INTRODUCTION

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PLASMA LACTATE INCREASES after ingestion or intragastric administration of carbohydrates. In response to intragastric glucose loads, plasma lactate concentration increased in unrestrained, chronically catheterized rats (23) and dogs (32). The increase was generally greater in the portal vein than in the periphery (14, 23), but the exact source of the portal vein lactate and whether it represents mainly first-pass or recirculated lactate is unclear. An increase in portal vein, systemic, and hepatic lactate concentration also accompanied feeding after mild food deprivation in rats (15), and, in humans, systemic plasma lactate increases in response to carbohydrate-containing meals (8, 33, 34). The increase in circulating lactate and, to a lesser extent, pyruvate reflects the low phosphorylating capacity of hepatocytes for glucose (9). Consequently, much of the ingested glucose enters hepatic glycogen by an indirect route that requires glucose transformation into three-carbon units by extrahepatic tissues (22).

Lactate reduces food intake after parenteral administration in monkeys (1) and rats (17, 18, 27). The meal-induced increase in plasma lactate may therefore contribute to the control of feeding. To further explore this possibility, we investigated the effects of remotely controlled, meal-contingent hepatic portal infusions of lactate on spontaneous feeding in undisturbed rats. In previous studies, the suppression of food intake in response to peripheral lactate administration was blocked after hepatic branch vagotomy (18, 21). This suggests that lactate inhibits feeding by acting in the liver; however, it is not proof for this assumption because the hepatic branch of the vagus also carries afferent fibers of extrahepatic origin (3). Therefore, as an alternative approach to examine whether or not lactate's effect on food intake originates in the liver, we compared feeding after infusing lactate directly in the liver through the hepatic portal vein or in the inferior vena cava just proximal to the junction of the hepatic veins. To judge the physiological relevance of the observed effects, we measured plasma lactate and glucose concentrations in the vena cava after hepatic portal vein lactate infusion.

	METHODS

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Animals. Male Sprague-Dawley rats (body weight: 280-300 g at study onset; Institute for Laboratory Animals, University of Zurich) were individually housed in a temperature-controlled (21 ± 2°C) colony room and were kept on a reversed 12:12-h light-dark cycle (dark onset: 1300). The rats had ad libitum access to water and ground rat chow (NAFAG 890, Gossau, Switzerland) with a metabolizable energy content of ~12 kJ/g. Rats were adapted to housing conditions for at least 8 days before surgery.

Surgery. Catheters were assembled as previously described and were sterilized in alcohol before use. The protruding dorsal end of a silicon catheter [Silastic (Dow Corning, Midland, MI); ID 0.508 mm, OD 0.940 mm, length 27 cm] was slipped on a headpiece (a modified cannula with a screw top), reinforced with a 2.5-cm piece of larger silicon catheter (ID 1.016 mm, OD 2.159 mm), and led through a folded polypropylene surgical mesh (Bard Marlex Mesh; Davol) to improve adhesion to the skin and fascia. The rats were anesthetized by intraperitoneal injection (1.25 ml/kg) of a mixture of ketamine (80 mg/ml; Ketasol-100), xylazine (4.66 mg/ml; Rompun; Bayer, Leverkusen, Germany), and acepromazine (0.05 mg/kg; Sedaline; Chassot, Berne, Switzerland). The topless headpiece was led subcutaneously from a 2-cm midline interscapular incision to a puncture wound 1 cm rostral to the incision, and the threaded end was exteriorized. The top was screwed on to secure the headpiece in place. The catheter was led subcutaneously from the neck to a 5-cm incision in the midline of the abdomen. The distal end of the catheter was implanted in the ileocolic vein and led rostrally to the hepatic portal vein. The catheter was fixed to the vein with sutures and Histoacryl glue (B. Braun, Melsungen, Germany). Skin and muscle were closed with resorbable sutures. A cap was put on the headpiece to close the catheter. Chloramphenicol (500 mg = 5 ml of Chloromycetin; Parke-Davis, Grub, Switzerland) was injected subuctaneously for infection prophylaxis, and novaminsulfon (50 mg = 0.1 ml of Vetalgin; Veterinaria, Zurich, Switzerland) was injected intramuscularly for analgesia. For further details of the surgical procedure refer to the work by Surina-Baumgartner et al. (35).

In one group of rats, a second catheter was placed in the vena cava within the same surgery to investigate whether hepatic portal vein and vena cava lactate infusion affect eating differently. The surgical procedure was the same as for the hepatic portal vein catheterization. The inferior vena cava was exposed and penetrated with one cannula rostral to the renal veins using Kaufman's (13) method. The distal tip of the cannula was advanced 3-4 cm so that it lay near the juncture of the hepatic vein with the inferior vena cava. The cannula was anchored to the abdominal muscles with nonresorbable sutures. The dorsal end of the vena cava catheter was exteriorized with a headpiece 1 cm rostral to the headpiece of the hepatic portal vein catheter. After a 3-day recovery period, the catheters were flushed every second day with 0.3 ml of 0.9% saline to ensure patency.

Test procedure. Rats were placed in individual open-topped Plexiglas infusion cages (37 × 21 × 41 cm) with stainless steel grated floors and hinged doors. Rats had ad libitum access to water, and ground chow was available ad libitum in food cups mounted on electronic balances (Mettler PM 3000). The rats could reach the food cups over a short bridge 5 cm from the cage floor. The balances were interfaced with a computer (Olivetti M 300) in an adjacent room, and a custom-designed program (VZM; Krügel, Munich, Germany) recorded the weights of the food cups every 30 s. In addition, a video surveillance camera (VSS 3440; Phillips) connected to a 13-in. monitor in the adjacent room allowed for continuous observation of the rats.

After at least 5 days of adaptation to the cages, the rats were adapted to the experimental procedure for 3 days. The procedure entailed connecting the catheter to a syringe pump (model A99; Razel, Stamford, CT) via two segments of Tygon tubing (0.76 mm ID, 2.29 mm OD) separated by a swivel joint suspended ~45 cm above the cage floor, which allowed the rat to move freely. The lower tubing segment was sheathed with a stainless steel spring. The syringe, both tubing segments, and the dead space of the indwelling cannula were filled with saline. In the rats with both hepatic portal vein and vena cava catheters, the two catheters were attached to the syringe pump alternately on consecutive days. The food cups were filled with fresh food, and the experimenter left the room. The whole procedure required ~30 min and was completed 2.5 h before dark onset.

The infusion pumps were remotely controlled from the adjacent room. The infusions started 2 min into the first spontaneous nocturnal meal and lasted for 5 min. If a meal was under way when the lights went out, the infusion was done during the next meal. The criterion for meal onset was a 0.3 g decrease in the food cup weight and visual verification of concomitant feeding activity on the monitor. After all rats had consumed the first and the second spontaneous meals, ~3 h after dark onset, the catheters were detached and flushed with 0.3 ml bacteriostatic saline, and the headset was capped. A similar procedure (20) had no detectable effects on spontaneous nocturnal feeding patterns.

Experiments. The effects of hepatic portal vein infusions of lactate on feeding were tested in a total of 32 rats (body wt on experimental days: 324-463 g), using individual crossover tests. A group of 12 rats was used to investigate the effect of 0.5 mmol lactate; these 12 plus an additional 10 rats were used in the test of 1.0 mmol lactate; and 10 of the original 12 rats plus an additional 10 rats were used in the test of 1.5 mmol lactate. On the day before each crossover test, rats received a 0.9% saline (mock) infusion. On the next 2 days, lactate (sodium L-lactate, pH 7.4) or saline (sodium chloride, pH 7.4) was infused in the hepatic portal vein, with the order randomized between rats. Infusion rates of 0.1 or 0.15 ml/min were combined with infusate concentrations of 1 or 2 M to yield doses of 0.5 mmol/rat (0.1 ml/min × 1 M × 5 min), 1.0 mmol/rat (0.1 ml/min × 2 M × 5 min), and 1.5 mmol/rat (0.15 ml/min × 2 M × 5 min). After each crossover test, catheter patency was tested by infusing 0.1 ml of the anesthesia mixture and flushing with 0.3 ml saline. Data from rats that were not anesthetized within 1 min (n = 5) were excluded from analysis, yielding 12, 19, and 18 rats for the 0.5, 1.0, and 1.5 mmol doses, respectively.

Two counterbalanced within-subjects tests, each on four consecutive days and preceded by a single mock infusion, were performed in the 12 rats with catheters in both the hepatic portal vein and the vena cava. One or 1.5 mmol were infused in either the hepatic portal vein or the vena cava, and feeding was recorded. The single mock infusion before each 4-day trial was supposed to be sufficient to lessen possible carryover effects because each rat received hepatic portal vein and vena cava infusions on alternate days and because vena cava infusions should not promote the learning effects observed after hepatic portal vein glucose infusion by Tordoff and Friedman (36). The patency of the hepatic portal vein catheter was tested as described above. The vena cava catheter was presumed to be patent if blood could be aspirated.

Blood sampling. The effects of hepatic portal vein lactate infusions on systemic plasma lactate and glucose levels were investigated in 13 rats (body wt 435-581 g) with only a hepatic portal vein catheter. Rats received infusions of 1.5 mmol lactate (n = 7) or saline (n = 6) beginning 2 min into the first spontaneous nocturnal meal, as described above. One minute after infusion end (i.e., 8 min after meal onset), the rats were anesthetized with ether, and a laparotomy was performed; blood was drawn from the vena cava in 2-ml sodium fluoride tubes (6.1 mg NaF added) and was centrifuged immediately (1,500 g, 15 min, 4°C). The plasma was stored at 20°C for later analysis of lactate and glucose concentrations by standard colorimetric and enzymatic methods adapted for the Cobas Mira autoanalyzer (Hoffman LaRoche; see Refs. 15 and 33).

Data collection and analysis. Meals were defined as weight changes 0.3 g, lasting 1 min, and separated by 15 min from other weight changes, as described previously (19). Meal duration was the time from the first to the last weight change in a single meal, and the intermeal interval (IMI) was the time from the last weight change in a meal to the first change in the next meal. Meals defined and recorded this way accounted for 94% of total daily food intake. Meal size divided by meal duration described the mean feeding rate within meals, and the IMI after a meal (min) divided by the size of that meal (g) described the satiety ratio. In the experiments that tested the effects of hepatic portal vein lactate infusions on meal patterns, only 8 of the 36 rats contributed data for all three lactate doses. Therefore, the data were analyzed in two ways: first, by paired t-tests of each of the three crossover tests, and, second, by a repeated-measures ANOVA of the data from the eight rats that completed all three crossover tests. The comparisons of the effects of hepatic portal and inferior vena cava lactate infusions were also analyzed with repeated-measures ANOVA for each dose separately and across the two doses for the eight rats that completed both trials. Bonferroni's modified t-test was used for post hoc comparisons. Student's t-test was used to detect treatment differences in systemic plasma glucose and lactate concentrations. P values 0.05 were considered significant.

	RESULTS

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Feeding responses to hepatic portal vein infusions. Separate analysis of the three crossover tests (n = 12, 19, and 18) indicated that meal-contingent hepatic portal vein infusions of 1.0 and 1.5 mmol lactate reduced the size of the first spontaneous nocturnal meal by 28% [1.0 mmol: t(18) = 2.45, P < 0.05] and 67% [1.5 mmol: t(17) = 4.19, P < 0.001], respectively (Fig. 1A). Infusion of 0.5 mmol lactate did not affect meal size. Only infusion of 1.5 mmol lactate/meal decreased meal duration (Table 1). None of the lactate doses affected the average feeding rate within the meal (Table 1) or the subsequent IMI (Fig. 1B). As a result of the reduced meal size and unaffected postmeal IMI, hepatic portal infusion of 1.5 mmol lactate increased [t(17) = 2.62, P < 0.05] the satiety ratio of the meal (Fig. 1C). The size of the subsequent (second) nocturnal meal (range: 0.9-7.1 g) increased from 2.4 ± 0.3 (mean ± SE) to 3.6 ± 0.4 g [t(18) = 2.45, P < 0.05] after hepatic portal infusion of 1.0 mmol but was not affected by infusion of 1.5 mmol lactate. None of these meal parameters differed between control infusions and mock (0.9% saline) infusions on the days before a lactate test, and none of the lactate doses affected 2-h cumulative food intake (data not shown). Overall analyses of these meal parameters in the eight rats that contributed data to all three crossover tests also indicated that 1.5 mmol lactate significantly decreased meal size [F(5,35) = 3.26, modified t = 2.71, P < 0.05] and meal duration [F(5,35) = 3.11, modified t = 2.64, P < 0.05] and increased the satiety ratio [F(5,35) = 2.58, modified t = 3.08, P < 0.05; Table 2].

View larger version (24K): [in this window] [in a new window]   Fig. 1.   Effects of prandial hepatic portal infusion of lactate on the size of the first spontaneous nocturnal meal (A), the duration of the subsequent intermeal interval (IMI; B), and the satiety ratio [subsequent IMI (min)/meal size (g); C] in ad libitum-fed rats. Data are means ± SE of 12 (0.5 mmol lactate/meal), 19 (1.0 mmol lactate), and 18 (1.5 mmol lactate) rats used in the individual crossover tests. * P < 0.05 (paired t-test for each dose).

Comparison between hepatic portal vein and vena cava infusions. Meal-contingent hepatic portal vein and vena cava infusion of 1.0 and 1.5 mmol lactate reduced the size of the first nocturnal meal similarly (Figs. 2 and 3). Independent of the infusion route, only the higher dose of lactate also decreased meal duration [repeated-measures ANOVA: F(3,21) = 3.43, P < 0.05]. None of the lactate infusions affected the average feeding rate within meals (data not shown). The subsequent IMI tended to decline after vena cava infusion of both lactate doses (Figs. 2 and 3), but the differences did not reach statistical significance [1.0 mmol: F(3,27) = 2.59, P = 0.07; 1.5 mmol: F(3,21) = 2.84, P = 0.06]. As a result of the reduced meal size and unaltered IMI, hepatic portal vein lactate infusion of 1.5 mmol increased the satiety ratio of the meal by 175 ± 32% [mean ± SE; F(3,21) = 4.45, modified t = 3.31, P < 0.01; Fig. 3]. This increase was much higher [t(7) = 4.08, P < 0.01] than the insignificant [F(3,27) = 1.37, P = 0.27] 43 ± 21% satiety ratio increase after vena cava infusion. After infusion of 1.0 mmol lactate through either route, the size of the second nocturnal meal increased from 2.6 ± 0.3 g (mean ± SE) in both control groups to 3.7 ± 0.4 g (lactate/hepatic portal vein) and 3.5 ± 0.3 g [lactate/vena cava; F(3,27) = 2.90, P = 0.053]. Again, overall analyses of these meal parameters in the eight rats that completed both trials revealed essentially the same effects, i.e., a significant reduction of meal size by both lactate doses independent of the infusion route [F(7,49) = 5.92, P < 0.001], and a significant increase of the satiety ratio by 1.5 mmol lactate infused in the hepatic portal vein [F(7,49) = 3.30, modified t = 3.36, P < 0.01].

View larger version (19K): [in this window] [in a new window]   Fig. 2.   Effects of prandial hepatic portal or vena cava infusion of 1.0 mmol lactate/meal on the size of the first spontaneous nocturnal meal (A), the duration of the subsequent IMI (B), and the satiety ratio [subsequent IMI (min)/meal size (g); C] in ad libitum-fed rats. Data are means ± SE of 10 rats used in a within-subject design. * P < 0.05 [Bonferroni t-tests after significant repeated-measures ANOVA: F(3,27) = 7.25, modified t for hepatic portal vein infusion = 2.59, modified t for vena cava infusion = 3.83].

View larger version (18K): [in this window] [in a new window]   Fig. 3.   Effects of prandial hepatic portal or vena cava infusion of 1.0 mmol lactate/meal on the size of the first spontaneous nocturnal meal (A), the duration of the subsequent IMI (B), and the satiety ratio [subsequent IMI (min)/meal size (g); C] in ad libitum-fed rats. Data are means ± SE of 8 rats used in a within-subject design. * P < 0.01 [Bonferroni t-tests after significant repeated-measures ANOVA: F(3,21) = 8.21, modified t for hepatic portal vein infusion = 3.72, modified t for vena cava infusion = 3.23].

Plasma glucose and lactate in response to hepatic portal vein infusion. One minute after the infusion of 1.5 mmol lactate in the hepatic portal vein, the vena cava plasma lactate concentration was significantly higher than after 1.5 mmol saline infusion [t(11) = 3.49, P < 0.05; Fig. 4]. Lactate infusion did not affect the plasma glucose concentration (Fig. 4).

View larger version (29K): [in this window] [in a new window]   Fig. 4.   Effects of prandial hepatic portal infusion of 1.5 mmol lactate on systemic plasma levels of lactate and glucose; n = 7 and 6 for lactate and control, respectively. * P < 0.05 (Student's t-test).

	DISCUSSION

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The results of this study demonstrate that hepatic portal vein and vena cava infusion of lactate (1.0 and 1.5 mmol) in the rat during the first spontaneous meal of the dark phase can prematurely end the meal. To our knowledge, this is the first evidence for such an acute effect of an intravenously administered metabolite on meal termination (satiation). In addition, hepatic portal vein infusion of 1.5 mmol lactate prolonged postprandial satiety. Because circulating lactate normally increases in response to carbohydrate-containing meals, the findings are consistent with the idea that endogenous lactate plays a role in the physiological control of meal taking.

Lactate has long been known to inhibit feeding after parenteral administration in animals (1, 17, 18, 27), but previous studies did not record meal patterns or address possible acute effects of lactate on feeding. Baile et al. (1) observed a reduction of cumulative food intake in monkeys within the first 30 min after a 30-min intravenous lactate infusion. Extending those findings, the present study revealed an immediate effect of intravenous lactate infusions on the size of the ongoing meal. This effect was more than caloric compensation for the administered lactate because the energy equivalent of the meal size suppression (~20 kJ after 1.5 mmol lactate) exceeded the gross energy of the administered lactate (~2 kJ) by a factor of 10. Moreover, feeding stopped after just 2 min of the 1.5 mmol lactate infusion, when only ~0.6 mmol lactate had been administered. Considering the weaker effect on meal size and the lower infusion rate of the 1 mmol lactate dose [1 mmol (0.1 ml/min) vs. 1.5 mmol (0.15 ml/min)], the rate of lactate delivery in the hepatic portal vein (i.e., portal vein lactate appearance) appears to be more important for meal termination than the administered lactate dose.

The smallest dose of intravenous lactate that reduced meal size under the present test conditions (1.0 mmol/meal) did not significantly affect the subsequent IMI or the 2-h cumulative food intake. Yet, the size of the second nocturnal meal increased, which may reflect a compensation for the decreased size of the first meal. Such an increase in the size of the second meal did not occur after hepatic portal infusion of 1.5 mmol lactate/meal, perhaps because a residual inhibitory effect of the higher lactate dose prevented a compensatory increase in the size of the second meal. As a result of the decrease in meal size and the unaffected IMI after the 1.5 mmol infusion, the satiety ratio of the first nocturnal meal [duration of the subsequent IMI (min)/amount eaten during the meal (g)] increased, indicating that hepatic portal infusion of 1.5 mmol lactate also enhanced the postingestive satiating effect of food, i.e., prolonged postprandial satiety.

Glucose has often been shown to reduce food intake after hepatic portal vein infusion. Russek (31) first demonstrated such a hypophagic effect of glucose in 22-h-fasted dogs. He postulated the existence of hepatic glucosensors involved in food intake control (30). Glucosensors have been shown to exist in the portal-hepatic area (24), and now it is widely accepted that they play a role in the control of food intake (see Ref. 16). In various past studies (36-38), hepatic portal vein glucose administration reduced cumulative food intake when glucose doses between 3 and 12 mmol were infused over 2 h. In contrast, tests of a possible acute effect of glucose on intake are scarce. One recent study addressed the time course of the glucose effect on intake by infusing similar doses of glucose over various times before and during an intraoral glucose solution feeding test (2). The primary finding in this study was a delayed effect of glucose infusion on oral ingestion. In all the studies mentioned, the food intake suppression by hepatic portal vein glucose infusion did not depend on the amount of glucose infused or the infusion rate. Also, glucose did not affect feeding after jugular vein infusion. Therefore, the effect of lactate on feeding reported here differed from the effect of glucose described above, as the effect of lactate on meal size appeared to be immediate, not restricted to the hepatic portal route of infusion, and dependent on the rate of lactate delivery. Also, preliminary studies of ours indicate that glucose doses of 0.5-3.0 mmol, delivered at equivalent infusion rates under similar conditions, do not consistently affect meal size and postprandial satiety. Together these findings suggest that hepatic portal glucose and lactate infusions affect food intake through at least partially separate mechanisms. However, a direct comparison between the effects of various lactate and glucose doses is difficult because the two metabolites have different plasma levels and turnover rates.

In previous studies, the hypophagic effect of lactate after subcutaneous (18) and intraperitoneal (21) injection depended critically on an intact hepatic branch of the vagus. Moreover, the critical lesion in these experiments appeared to be afferent and not efferent because subcutaneously injected lactate still reduced food intake after peripheral atropinization (18). The hepatic branch of the vagus is not the only afferent connection between liver and brain and does not only carry fibers originating in the liver (3). Therefore, differential feeding effects after hepatic portal and jugular vein or vena cava infusions provide more compelling evidence for a hepatic origin than data from hepatic branch vagotomy studies (see Ref. 10). In the present study, lactate's similar effect on meal size after the hepatic portal and the vena cava infusion does not support the hypothesis that lactate acts in the liver to limit meal size. On the other hand, the data also raise a critical question concerning the possibility that lactate reduces food intake by acting at an extrahepatic site. Because the liver presumably removes lactate from the portal vein (see also below), more lactate should reach an extrahepatic site of action and thus reduce meal size more after vena cava than after hepatic portal vein infusion. However, this did not occur, and this discrepancy requires further investigation. Perhaps the amount of lactate reaching the liver after vena cava infusion was sufficient to inhibit feeding, or lactate can reduce meal size through both a hepatic and an extrahepatic mechanism. Although lactate reduced meal size through both routes of infusion similarly, the 1.5 mmol dose increased the satiety ratio of the meal and hence prolonged postprandial satiety more after hepatic portal vein than after vena cava infusion, for which the increase was not significant. This suggests that the effect of lactate on postprandial satiety is due to a hepatic action. Thus lactate may terminate a meal and enhance postprandial satiety by acting at different sites and perhaps through different mechanisms. Interestingly, results of hepatic portal vein glucose infusion indicate that a glucose-related hepatic metabolic signal controls postprandial satiety rather than meal termination (2).

Lactate (1.5 mmol) infused in the hepatic portal vein did not appear to increase the systemic plasma lactate concentration above the level usually seen after a carbohydrate-containing meal in the rat (15, 34) or in humans (8, 33). This corresponds with previous findings of a moderate increase in circulating lactate after intravenous L-lactate infusion in monkeys (1). Yet, in the present study, the infusion-induced increase in portal vein lactate concentration and appearance rate were probably higher than normal. Wet liver weight in the rat is ~3.5% of body weight (23), and total liver blood flow is around 2 ml · min¹ · g liver¹ (28), with ~75% of that provided through the portal vein (11). Thus, after a meal, when splanchnic blood flow usually increases (12), hepatic portal vein blood flow in a 350-g rat can presumably reach ~20 ml/min (2 ml × 12.25 g liver × 0.75 = 18.4 ml + x ml, due to meal-induced increase). Consequently, the 0.3 mmol lactate/min administered in this study should yield portal vein blood concentrations of ~15 mmol/l lactate (1,000 ml: 20 ml × 0.3 mmol/l). This estimate corresponds very well with the systemic plasma lactate concentration measured at the end of a 5-min, 1.5 mmol infusion of lactate in the jugular vein of rats without concomitant access to food; in this situation, plasma lactate rose from a baseline of 0.9 ± 0.1 (mean ± SE, n = 6) to 16.5 ± 0.7 mmol/l at infusion end and was still at 5.2 ± 0.6 mmol/l 5 min thereafter. Therefore, the comparatively low systemic lactate level found 1 min after the end of hepatic portal vein infusion was presumably, at least in part, due to hepatic lactate removal from the portal blood. In any case, in response to a meal, the rate of portal vein lactate appearance and the maximum lactate levels in the portal vein and in the vena cava are lower (14, 15, 23) than after lactate infusion. Because the meal-induced increase in circulating lactate is certainly not the only feedback signal that controls food intake, supraphysiological increases in lactate appearance and/or level are presumably necessary to affect meal termination and/or postprandial satiety.

Lactate did not affect the average feeding rate within the meal, and no signs of illness in response to the 0.5-1.5 mmol/meal lactate infusions were noticed by frequent observation. The rats merely stopped feeding earlier. The observed reduction in meal size by lactate was unlikely to be an osmotic effect because the control infusion consisted of equiosmotic saline in all experiments. Higher doses of lactate (2 mmol/rat) reduced meal size even more; yet, the data are not reported here because these lactate doses and the corresponding saline control infusions occasionally triggered some unusual behavior such as intensive grooming. Again, however, no signs of illness were observed. All in all, it appears unlikely that the effect of lactate on feeding is due to malaise or some other nonspecific effect of the infusion. To absolutely exclude this possibility, however, it should be addressed directly in further studies.

Control meal size appeared to be smaller in the trials in which the higher doses were administered. Because the dose was increased with each test, it is possible that learning somehow influenced meal sizes (36). In addition, a high osmotic load with the high dose might decrease meal size. In any case, however, the phenomenon did not significantly influence the feeding-suppressive effect of lactate, and also control meal size did not differ significantly across doses in any of the trials [hepatic portal vein infusion: F(5,35) = 3.26, modified t = 0.10, P > 0.05; comparison between hepatic portal vein and vena cava infusion: F(7,49) = 5.92, modified t = 1.17 and 2.18 for hepatic portal vein and vena cava infusion, respectively, P > 0.05]. The results do not allow determination of whether lactate acted as a signal or whether it inhibited feeding through its metabolic actions. Yet, the failure of lactate infusion to increase the systemic blood glucose level corresponds with previous observations in monkeys (1) and indicates that the reduction of food intake by lactate was not based on an increase in circulating glucose subsequent to conversion of the infused lactate to glucose. This is consistent with previous findings suggesting that oxidation of lactate beyond pyruvate, rather than stimulation of gluconeogenesis, contributes to the inhibition of feeding (17). Yet, a stimulation of liver glycogen synthesis by lactate may also play a role, at least for the enhancement of postprandial satiety. In the perfused rat liver, addition of lactate to the perfusion medium increased hepatic insulin clearance (26) and greatly enhanced, and in fact determined, the rate of glycogen synthesis from glucose (40). Interestingly, an increase in liver glycogen was the only metabolic measure related to changes in food intake after hepatic portal vein glucose infusions in another study (38). It is presently unknown, however, how changes in liver glycogen could affect feeding. Other possible mechanisms for the effect of lactate on food intake include a hyperpolarization of hepatocyte membranes, which has also been shown to occur in response to lactate addition to the perfusate in the perfused rat liver preparation (4, 29), and a decrease in the discharge rate of hepatic branch vagal afferents. The latter effect could be subsequent to or independent of an effect on hepatocyte membranes. A decrease in hepatic vagal afferent activity has been shown in response to hepatic portal infusion of pyruvate (25), which forms easily from lactate.

Lactate is used by astrocytes and neurons as a source of energy, and it can replace glucose as a fuel for the brain under certain conditions (e.g., see Ref. 39). As lactate is rapidly taken up by neurons through a saturable transport system (7), a central mechanism may contribute to the acute meal size effect of intravenously infused lactate. This interesting possibility merits further investigations. To the best of our knowledge, the effect of intracerebroventricular lactate infusion on food intake has not yet been investigated, and intracerebroventricular administrations of glucose and other metabolites that inhibited eating were usually sustained for several hours (5).

In conclusion, intravenous infusion of lactate acutely reduced spontaneous nocturnal meal size in the rat. When infused in the hepatic portal vein, lactate also prolonged postprandial satiety. These results are consistent with a physiological role of endogenous lactate in the control of meal taking and suggest that the liver is crucial for the effect of lactate on postprandial satiety. The exact mechanisms of these inhibitory effects of lactate on feeding and the site where lactate acts to terminate meals remain to be identified.

Perspectives