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Am J Physiol Gastrointest Liver Physiol 294: G655-G659, 2008. First published January 3, 2008; doi:10.1152/ajpgi.00484.2007
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NEUROREGULATION AND MOTILITY

Fixed feeding potentiates interdigestive gastric motor activity in rats: importance of eating habits for maintaining interdigestive MMC

Hajime Ariga, Kenji Imai, Cindy Chen, Christopher Mantyh, Theodore N. Pappas, and Toku Takahashi

Department of Surgery, Duke University Medical Center, Durham, North Carolina

Submitted 20 October 2007 ; accepted in final form 1 January 2008


   ABSTRACT
TOP
 ABSTRACT
MATERIAL AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Endogenous ghrelin regulates the occurrence of interdigestive gastric phase III-like contractions in rats. However, the fasted motor pattern is not as regular and potent in humans and dogs. We hypothesize that eating habits play an important role in maintaining a regular interdigestive gastric contractions. We studied the effect of fixed-feeding regimen on interdigestive gastric contractions and plasma acyl ghrelin levels. The fixed-fed rats were trained to the assigned meal feeding regimen, once daily at 12:00 PM to 4:00 PM for 14 days. Free-fed rats were maintained with free access to food. As ghrelin regulates gastric emptying as well, solid gastric emptying was also studied in fixed-fed rats and free-fed rats. In free-fed rats, two of six rats did not show interdigestive gastric phase III-like contractions. In contrast, phase III-like contractions were observed in all rats 14 days after starting the fixed-feeding regimen. The maximal amplitude of phase III-like contractions significantly increased from 8.4 ± 0.6 to 16.3 ± 1.8 g (n = 6, P < 0.05) 14 days after the start of the fixed feeding. Fasted and postprandial plasma ghrelin levels were significantly increased after 14 days of fixed feeding. Solid gastric emptying was significantly accelerated in fixed-fed rats (72.1 ± 4.2%) compared with that of free-fed rats (58.7 ± 2.7%, n = 6, P < 0.05). Our present findings suggest that fixed feeding increases plasma ghrelin levels, potent interdigestive contractions, and acceleration of gastric emptying.

migrating motor complex; ghrelin; scheduled feeding

DURING THE INTERDIGESTIVE state, the stomach and small intestine show a remarkable motor pattern, known as the migrating motor complex (MMC) in dogs (20) and humans (22). MMC consists of three phases: phase I (period of motor quiescence), phase II (period of irregular low-amplitude contractions), and phase III (period of regular high-amplitude contractions). The pulsatile flow immediately ahead of phase III of the MMC implied physiological importance for clearance of secretions, debris, and microbes during fasting and let the stomach be ready to receive a next meal (22).

The characteristic feature of gastric MMC is different among species. In humans and dogs, MMC is usually observed every 90–120 min in the interdigestive state. In contrast, in rats, the MMC cycle is short (less than 20 min) and not as regular as that of humans and dogs (2, 7, 21). It is rather difficult to distinguish three phases, especially phase III(-like) contractions. Furthermore, sometimes phase III-like contractions are not clearly observed even after a 24-h fasting in rats.

These may be explained by the difference of the peptides that regulate MMC. In dogs and humans, gastric phase III is highly associated with the peak of plasma motilin levels. Intravenous administration of motilin causes gastric phase III in humans and dogs (13). In rats, on the other hand, the peak of plasma ghrelin level is correlated with gastric phase III-like contractions (2). Phase III-like contractions are induced by the intravenous administration of ghrelin in rats (2, 7). Motilin does not cause any gastric contractions in rats (5), whereas ghrelin does not induce any gastric phase III in dogs (17). Therefore, the differences of interdigestive motor pattern between rats and dogs can be explained, at least in part, by the difference of the responsible peptides which regulate gastric phase III.

Another possible factor that affects the character of MMC is eating habit. Humans and dogs are placed on a fixed or scheduled regimen of meals; humans and dogs usually have their meals three times and once a day, respectively. In contrast, rats usually eat all through the day and do not have the daily rhythm of feeding behavior. This may disrupt interdigestive gastric motor pattern once they are fasted.

Fixed-feeding regimen, in which food is available only restricted hours, has been established in rats (14, 18, 19). Recent studies demonstrated that fixed-fed rats show higher plasma ghrelin levels than do free-fed rats in the fasted state (3, 6).

We hypothesize that fixed feeding stimulates more ghrelin release than does free feeding during the fasted state and that elevated plasma ghrelin levels may induce more regular and potent gastric phase III-like contractions. In addition to stimulating interdigestive gastric motor activity, ghrelin is also showed to accelerate gastric emptying (1, 8). Therefore, we further hypothesize that elevated plasma ghrelin levels may accelerate gastric emptying in fixed-fed rats.

On the basis of our hypothesis, we studied 1) whether a fixed-feeding regimen affects nutrient status, especially daily food intake and body weight (BW); 2) whether fasted plasma ghrelin level and gastric motor pattern are affected after fixed-feeding regimen; and 3) whether solid gastric emptying is altered after fixed-feeding regimen in rats.


   MATERIAL AND METHODS
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 ABSTRACT
 MATERIAL AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Animals. Male Sprague-Dawley rats weighing 240–260 g were housed in group cages under conditions of controlled temperature (22–24°C) and illumination (12-h light cycle starting at 7:00 AM). Before the experiment, rats were fasted for 24 h but given free access to water. Protocols describing the use of rats were approved by the Institutional Animal Care and Use Committee of Duke University Medical Center and were in accordance with the National Institutes of Health "Guide for the Care and Use of Laboratory Animals."

Surgery. Six days before the experiments, rats were fasted for overnight and anesthetized with intraperitoneal injection of pentobarbital (45 mg/kg). Through the midline incision, a strain-gauge transducer was implanted onto the serosal surface of the antrum for recording the circular muscle contractions, as previously described (12, 16). An intravenous catheter filled with heparinized saline was inserted into right jugular vein. All wires and a catheter were tunneled subcutaneously to exit at the back of the rat's neck and protected by specialized jacket (Star Medical, Tokyo, Japan).

Feeding schedule and experimental protocol. Rats were fasted for 24 h before the experiments. At day 1, gastric contraction was monitored and blood was taken before and after feeding for 7–8 h.

Rats received food at 12:00 PM and were divided into two age-matched groups: the rats of the fixed-fed group were deprived of food at 4:00 PM, whereas the rats of free-fed group were still allowed free access to rat chow. The fixed-fed rats were trained to the assigned feeding regimen, once daily at 12:00–4:00 PM for 14 days. Free-fed rats were maintained with free access to food for the whole day. Food intake and BW were monitored daily in both groups.

At day 14, gastric contractions were monitored and blood was taken, after a 24-h fasting in a same manner as that of day 1.

Monitoring of gastric contractions. After a 24-h fasting, gastric contraction was monitored from 8:00 to 9:00 AM at days 1 and 14. The wires from a transducer were connected to a recording system. During a fasted period, the frequency and the maximal amplitude of phase III-like contraction were evaluated.

Phase III-like contractions were defined as clustered potent contractions with amplitude of more than 4 g, as previously reported (2, 21). Gastric contraction was still monitored for several hours after the feeding.

Measuring plasma ghrelin concentration. Since the character of interdigestive gastric contractions was altered from day 1 to day 14 in fixed-fed rats, we investigated plasma ghrelin levels at days 1 and 14 in fixed-fed rats. An implanted jugular vein catheter, filled with heparinized saline, was used for blood collection. At days 1 and 14, blood samples (0.2 ml) were taken at 11:00 AM, 11:15 AM, 11:30 AM, 11:45 AM, 12:00 PM, 12:15 PM, 12:30 PM, 1:00 PM, and 2:00 PM. The blood was collected in tubes containing EDTA and aprotinin (500 kIU/ml) and immediately centrifuged at 4°C. As previously reported (2), plasma was aliquoted and added 1.0 N HCl (10% of sample volume) then stored at –80°C until assayed.

Plasma level of acyl ghrelin was measured by radioimmunoassay (RIA) using a RIA kit (LINCO Research, St. Charles, MO), as previously reported (2). This kit is designed to measure active form of ghrelin (acyl ghrelin) and does not have cross reactivity to des-acyl ghrelin. Intra- and interassay coefficient variances were less than 10% (2).

Measuring gastric emptying. Rats were maintained under free-fed or fixed-fed conditions for 14 days. After a 24-h fasting, rats were given 1.5 g of solid meal at 12:00 PM, as previously described (12). Ninety minutes after feeding, rats were euthanized by the intraperitoneal injection of pentobarbital sodium (200 mg/kg). The stomach was surgically isolated and removed. The gastric content was recovered from the stomach, dried, and weighed. Solid gastric emptying was calculated according to the following formula, as previously described (12): Gastric emptying (%) = [1 – (dried weight of food recovered from stomach/weight of food intake)] x 100.

Statistical analysis. All results were expressed as means ± SE. Statistical analysis was performed by analysis of variance, Student's t-test, paired t-test, or Mann-Whitney U-test. P values <0.05 were considered significant.


   RESULTS
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DISCUSSION
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Daily food intake and BW in free-fed and fixed-fed rats. As shown in Fig. 1A, daily food consumption of free-fed rats at day 1 and day 14 was 20.3 ± 1.0 g (n = 6) and 23 ± 0.5 g (n = 6), respectively. In contrast, fixed-fed rats could eat food only 8.9 ± 1.0 g at day 1 (Fig. 1A). Because fixed-fed rats could not eat enough food in the first several days, they lost BW at days 1 and 2 (Fig. 1B). From day 3, fixed-fed rats started to gain BW (Fig. 1A).


Figure 1
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  		Fig. 1. Daily food intake (A) and body weight (BW; B) in free- and fixed-fed rats. In free-fed rats, daily food consumption was 20–23 g over the experimental periods. In contrast, fixed-fed rats ate only 8.9 ± 1.0 and 11.1 ± 1.2 g of food at day 1 and day 2, respectively, which was less than half of daily food intake in free-fed rats. However, the amount of daily food intake was gradually increased in fixed-fed rats. From day 0 to day 1, both free- and fixed-fed rats were fasted for 24 h, which decreased their BW on day 1. From day 2, free-fed rats gained BW constantly and finally reached 324.2 ± 2.4 g, at day 14. In contrast, fixed-fed rats still reduced their BW at day 2. However, because of the increment of daily food intake, they began to increase their BW from day 3 and reached 309.4 ± 4.6 g of BW at day 14 (n = 6, *P < 0.05 and **P < 0.01, vs. fixed-fed rats).

 
Daily food intake increased day by day in fixed-fed rats, especially during the first week. Daily food intake was 18.2 ± 0.9 g at day 7 in fixed-fed rats. The BW of free-fed and fixed-fed rats at day 14 reached 324.2 ± 2.4 and 309.4 ± 4.6 g, respectively (Fig. 1B). The BW of fixed-fed rats was significantly lower than that of free-fed rats, whereas the daily increments did not show any significant differences between the two groups except on day 2.

Fasted gastric contractions before and after fixed feeding. In free-fed group, two of six rats did not show any distinctive phase III-like contractions at day 1. The pattern of phase III-like contractions was not significantly affected at day 14 in free-fed rats (data not shown).

In the fixed-fed group, two of six rats did not show any distinctive phase III-like contractions at day 1 (rats C and E; Fig. 2). However, at day 14, all of six rats showed potent phase III-like contractions.


Figure 2
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  		Fig. 2. Change of interdigestive gastric contraction in 6 fixed-fed rats at days 1 and 14. Asterisks and arrows indicate phase III-like contractions and feeding time (12:00), respectively. In the fixed-fed group, 2 of 6 rats did not show any distinctive phase III-like contractions at day 1 (rats C and E). However, at day 14, all 6 rats showed potent phase III-like contractions. Two types of interdigestive gastric motor pattern were observed 14 days after fixed feeding: potent phase III-like contractions observed just before the meal (rats A, C, and F) and regular phase III-like contractions throughout the recording period (rats B, D, and E).

 
Two types of interdigestive gastric motor pattern were observed 14 days after fixed feeding: 1) potent phase III-like contraction observed just before the meal (rats A, C, and F), and 2) constant regular phase III-like contractions throughout the recording period (rats B, D, and E) (Fig. 2).

At day 1, the frequency of phase III-like contractions did not differ between fixed-fed rats (3.8 ± 1.2 times/2 h, n = 6) and free-fed rats (4.2 ± 1.4 times/2 h, n = 6) (Fig. 3A). At day 14, the frequency of phase III-like contractions of fixed-fed rats (6.1 ± 1.0 times/2 h, n = 6) was slightly higher than that of free-fed rats (3.9 ± 1.2 times/2 h, n = 6) (Fig. 3A).


Figure 3
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  		Fig. 3. Frequency (A) and maximal amplitude (B) of phase III-like contractions at days 1 and 14 in free- and fixed-fed rats. In free-fed rats, frequency of phase III-like contractions were not different between day 1 and day 14 (4.2 ± 1.4 and 3.9 ± 1.2 times/2 h, respectively). In contrast, frequency of phase III-like contraction in fixed-fed rats tended to be higher at day 14 (6.1 ± 1.0 times/2 h) than day 1 (3.8 ± 1.2 times/2 h). Similarly, the maximal amplitude of phase III-like contraction in free-fed rats was not different between day 1 and day 14. However, in fixed-fed rats, the maximal amplitude at day 14 (16.3 ± 1.8 g) was significantly higher than that of day 1 in fixed-fed rats (8.4 ± 0.6 g, **P < 0.01) and that of day 14 of free-fed rats (8.6 ± 1.2 g, ##P < 0.01) (n = 6).

 
The maximal amplitude of phase III-like contractions was not significantly different between day 1 (7.4 ± 0.5 g, n = 6) and day 14 (8.6 ± 1.2 g at day) in free-fed rats (Fig. 3B).

In contrast, the maximal amplitude of phase III-like contractions was significantly increased from 8.4 ± 0.6 g (n = 6) at day 1 to 16.3 ± 1.8 g (n = 6, P < 0.01) at day 14 in fixed-fed rats (Fig. 3B).

Plasma ghrelin level before and after fixed feeding. We measured plasma ghrelin level in fixed-fed rats at day 1 and day 14. As previously reported (2), the plasma ghrelin concentration fluctuated in individuals during the fasted state at day 1 and day 14 in fixed-fed rats. However, the difference between the peak and the bottom of plasma ghrelin concentration was not detected when the average was calculated.

In fixed-fed rats, the plasma ghrelin levels at day 14 reached its peak 30 min before feeding (at 11:30 AM; 281.6 ± 28.5 pg/ml, n = 6), which were significantly higher than that of day 1 (at 11:30 AM; 143.1 ± 23.1 pg/ml, n = 6, P < 0.01, Fig. 4). At day 14, plasma ghrelin level was decreased from 275.4 ± 16.2 pg/ml (at 11:45 AM) to 192.9 ± 17.9 pg/ml just before feeding time in fixed-fed rats (at 12:00 PM).


Figure 4
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  		Fig. 4. Plasma acyl ghrelin levels before and after feeding in fixed-fed rats at days 1 and 14. The plasma ghrelin levels at day 14 reached their peak 30 min before feeding (281.6 ± 28.5 pg/ml, n = 6) and were significantly higher than those of day 1 (143.1 ± 23.1 pg/ml, n = 6, P < 0.01). At day 14, plasma ghrelin level was decreased from 275.4 ± 16.2 pg/ml (at 11:45 AM) to 192.9 ± 17.9 pg/ml just before feeding time in fixed-fed rats (at 12:00 PM). Plasma ghrelin level decreased after feeding at day 14 as well as day 1. However, the postprandial plasma ghrelin levels at day 14 were significantly higher than those of day 1. Two hours after start of feeding (at 2:00 PM), the plasma ghrelin level decreased to 48.1 ± 10.8 pg/ml at day 1 and 69.0 ± 17.7 pg/ml at day 14 in fixed-fed rats (n = 6, *P < 0.05, **P < 0.01, compared with day 1).

 
Although plasma ghrelin level decreased after feeding at day 14 as well as day 1, the postprandial plasma ghrelin levels at day 14 stayed at the same levels of fasted plasma ghrelin levels of day 1 for 1 h after starting feeding (Fig. 4). Two hours after staring feeding (at 2:00 PM), the plasma ghrelin level decreased to 48.1 ± 10.8 pg/ml (n = 6) at day 1 and 69.0 ± 17.7 pg/ml (n = 6) at day 14 in fixed-fed rats (Fig. 4).

Fasted and postprandial plasma ghrelin levels in normal-fed rats were almost the same as those of fixed-fed rats at day 1.

Gastric emptying in ad libitum- and fixed-fed rats. Fourteen days after fixed feeding, solid gastric emptying rate was significantly accelerated (72.1 ± 4.2%), compared with that of free-fed rats (58.7 ± 2.7%, n = 6, P < 0.05) (Fig. 5).


Figure 5
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  		Fig. 5. Solid gastric emptying rate in free- and fixed-fed rats. In free-fed rats, gastric emptying was 58.7 ± 2.7% (n = 6). In contrast, fixed-fed rats showed accelerated solid gastric emptying (72.1 ± 4.2%, n = 6, *P < 0.05).

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIAL AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
When rats were allowed to eat only 2 h a day, their BW was reduced from 221.8 to 206.1 g (11). When rats receive food only 2 h a day, the amount of their food intake was less than that of ad libitum-fed rats (10). In these rats, high peak of plasma corticosterone level was observed in a fasting period. However, the high peak of plasma corticosterone level was no more observed when rats were allowed to eat 6 h a day (10).

In our experiment (4 h feeding per day), BW was decreased because they could not eat enough as free-fed group during first 2 days. However, from day 3, they could gain their BW and daily growth of their BW is almost same as that of free-fed rats. This suggests that fixed-fed rats in our experiment could consume their essential daily food within 4 h and our fixed-feeding regimen did not cause any severe nutritional problems.

Our present study demonstrated that a fixed-feeding regimen could facilitate the occurrence of regular and potent phase III-like contractions in conscious rats. The rats received their food all day long before the fixed-feeding instruction. Two of six rats did not show the distinctive phase III-like contractions at day 1, even though they were fasted for 24 h. However, these two rats showed obvious phase III-like contractions at day 14 and the other four rats had more regular and potent phase III-like contractions at day 14 than at day 1.

Utilizing the fixed-feeding regimen, we characterized the pattern of plasma ghrelin changes before and after feeding. Fourteen day after the start of the fixed feeding (at day 14), the fasting plasma ghrelin levels were significantly increased compared with those before the start of the fixed feeding (at day 1).

We have previously shown that ghrelin receptor antagonists significantly attenuated spontaneous phase III-like contractions of the rat stomach (2). This suggests that endogenous ghrelin plays an important role to mediate spontaneous phase III-like contractions in rats. It is conceivable that increased plasma ghrelin levels of the fasting state may induce potent interdigestive contractions in fixed-fed rats.

Interestingly, preprandial ghrelin level was decreased before the feeding time (at 12:00 AM), which was compatible with the previous data (6). The authors (6) showed that plasma total ghrelin level reached a peak of 2,192 ± 218 pg/ml half an hour before the time food is anticipated.

These findings indicate that plasma ghrelin levels in a fasted state are mediated by two factors. One possible factor is that lack of nutrients may stimulate ghrelin secretion. Ingested nutrients may exert their inhibitory effects on ghrelin secretion, locally or systemically (9). Another feasible factor is anticipation of feeding at a predictable time. Especially, the transient surge in ghrelin secretion occurs just before expected time for food (Fig. 4). This may be explained by a conditioned psychophysiological response to stimulate ghrelin release.

In a fed state, ghrelin administration can increase solid gastric emptying in humans (15), rats (8), and mice (4). Postprandial antropyloric coordination plays an important role in mediating solid gastric emptying. We have recently showed that ghrelin administration enhances antropyloric coordination and accelerates gastric emptying in rats (1).

In our present study, postprandial acyl ghrelin levels at day 14 were significantly higher than those of day 1. Solid gastric emptying rate in fixed-fed rats was significantly accelerated compared with that of free-fed rats. Elevated postprandial elevated ghrelin level in fixed-fed rats may be responsible for the accelerated gastric emptying in fixed-fed rats.

The physiological importance of gastric MMC is a mechanical and chemical cleansing of the empty stomach in preparation for the next meal. When gastric phase III is impaired, the gastric content may stay for a longer period. Impaired gastric phase III may cause retention of the gastric contents, bacterial overgrowth, and delayed gastric emptying, resulting in various symptoms (22). The impaired and/or irregular gastric MMC may aggravate dyspeptic symptoms following food ingestion. The dyspeptic symptoms in the postprandial state may be reduced when impaired gastric MMC in the interdigestive state is improved.

This is a novel demonstration that directly revealed the influence of eating habit on the interdigestive gastric motor function in rats. It still remains unknown whether irregular eating habits may impair gastric MMC and induce postprandial symptoms in humans. Rigorous studies are needed to investigate the correlation between eating habit and gastric MMC in humans.


   FOOTNOTES
 

Address for reprint requests and other correspondence: T. Takahashi, Zablocki VA Medical Center, 5000 W. National Ave., Milwaukee, WI 53295 (e-mail: ttakahashi{at}mcw.edu)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
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 ABSTRACT
 MATERIAL AND METHODS
 RESULTS
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