficiency of the response is altered if arterial blood
pressure is controlled or allowed to alter secondary the portal flow changes.
to
Materials and Methods Cats (18-h fasted) of either sex were anesthetized with sodium pentobarbital (32.5 mg/kg) (Somnotol, MTC Pharmaceuticals, Mississauga, Ontario] given intraperitoneally. Supplemental doses (6.5 mg] were given iv. as required. The following preparation has been described and evaluated (41.After laparotomy and splenectomy, electromagnetic flow probes were placed to measure HA blood flow and superior mesenteric arterial blood flow. All blood entering the portal vein was derived from the superior mesenteric artery; this flow is, therefore, referred to as PVF. A micrometer-controlled screw clamp was used to produce graded reductions in blood flow. Portal flow was reduced in random order to 20%, 40%, 6070, 80%‘, and 100% of control flow, returning to control flow between each step. When the portal flow was reduced, HA blood flow and arterial pressure increased. The increase in blood flow was allowed to stabilize (about 1 min), and then the screwclamp on the HA was used to return HA pressure to control level. The HA flow measured at that point is the true or active buffer response. Each flow change required about 3 min for all measurements to be made. The second preparation used was identical to the first, except that a catheter in the femoral artery diverted blood through a rotary pump (Cole-Parmer, Chicago, Ill.] to a silk filter and a small windkessel chamber, constructed of a glass test tube, to trap any bubbles and to buffer pressure oscillations from the pump. The blood was pumped to the superior mesenteric artery via a catheter. Blood pressure downstream from the catheter tip was measured from a catheter in the cecal artery. Blood flow in the portal vein was increased to roughly double basal levels and then decreased __.--___ Abbreviations used in this paper: HA, hepatic artery: portal venous flow. @ 1990 by the American Gastroenterological Association 0016-5065/90/$3.00
PVF,
April 1990
HEPATIC ARTERIAL BUFFER RESPONSE
in a series of six roughly equal steps. Systemic blood pressure, HA conductance, and HA flow were measured at each point. Curves for each experiment were drawn, and values were read at PVF changes of 5 ml/min/kg. The mean data shown in Figures 1, 2, and 3 were obtained from points on each individual curve. Body temperature, systemic arterial pressure, central venous pressure, and portal venous pressure were monitored (4). Vascular conductance was calculated from flow per kilogram body weight divided by the pressure gradient between the arterial and venous pressures. Portal venous pressure was used for both the superior mesenteric and HA conductance because portal venous pressure is insignificantly different from hepatic sinusoidal pressure (5). The hepatic arterial buffer response is the inverse change in HA blood flow in response to changes in PVF. The buffer capacity is calculated as the change in HA flow expressed as a percent of the change in PVF; 100% would thus represent complete compensation. In the present context, the buffer capacity is measured with blood pressure controlled to quantitate the active adenosine-mediated dilation of the HA in response to reduced portal flow. The buffer capacity is also determined with blood pressure uncontrolled so that this total buffer capacity represents the effect of the active buffer response and the effects of altered arterial blood flow. Means and standard errors are reported throughout, with statistical comparisons made using blocked ANOVA with multiple comparisons by Duncan’s test or Z-sided paired Student’s t-test.
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Figure 1. Relationship between hepatic arterial flow (HAFJ and PVF with uncontrolled blood pressure and using a superior mesenteric arterial long circuit to control portal blood flow. Linear regression (R’ = 0.99) indicated a slope of -0.442. The negative slope expressed as percent (44.2%) is equivalent to the mean HA buffer capacity over the full range of superior mesenteric arterial flows. The hepatic arterial buffer response (fIABR) calculated for each 5-ml change in SMAF is shown beneath the curve. Buffer capacity is shown as (change in HA flow)/(change in portal flow) as a percentage. Although the buffer capacity at the highest flow range tends to be reduced, the differences are not significant (blocked ANOVA). Each of the 7 curves used for the mean data were highly linear (B” = g.gl-o.gtt), and the buffer capacity calculated from the slope ranged from 54.6%~64.5%.
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The first series, with no arterial long circuit, consisted of 7 cats. Control parameters (mean + SE] were arterial blood pressure (femoral), 143.8 f 11.1 mmHg; portal venous pressure, 7.9 + 0.5 mmHg; superior mesenteric arterial blood flow, 40.1 + 7.1 ml min-’ - kg-r; hepatic arterial flow, 19.7 2 2.9 ml min-l - kg-‘; hepatic arterial conductance, 0.183 + 0.031 ml - min-’ . kg-l . mmHg-‘; superior mesenteric arterial conductance, 0.299 + 0.085 ml - min-’ kg-’ . mmHg-‘. The HA buffer responses to reduced portal blood flow were calculated during uncontrolled arterial
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Figure 2. The HA pressure (HAP) at various PVF obtained by shunting blood from the femoral artery into the superior mesenteric artery. The combination of the passive effect of the altered blood pressure and the active vascular response (Figure 2) leads to the relatively constant buffer capacity seen over a wide range of PVF (Figure 1).
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Figure 3. The active response of the HA as reflected by the HA conductance (HAC) at various PVF. At the highest and lowest PVF, the HAC approaches its maximal range of response. The HAC did not decrease significantly more than the value shown at 55 ml/min PVF when norepinephrine was infused into the HA.
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GASTROENTEROLOGYVol.98.No.4
pressure, which includes the HA flow response to the combined increase in blood pressure and the buffer response. Similar calculations were done during controlled HA pressure, which represents the true active buffer response to the changes in PVF. Figure 4 shows the buffer responses at all PVF reductions, with arterial pressure controlled and uncontrolled. With pressure controlled, the buffer capacity of the HA was highest (24.2% i-2.9%) at a 60% reduction in PVF, and significantly higher than the buffer capacity seen with 20% and 100% reductions in PVF (15.1% 2 2.7Y~, 17.7% + 3.5%, respectively, F [4,24] = 7.8). The PVF at the maximal response was approximately 16 ml/ min - kg-*. In Figure 5, the response of the HA is expressed as percent change of arterial conductance from the control level to the peak dilation of the HA. The change in HA conductance increased sharply from a value of 4.8Y0 * 1.0% (at 20% decrease in PVF) to a value of 28.7Y0+ 3.6% (at 100% decrease in PVF). These data were calculated for the true buffer, with arterial pressure held at control levels. When pressure was uncontrolled, the conductance was lower, indicating some degree of autoregulation (vasoconstriction in response to the increased arterial pressure). The differences were small, with the maximal difference between pressure controlled and uncontrolled being reached at the 80% decrease in PVF (percent change in HA conductance was 26.00/o + 2.3% with pressure
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0 UNCONTROLLED PRESSURE
t
N=7
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Figure 4. The calculated buffer capacity (HA flow/PVF x 100%) of the hepatic arterial buffer response (HABR) in response to reductions in PVF with arterial blood pressure held at control level or uncontrolled. The buffer capacity with pressure uncontrolled is similar for all changes in PVP, whereas the active (true) buffer response is maximal at superior mesenteric arterial flow reductions of 60% (in the range of 16 ml . min-’ e kg-l). The buffer capacity at 20% and 100% reduction is significantly greater than that at 60%. Superior mesenteric arterial flow in this preparation is measured and controlled and is equal to total portal blood flow.
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Figure 6. The change in hepatic arterial conductance (HAC) with changes in PVF calculated from the responses shown in Figure 4 with pressure controlled. The changes in HAC represent the active buffer response to change in portal flow, reaching nearmaximal dilation at very low portal flow, as confirmed by inability of the artery to dilate significantly more in response to infused adenosine.
controlled and 20.3% i 2.29’0 with pressure uncontrolled: p < 0.01). To determine if the highest buffer capacity was limited because of full dilation of the HA, a paired comparison was made between the highest conductance reached during the buffer tests (0.254 + 0.043 ml . min-’ - kg-l . mmHg-l] and that reached during an intraportal infusion of adenosine (0.4-l mg . kg-’ . min-‘1. Adenosine caused a similar dilation (to a conductance of 0.299 i 0.045 ml . min? - kg-l . mmHg_-‘; p > 0.05 by Z-sided paired t-test) to that seen during the largest buffer-induced dilation. For the second series, with the superior mesenteric arterial (portal) blood flow controlled by the long circuit, a basal flow was established to provide estimated normal blood flow levels. The basal parameters (mean t SE], measured before obtaining a full pressure-flow curve (n = 71, were: arterial blood pressure, 105 5 8 mm Hg; blood pressure in the superior mesenteric artery (distal to the circuit catheter tip), 82 + 14 mm Hg; portal venous pressure, 10.3+ 1.1mm Hg; portal venous flow (PVF), 15.5f 1.2ml . min-’ . kg body wt-I; superior mesenteric arterial conductance, 0.31 + 0.07 ml . min-’ . kg body wt.-’ . mmHg; HA flow, 19.8 + 1.4ml . min-’ . kg body wt-‘; and HA conductance, 0.25 k 0.02 ml - min-’ . kg-* . mmHg-I. Figure 1 shows the HA flow as PVF was increased and decreased from the mean control flow of 15.5 + 1.2 ml . min-’ - kg-‘. The change in HA flow expressed as a percentage of the change in PVF is the buffer capacity of the HA and ranged from 51.2(7r25.29’; at different points on the curve. Mocked analy-
April 1990
sis of variance indicated that the differences in buffer capacity did not change significantly over the entire flow range (F[5,30] = 1.94). At 95% statistical power, buffer capacities over the full range of portal flow were not significantly different. The highest and lowest buffer capacities became significant at 90% power. The next largest difference would have required one additional experiment, and all subsequent differences would have required 11 or more experiments to achieve statistical significance at 90% power. The average buffer capacity (uncorrected for arterial pressure effect] over the full range is 44.2%, calculated from the slope of Figure 2 (R’ = 0.99; linear regression]. When each of the seven curves of the relationship between PVF and HA flow was analyzed separately by linear regression, R2 varied from 0.91-0.99, with the slopes (negative slope expressed as percent to represent the mean buffer capacity over the full range of changes of PVF) ranging from 34.6%-64.5% (mean, 44.2 -+ 3.8). The buffer capacity thus calculated incorporates two mechanisms of HA flow change: active changes in vascular conductance and passive changes secondary to arterial blood pressure. Figure 2 shows that as blood was diverted from the systemic circulation into the superior mesenteric artery, arterial blood pressure decreased. Figure 3 shows the change in HA conductance, indicating that at the highest and lowest PVF, the active vascular response approached its limits, thus producing a sigmoid curve. To determine if the conductance plateaued because the artery was nearly maximally constricted, norepinephrine (1.25 pg/kg/ min) was infused into the HA. Vascular conductance decreased by 78% + 8% to a minimal level of 0.0618 + 0.0242, which was not significantly lower than the conductance reached at a PVF of 35 ml . min-’ . kg (0.115 +-0.02; p > 0.05; &sided paired t-test). Discussion
The HA buffer response is the adenosinemediated inverse change in HA blood flow in response to changes in PVF (1). For mechanistic studies, it is essential that the HA flow response be caused only by the active conductance changes imposed by the buffer response. The 25% buffer capacity usually found, however, is not a good index of the most important physiological response, i.e., the actual change in HA blood flow that would occur with pressure uncontrolled. If PVF undergoes a large increase, for example, the HA flow will decrease because of the buffer mechanism but also will be affected by the ensuing decrease in blood pressure that occurs with dilation of the splanchnic blood supply. A further complication arises from the active autoregulatory response of the HA to altered blood pressure, a response also mediated by adenosine (6).
HEPATIC ARTERIAL
BUFFER RESPONSE
1027
The first series of experiments used multiple graded reductions in PVF and showed that as blood flow in the portal vein is reduced, the HA conductance increases rapidly to approach a plateau at the largest reductions in PVF. Despite the observations that the active buffer capacity is dependent on the degree of change in PVF, being most efficient at PVF reductions of about 60%, the buffer capacity remained remarkably stable if blood pressure was uncontrolled. The blood pressure effect increased the net buffer capacity despite the demonstration that the elevated blood pressure also caused the HA to actively constrict, thus showing an interaction between the buffer response, the passive effect of the increased blood pressure, and HA autoregulation in response to the increased blood pressure. A more rigorous vascular model was then used wherein the blood flow in the portal vein could be increased as well as decreased. This model avoided the need for multiple mechanical occlusions of the superior mesenteric artery and multiple manipulations of the HA pressure. The relationship between HA and PVF is linear, with a mean buffer capacity over the entire range of PVF from 5-35 ml - min-’ kg-’ of 44.2% +- 3.8% (range for individual animals, 34.6%-64.5%). Previous studies have suggested both linear (7) and nonlinear (8,9) relationships between HA and PVF. The linear relationship reported by Mathie and Blumgart (7) used a portocaval surgical shunt to control PVF. Linearity was determined by regression fit of 2-4 data points from each of 10 dogs. The mean buffer capacity calculated from the regression equation was 22.7%. Hanson and Johnson (8) showed a curve in which the HA flow plateaued at low PVF. Their buffer capacity was, however, very low; about 8%. Gelman and Ernst (9) reported a greater increase in HA flow at the lowest PVF. These studies also used regression of multiple points from different animals. However, by eliminating only 3 data points (of 45) with unusually high arterial flows at the lowest PVF, their data are rendered clearly linear, with a mean buffer capacity of roughly 3570, similar to the 44.2% + 3.8% capacity reported here. In the current experiments, a full curve was obtained for each animal, allowing for blocked ANOVA at various points as well as calculation of mean buffer capacity over the full range of PVF for each curve. Although the relationship of HA and portal flows was clearly a linear relationship, the buffer response showed a tendency toward a lesser buffer capacity (25.2%) at the highest PVF compared with the highest buffer capacity (51%) seen for portal flow changes in the middle range. Thus, overall, the relationship between PVF and HA flow is highly linear over a wide range of flows when blood pressure is uncontrolled. The active buffer capacity (pressure controlled) be-
1028 LALJTT ET AL.
comes less efficient at the extremes of PVF change. The dilation of the HA is near maximal at fully reduced PVF, as shown by the lack of significantly greater dilation induced by infusion of adenosine. Similarly, the maximal reduction in HA conductance seen at the highest PVF represents near-maximal vasoconstriction because intra-arterial infusion of norepinephrine did not lead to significantly greater contraction. Thus, it seems that the HA buffer response is able to lead to maximal or near-maximal constriction and dilation of the HA at very high and low PVF, respectively. A major question that remains unanswered related to quantitation of this response is the effect of surgery and species. Data on the efficiency of the buffer capacity in conscious animals are not available. Anesthetics, however, may alter the response. Halothane, for example, seems to eliminate the buffer response (lo), which may be the reason for the liver being much more susceptible to damage using that anesthetic. No different buffer capacity is seen using pentobarbital or chloralose with ketamine induction (unpublished observation]: however, in surgery using pentobarbital anesthesia, total hepatic blood flow has been shown to be greatly reduced as determined by very-low-dose ethanol clearance (11) and may thus produce quantitative data that are not representative of those seen in the conscious state. It is important to quantitate the buffer capacity in the conscious state and in humans because of the need to fully assess the physiological role of the buffer response. The presence of the dilation of the HA in response to brief occlusion of the portal vein is said to be the best predictor of successful outcome of a portocaval shunt in humans (12). However, if various anesthetics can modulate the buffer capacity, this tool becomes less reliable. Pharmacological studies are also confounded by the presence and magnitude of the buffer response. For example, iv. glucagon, isoproterenol, and adenosine cause dilation of the superior mesenteric artery, but, at appropriate doses, may actually decrease HA flow, thus leading to the impression that the HA is pharmacologically uniquely insensitive to these agents. However, if the buffer response is mechanically prevented (by holding portal flow steady) or blocked with adenosine-receptor antagonists, the HA shows normal dose-related dilations to these agents (13,14). The great variability in reported responses of the HA to iv. glucagon (15-18) is likely to be explained by variable buffer capacities in the different studies rather than by intrinsic pharmacological differences in response to glucagon. Therefore, pharmacological studies of the splanchnic circulation must avoid or take into account the impact of altered portal blood flow on the HA.
GASTROENTEROLOGY
Voi. 98, No. 4
References 1. Lautt
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3. 4.
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6. 7.
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WW. Mechanism and role of intrinsic regulation of hepatic arterial blood flow: the hepatic arterial buffer response. Am J Physioll985;249:G549-G556, Lautt WW. Role and control of the hepatic artery. In: Lautt WW, ed. Hepatic circulation in health and disease. New York: Raven, 1981, pp 203-220. Lautt WW, Greenway CV. Conceptual review of the hepatic vascular bed. Hepatology 1987;7:952-963. Lautt WW, Legare DJ, d’iilmeida MS. Adenosine as putative regulator of hepatic arterial flow (the buffer response). Am J Physioll985;248:H331-H338, Lautt WW, Greenway CV. Legare DJ, Weisman H. Localization of intrahepatic portal vascular resistance. Am J Physioll986;251: G375-G381. Ezzat WR, Lautt WW. Hepatic arterial pressure-flow autoregulation is adenosine mediated. Am J Physioll987;252:H836-845. Mathie RT, Blumgart LH. The hepatic haemodynamic response to acute portal venous blood flow reductions in the dog. Pflugers Arch 1983;399:223-227. Hanson KM, Johnson PC. Local control of hepatic arterial and portal venous flow in the dog. Am J Physiol 1966;211:712-720. Gelman S, Ernst EA. Role of pH, PCO, and 0, content of portal blood in hepatic circulatory autoregulation. Am J Physiol 1977; 233:E255-E262. Gelman S. General anesthesia and hepatic circulation. Can J Physiol Pharmacol1987;65:1762-1779.
11 Greenway CV, Lautt WW, Sitar DS. Hepatic blood flow: estimation from clearance of very low dose infusions of ethanol in cats. Can J Physiol Pharmacol1988;66:1192-1197. 12 Burchell AR, Moreno AH, Panke WF, Nealon TF. Hepatic artery flow improvement after portacaval shunt: a single hemodynamic clinical correlate. Ann Surg 1976:184:289-300. 13. Lautt WW, d’Almeida MS, McQuaker J. D’Aleo L. Impact of the hepatic arterial buffer response on splanchnic vascular responses to intravenous adenosine, isoproterenol and glucagon. Can J Physiol Pharmacol 1988;66:807-813. MS, McQuaker JE, D’Aleo L, Lautt WW. Competing 14. d’Almeida effects of intravenously infused dilator agents and raised portal blood flow on hepatic arterial conductance. Proc Western Pharmacol Sot 1988;31:113-115. 15. Gelman S, Dillard E, Parks DA. Glucagon increases hepatic oxygen supply-demand ratio in pigs. Am J Physiol1987;252:G648G653. 16. Krarup N, Larsen JA. The effect of glucagon on hepatosplanchnit hemodynamics, functional capacity, and metabolism of the liver in cats. Acta Physiol Stand 1974;91:42-52. 17. Lindberg B, Darle N. Effect of glucagon on hepatic circulation in the pig. Arch Surg 1976;111:1379-1383, 18. Richardson PDI, Withrington PG. Liver blood flow. II. Effects of drugs and hormones on liver blood flow. Gastroenterology 1981;81:356-375.
Received October 14,1988.Accepted September l&1989. Address requests for reprints to Dr. W. W. Lautt, Professor and Head, Department of Pharmacology, Faculty of Medicine, University of Manitoba, Winnipeg, Manitoba R3E 0W3, Canada. Waleed R. Ezzat was supported by a scholarship from the University of Bhagdad, Iraq. This study was financed by a grant from the Manitoba Heart Foundation. Graphics were done by Janet McQuaker; manuscript preparation was done by Karen Sanders.