Influence of Exogenous Avian Pancreatic Polypeptide on Gastrointestinal Motility in Turkeys

Influence of Exogenous Avian Pancreatic Polypeptide on Gastrointestinal Motility in Turkeys G. E. DUKE 1 , J. R. KIMMEL2 , P. T. REDIG1 , and H. G. POLLOCK2 1

Department of Veterinary Biology, University of Minnesota, St. Paul, Minnesota 55108 and

^Department of Biochemistry, Kansas University Medical Center, Kansas City, Kansas 66103 (Received for publication July

6,1978)

ABSTRACT To determine the influence of avian pancreatic polypeptide (APP) on avian GI motility, strain-gauge transducers were implanted on the glandular stomach, thick caudodorsal and thin caudoventral muscles of the muscular stomach, and on the duodenum (cranial tract) of five young turkeys. Implants were also made on the ileum, cecum, and colon (caudal tract) of three other turkeys. Isovolumic injections of APP at six (cranial tract preparations) or four (caudal tract preparations) levels were made via a chronic jugular catheter while recording GI contractile activity in fasted birds. Injections of 2 or 5 Mg/kg caused no statistically significant change in motility of the cranial tract. Significant depression in contraction frequency during the first 10 min post-injection resulted from an injection of 8 Mg/kg. Injections of 10, 20, and 30 Mg/kg depressed motility throughout the entire 30 min post-injection period. Motility of the caudal tract usually was not significantly affected by injections of 5 and 10 Mg/kg doses. Larger doses (20 and 30 Mg/kg) significantly depressed caudal tract motility during the first 10 min post-injection but not throughout the 30 min post-injection period. In both cranial and caudal portions of the tract, depression of contractile activity by injections of APP persisted longer following larger doses. The highest plasma APP levels in turkeys, found at about 1 hr post-prandially, were still less than plasma levels following IV injection of 5 Mg/kg. Since the latter injection caused no apparent alteration in GI motility, APP may have little or no physiological role in regulation of avian GI motility.

INTRODUCTION Regulation of gastrointestinal (GI) motility (Duke et al, 1 9 7 5 b ; Lai and D u k e , 1 9 7 8 ) , particularly of gastric motility (e.g. D u k e and Evanson, 1 9 7 2 ; D u k e et al, 1976a., 1 9 7 6 b , 1 9 7 7 , 1 9 7 5 a ; Duke and R h o a d e s , 1977) of turkeys and of several o t h e r avian species has recently been studied. However, previous studies were primarily concerned with regulation b y n o n - h u m o r a l m e a n s and a p p a r e n t l y almost n o t h i n g is k n o w n a b o u t regulation of motility by GI h o r m o n e s in birds ( D u k e , 1 9 7 7 ) . Avian pancreatic p o l y p e p t i d e (APP), recently proposed t o be a GI h o r m o n e (Kimmel et al., 1 9 6 8 , 1 9 7 1 ; Larsson et al, 1 9 7 4 ) , has been shown t o be stimulatory t o gastric secretion (Kimmel et al, 1 9 7 1 ; Hazelwood et al, 1973) in chickens b u t has n o t y e t been studied with regard t o its influence o n avian GI m o t i l i t y . Analogs of APP in m a m m a l i a n species, e.g. Bo-

3 Turkey Formula 22, Land-O-Lakes Creameries, Inc., Minneapolis, MN.

1979 Poultry Sci 58:239-246

vine PP (BPP) (Lin and Chance, 1 9 7 2 ; Lin et al, 1 9 7 7 ) , are a p p a r e n t l y involved in regulation of GI motility as well as in regulation of secretion. T h e objective of this s t u d y was t o d e t e r m i n e t h e influence of APP from chickens o n GI m o tility in t u r k e y s .

METHODS Eight Wrolstad Medium-White t u r k e y s , 6 t o 12 weeks old and weighing 1.3 t o 2.8 kg were used. T h e y were housed in individual cages in animal r o o m s in which t e m p e r a t u r e , h u m i d i t y , and p h o t o p e r i o d were automatically controlled. A mash d i e t 3 was fed ad libitum e x c e p t for a 12 hr fasting period prior t o recording experim e n t s and 12 or 2 4 hr prior t o b l o o d sampling e x p e r i m e n t s (see b e y o n d ) . E x p e r i m e n t s were performed in a l a b o r a t o r y adjacent t o t h e animal holding r o o m . Extraluminal strain gauge transducers (SGT) were used t o m o n i t o r GI m o t i l i t y ; p r e p a r a t i o n and use of these devices in avian species have been previously described ( D u k e et al, 1976a-,

239

240

DUKE ET AL.

Duke and Kostuck, 1975). SGT were surgically implanted on the glandular stomach, thick caudodorsal and thin caudoventral muscles of the muscular stomach (Duke, 1977), and on the mid-proximal duodenum (cranial tract) of five -8 to 10-week-old turkeys. They were also implanted on the mid-ileum, distal portion of the left cecum, mid-colon (caudal tract) and thick caudodorsal muscle of three -10 to 12-week-old turkeys. Leads from the SGT were passed subcutaneously from the abdomen to the mid-back, where they were attached to 12 pins of a 25 pin connector which was sutured to the skin on the back. The right jugular vein was permanently cannulated with polyethylene tubing (1.77 mm id, 2.80 mm od) to permit injection of APP solutions during recording. Surgical procedures were accomplished under sodium pentobarbital induced anesthesia (approximately 40 mg/kg). Turkeys were given a recovery period of 5 to 7 days following surgery before being used in recording experiments. Information on contractile forces detected by the implanted SGT was recorded on an eight-channel recorder using four carrier amplifiers4 . During recording experiments, turkeys were connected to the recorder via the plugs on their back and wrapped loosely in a towel for restraint. After 35 min of recording, an APP solution (see beyond) was injected into the jugular vein. After another 30 min of recording, or upon recovery from the APP injection if more than 30 min was required, an equivalent volume of physiological saline was injected as a control injection. Recording was terminated 15 min after the saline injection and the turkey was returned to its cage. The saline injections produced no significant responses in this study. Aliquots of purified lyophilized APP (Kimmel et ah, 1975) (containing .027% insulin and .35% glucagon by weight as determined by radioimmunoassay) were prepared at a concentration of 30 Mg/ml in physiological saline. Isovolumic doses (1 ml/kg) of 2, 5, and 8 £ig/kg (six experiments at each dose) and of 10, 20, and 30 jug/kg (nine experiments each dose) were injected as rapidly as possible into the five turkeys with SGT implanted in the cranial tract. Doses of 5, 10, 20, and 30 Mg/kg (four, four, five, and six experiments, respectively)

4 Hewlett-Packard, Models 7788A and 8805B, respecitvely; Waltham, MA.

were injected into the three birds with the caudal tract implants. In order to determine basal plasma concentrations of APP and concentrations following our APP injections, turkeys were fasted for 12 hr, then approximately 2 ml of blood was collected (iv) in heparinized syringes. Ten minutes after blood sampling 30, 10, or 5 Mg/kg of APP were injected (iv), and after a 2 min wait another 2 ml of blood was taken. A similar procedure was used for injection of a saline control to determine if the injection itself may influence plasma APP concentrations. To determine whether handling and restraint may alter plasma APP levels, two blood samples were taken at 12 or 20 min after the initial sample with no intervening injection. Plasma APP levels increase post-prandially in chickens reaching a peak response at about 1 hr post-prandially (Kimmel and Pollock, 1975). Therefore, blood samples were also taken at 30 min and 1 hr after refeeding birds fasted 12 or 24 hr to ascertain whether the influence of feeding on release of APP is similar in turkeys and chickens. All samples were taken during a 1 to 1.5 hr period immediately after dawn on successive days to minimize possible affects of a diurnal cycle of plasma APP levels. Plasma (.2 ml) was added to 1 ml of phosphate buffer (Hales and Randle, 1963) and analyzed for APP content (Langslow etah, 1973). lyzed to determine the frequency and amplitude of all contractions for a 30 min period prior to the APP injection and for three successive 10 min periods after injection for each bird in each experiment. A mean contraction frequency was determined for each dose level, each implant site, and for each of the above time periods, plus an overall mean for the 30 min post-injection period (Tables 1 and 3). Then, using paired comparison's t-tests, differences between the mean frequencies for each of the three 10 min post-injection periods and for the total 30 min post-injection period were compared to the pre-injection mean for each implant site at each dose level. Mean contractile amplitudes for all birds in each treatment for pre- and post-injection periods were not calculated because of significant variability in mean amplitudes between birds. This variability apparently is mainly due to differences in the exact location and orientation of implanted SGT between birds as well as to the quality of the signal received from each

AVIAN PANCREATIC POLYPEPTIDE IN TURKEYS

of the implants. When contractile amplitudes were expressed as percent change in mean amplitude from one time period to the next it was evident that amplitude tended to decrease following injection of the higher doses of APP, but decreases were generally less than decreases in contractile frequency following APP injection, and statistical variability was still great (e.g., standard deviations were nearly always higher than means). For these reasons, amplitude data are not included. Two types of colonic contractions occur in a ratio of about 1:5. These have been called "large" and "small" waves (Lai and Duke, 1978), respectively, and large waves were used in determination of frequency and amplitude of contractions herein. Periodically a major concentraction wave occurs in the ceca (Duke, unpublished observation), but since these did not appear to be affected by APP injections they were not included in analysis. Smaller, more regular cecal contractions, large colonic contractions, and ileal contractions usually appeared to be coordinated with each other and with gastric contractions. The influence of APP on this coordination was assessed during analysis of data. We decided that coordination between the stomach and caudal tract existed during a given time period pre- or post-injection in a given experiment if mean contraction frequencies for the thick caudodorsal muscle of the muscular stomach and ileal, cecal, or colonic frequency were within 5% of being equal. Then we determined the proportion of time periods from all experiments at a given dose in which coordination existed (Table 4). Latent periods and durations of responses to the APP injections were also determined during the record analysis. Latent period was defined as the time elapsing from the end of injection of APP until the first noticeable and persistent change in muscular stomach contraction frequency. Duration was defined as the time required for contractile frequency to "recover" to two-thirds of the pre-injection level. In previous studies (e.g. Duke et al, 1975a, 1975b) it was shown that GI motility of turkeys often began to decline after about 1.25 to 1.5 hr of restraint during a recording experiment. When 20 or 30 Mg/kg doses were administered, a depression of motility often persisted for more than 30 min but recovery of contractile frequency to two-thirds of pre-injection frequency was usually complete within 30 min. Therefore, choice of two-thirds recovery generally avoided

241

the possible confusion of the APP response with the restraint response during record analysis. Questionable records (4.1% of all records) were omitted from analysis. RESULTS

Injections of 2 or 5 jUg/kg of APP caused no statistically significant change in contraction frequency of the cranial tract (Table 1). An injection of 8 jug/kg significantly depressed frequency during the first 10 min after injection but not during subsequent post-injection periods. Injections of 10, 20, and 30 Aig/kg depressed frequency in every trial and some degree of depression persisted during most of the 30 min post-injection period (Tables 1 and 2). The contractile frequency of the entire caudal tract was significantly reduced during the first 10 min post-injection period following injections of 30 /Ug/kg. Only the cecum was not similarly affected by injections of 20 Mg/kg (Table 3). However, on the average, statistically significant reduction did not continue after this period even in the thick caudodorsal muscle which was significantly depressed up to 30 min post-injection in those experiments involving cranial tract implants. Doses of 5 and 10 Mg/kg produced little depression of lower gut frequency. In general, the caudal portions of the GI tract were less affected by APP than were the cranial portions (Tables 1 and 3) both initially and throughout the 30 min post-injection period. In both portions of the tract, the degree of initial depression of motility was directly dose related. This dose dependent relationship was also evident in mean latent periods and duration of responses following APP injections (Table 2). Examination of recordings revealed that a significant depression of caudal tract motility in response to 20 and 30 jug/kg APP injections nearly always occurred when the contractile activity of the caudal tract was coordinated with that of the muscular stomach. Depression was less likely to occur when motilities of the stomach and caudal tract were not coordinated. Also, APP injections at the 20 or 30 Mg/kg level tended to slightly reduce this coordination (Table 4). During studies on the ileum, cecum, and colon contractile patterns seen previously in the ileum of turkeys (Duke et al, 1975b) in association with a "migrating electric complex"

D

Tk

Tn

G

D

Tk

Tn

G

D

Tk

Tn

G

(

1.7* ( .95)

2.7

2.8

7.8

.97)

2.8

7.8

( 2.0)

7.9

( 2.7)

.12)

5.1* (3.3)

7.7

( 1-4)

.60)

(

1.9

2.7

.16)

2.6

.16)

2.6

( .78)

(

(

(

( 4.7)

(

( 1.0)

2.7

1.4

.98)

2.2

2.4

( 1.5)

( .21)

( .75)

.99)

2.3

(

1.7* ( .93)

2.6

7.1

( 4.2)

7.6

(3.6)

( 1.1)

2.8

( 1.1)

2.8

( 1.3)

2.5

10.6 ( 2.4)

.89)

3.5

( 1.1)

3.8

10.1 ( 2.4)

8.3

( .76)

3.8

( 1.1)

(

3.6

.69)

3.6

.72)

3.6

3rd 10 min after

( 1.2)

(

(

2nd 10 min after

( 4.9)

2.6

(1.1)

2.7

( 1.3)

2.6

( .95)

2.8

( 1.5)

2.2

(1.1)

2.1

9.3

(1.9)

L1.0

( 1.6)

( 1.4)

3.3

(1.7)

3.4

( .88)

( .44)

( .77)

3.4

3.3

3.4

( .88)

3.4

( ,43)t

1st 10 min after

3.4

.67)

3.6

.63)

3.6

2.7

.99)

2.7

7.0

.32)

1.9

.55)

2.1

.56)

2.2

( 2.0)

(

(

(

( 3.7)

7.4

( 1.0)

(

( 1.4)

2.3

10.1 ( 1.6)

( 1.2)

(

(

All 30 min after Mg

30 Mg

2 0 Mg

kg

10

D

Tk

Tn

8.2

.78)

2.9

.81)

2.9

( 2.9)

(

(

2.7

G

.59)

12.5 3.0)

.73)

3.9

.70)

3.7

.66)

3.8

3.2)

9.1

.86)

3.1

1.2)

3.5

.48)

2.8

D

Tk

Tn

D

D

Tk

Tn

G

30 min before

1.

5. 3.

2. 1.

1. 1.

1. 1.

5. 4.4

(

2. 1.

( .

1.

( .

1.

( .

(

(

(

(

(

( .

2.0

( .

1.7

( .

1.

1st min

tNumber in parenthesis is standard deviation. •Indicates mean significantly different (P-C05) from the mean for the period 30 min prior to injection of APP for 2— *'Indicates mean not significantly different (P<.05) from the mean for the period 30 min prior to injection for 10 cantly different).

kg

8ng

kg

5jUg

2Mg

30 min before

TABLE 1. Mean contraction frequencies (number of contractions/min) for the glandular stomach (G), thin caudo muscles of the muscular stomach, and for the proximal duodenum (D) of turkeys for a 30 min period prior to in and for three 10 min periods after injection

AVIAN PANCREATIC POLYPEPTIDE IN TURKEYS

TABLE 2. Mean duration (min) and latent period (min) of the response of the thick caudodorsal muscle of the muscular stomach to APP injected (IV) at six doses APP dose levels (Mg/kg)

Duration Latent period

2

5

8

10

20

30

a1 b

a b

11.0 .93

18.9 .71

17.1 24.3 .51 .28

1 a = Duration was not detectable in most experiments; b = Latent period was not detectable in most experiments.

(Szurszewski, 1970) were recorded occasionally from all three organs. APP was injected during this activity in the colon (30 /ig/kg) once and in the ileum (10 Mg/kg) once. The activity did not appear to be altered in either case, although motility in the organs not involved in this activity was depressed following the 30 jug/kg injection. During two experiments this contractile activity appeared in the record from the ileal implant within one minute after injection of APP (20 and 30 Aig/kg while motility was simultaneously depressed in the stomach, cecum and colon. Apparently this activity is not prevented nor depressed by APP at levels used in this study. Basal plasma concentrations of APP averaged from 3.20 to 6.55 ng/ml in fasted turkeys (Table 5). Plasma concentrations increased postprandially and in increments following injections of increasingly higher doses of APP but did not change significantly after saline injections nor in response to handling and blood withdrawal. Plasma levels of APP were lower and the post-prandial increase in APP concentration was greater in birds fasted for 24 hr than in those fasted for 12 hr. DISCUSSION Hormonal regulation of GI secretion, especially gastric secretion, has been fairly well studied in birds (e.g. Burhol, 1974; Gibson et al, 1975). However, we believe the present study is the first to deal directly with hormonal regulation of GI motility in an avian species. Plasma levels of APP in turkeys fasted for 12 or 24 hr were found to be slightly higher than levels for fasted chickens (2 to 4 ng/ml; Kimmel et al, 1971). As in fasted chickens, plasma concentration of APP increased upon eating in turkeys but much less than the five- to ten-fold increase observed after eating in chickens (Kim-

243

mel and Pollock, 1975). "Tease" feeding chickens did not cause a similar increase in plasma concentration of APP (Kimmel and Pollock, 1975). Injections of APP (12.5, 25, 50 jUg/kg, iv) have been shown to cause an increase in volume, acid, pepsin, and total protein secretion from the glandular stomach of fed chickens; however, pancreatic and biliary flow were not affected by APP (Hazelwood et al, 1973). In dogs pancreatic peptide is apparently released upon eating or in anticipation of eating ("tease" feeding) (Lin and Chance, 1973; Chance et al., 1975). At physiological levels it appears to stimulate gastric secretion in fasted dogs but to suppress it in fed dogs (Lin and Chance, 1973; Lin et al, 1977). Pancreatic and biliary secretion and GI motility are also suppressed by pancreatic polypeptide in dogs (Lin and Chance, 1973; Lin et al, 1977). Since increases in the plasma concentration of APP caused by refeeding of fasted turkeys were less than increases caused by iv injection of 5 Mg/kg of APP (Table 5), and since 5 £tg/kg injections caused no apparent change in GI contractile activity (Tables 1 and 3), it seems unlikely that APP has a physiological influence on GI motility in turkeys. Likewise, plasma concentrations of APP used to stimulate gastric secretory activity in chickens (Hazelwood et al, 1973) were much higher than post-prandial plasma concentrations of APP. Therefore, APP appears to have little, if any, physiological role in regulation of avian GI activity. Because the avian stomach serves not only digestive functions, as in mammals, but masticatory functions as well, different regulating mechanisms for gastric activity might be required. During experiments on the caudal tract, the response of the thick caudodorsal muscle to APP was being used as a "control" to help in interpreting the responses of the ileum, cecum, and colon. This muscle was less inhibited during caudal tract studies as compared to its responses in the caudal tract experiments were about three weeks older than those used in the cranial tract experiments which might be interpreted to mean that the caudal tract would normally be more depressed than was indicated in this study. However, turkeys used in the cranial tract study and appeared to be slightly less "nervous" than the younger birds. Possibly the nervousness of the younger birds contributed slightly to the postinjection response observed during the cranial tract experiments. Since no significant response occurred following

3.0 ( .06)

2.9 ( .28)

3.1 ( .19)

3.0 ( .35)

Ce

Co

2.5 ( .13)

2.4 ( .50)

2.5 ( .31)

2.4 ( .95)

Ce

Co

I

1.8 (1.3) 2.8 ( .29)

Tk

1.6* (1.3) 2.4 ( .40)

2.9 ( .23)

3.0 ( .29)

I

3.0 ( .12)

3.1 ( .42)t

Tk

1st 10 min after

2.6 ( .36) 2.3 ( .74)

2.8 ( .33) 2.2 (1.1)

2.4 ( .64)

2.9 ( .23)

2.8 ( .30)

2.3 ( .47)

3.1 ( .19)

3.2 ( .33)

2.3* ( .59)

3.0 ( .34)

3.1 ( .55)

1.7 (1.4)

3.1 ( .24)

3.1 ( .56)

1.9 (1.3)

All 30 min after

3rd 10 min after

1.8 (1.5) 2.1 (1.1) 2.4 ( .67)

2.9 ( .10)

3.1 ( .25)

2.9 ( .31)

3.1 ( .25)

2nd 10 min after

30 Mg kg

20 Mg kg

Co

Ce

2.8 ( .42)

3.0 ( .52)

3.0 ( .38)

3.0 ( .49)

Tk I

2.7 (1.1)

2.3 (1.3)

2.6 ( .88)

2.3 (1.5)

30 min before

Co

Ce

I

Tk

tNumber in parentheses is standard deviation. •Mean significantly different from the mean for the period 30 min prior to injection of APP (P<.05).

10 Mg kg

5 Mg kg

30 min before

2

1

(

(

(

(

1

1

1

1

2 (1

(

(

1 (1

1s m

TABLE 3. Mean contraction frequencies (number of contractions/min) for the thick caudodorsal (Tk) muscle of portion oftheleft cecum (Ce), and mid-colon (Co) of turkeys for a 30 min period prior to injection (iv) of APP periods after injection

AVIAN PANCREATIC POLYPEPTIDE IN TURKEYS

245

TABLE 4. Influence of an injection (iv) of APP at four doses on the proportion of time during which coordination of contractile frequency existed between the thick caudodorsal muscle of the muscular stomach and the mid-ileum (Tk:I), distal, left cecum (Tk-Ce), and mid-colon (Tk:Co) % o f time coordination exists 30 min before injection

1st 1 0 min after injection

2nd 10 min after injection

3rd 1 0 min after injection

5 Mg/kg Tk-I Tk-Ce Tk-Co

100 75 100

75 100 100

75 100 75

100 75 50

10 Mg/kg Tk-I Tk-Ce Tk-Co

50 25 50

25 50 50

25 50 50

25 50 50

2 0 Mg/kg Tk-I Tk-Ce Tk-Co

40 60 40

20 20 20

20 60 20

40 40 40

30 Mg/kg Tk-I Tk-Ce Tk-Co

100 100 100

100 100 67

67 100 67

100 100 100

injections of 2 and 5 Mg/kg. however, t h e effect of nervousness m u s t have been m i n o r relative t o t h e effect of APP. In any case, t h e ileum, cec u m , and colon were relatively less depressed b y APP t h a n t h e s t o m a c h and d u o d e n u m even during t h e first 10 m i n postinjection period, permitting t h e conclusion t h a t t h e caudal tract is less influence b y APP t h a n t h e cranial.

C o o r d i n a t i o n b e t w e e n gastric a n d caudal tract motility has been previously described in chickens ( R o c h e , 1 9 7 4 ) . Since t h e caudal t r a c t was s o m e w h a t less inhibited b y APP t h a n t h e cranial and since m o t i l i t y of cranial and caudal p o r t i o n s of t h e t r a c t m a y b e c o o r d i n a t e d , it is possible t h a t t h e caudal tract is n o t indirectly inhibited b y APP b u t is only depressed as a re-

TABLE 5. Mean plasma levels of APP (ng/ml) in blood samples taken 10 min before and at various times after (see below) one of four treatments using turkeys fasted for 12 (treatments A, B, and C) or 24 (treatment D) hours1

Treatment

C.

Saline 5 Mg/Kg 10Mg/Kg 30 Mg/Kg 12 min after 20 min after 30 min post-prandial 60 min post-prandial 30 min post-prandial 60 min post-prandial 1

[APP] before 3.85 5.20 6.55 4.12 3.61 3.41 4.38 4.57 3.20 3.66

( .23)* ( .12) (1.74) ( .32) ( .50) ( .17) ( .13) ( .30) ( .26) ( .30)

[APP] after 3.90 22.7 32.9 105.0 3.64 3.52 5.35 5.96 7.92 8.77

( .54) ( 5.40) (11.8 ) (15.6 ) ( .26) ( .12) ( .99) ( .37) ( 1.05) ( 2.32)

A, isovolumic injections of saline, 5, 10, or 30 Mg/kg of APP with blood sampled at 2 min after injection; B, no treatment, but samples taken at 12 or 20 min after "before" sample; C, refeeding with samples taken at 30 and 60 min post-prandial (12 hr fast); D, refeeding with samples taken at 30 and 60 min post-prandial (24 hr fast). •Numbers in parentheses are standard deviations.

DUKE ET AL.

246

suit of gastric or cranial tract inhibition by APP. Contractile activity, apparently associated with a migrating electric complex, has not previously been observed to occur in the ceca or colon of birds. Perhaps this activity was not observed in previous studies of contractile and electrical activity in the colon of turkeys (Lai and Duke, 1978) because turkeys were not fasted. The activity is more common in fasted than in fed dogs (Szurszewski, 1970). Since contractile activity associated with the migrating electric complex does not appear to be affected by APP while other types of contractile activity are affected, a better understanding of the origin and regulation of the complex would help greatly in understanding the mechansim by which APP inhibits GI motility. This problem, as well as several others arising from the present study, should be further investigated.

REFERENCES Burhol, P. C , 1974. Effect of glucagon on gastric secretion in fistula chickens. Scand. J. Gastroenterol. 9:411-414. Chance, R. E., T. M. Lin, M. D. Johnson, N. E. Moon, and D. C. Evans, 1975. Page 183 in Studies on a new recognized pancreatic hormone with gastrointestinal activities. 57th Annu. Meeting Endocrine Soc. Duke, G. E., 1977. Avian digestion. Page 313-320 in Dukes' physiology of domestic animals. 9th Ed. M. J. Swenson, ed. Cornell University Press, Ithaca, NY. Duke, G. E., and O. A. Evanson, 1972. Inhibition of gastric motility by duodenal contents in turkeys. Poultry Sci. 51:1625-1636. Duke, G. E., O. A. Evanson, and P. T. Redig, 1976a. A cephalic influence on gastric motility upon seeing food in domestic turkeys, great-horned owls (Bubo virginianus) and red-tailed hawks (Buteo jamaicensis). Poultry Sci. 55:2155-2165. Duke, G. E., O. A. Evanson, P. T. Redig, and D. D. Rhoades, 1976b. Mechanism of pellet egestion in great-horned owls (Bubo virginianus). Amer. J. Physiol. 231:1824-1830. Duke, G. E., M. R. Fedde, and W. D. Kuhlmann, 1977. Evidence for mechanoreceptors in the muscular stomach of the chicken. Poultry Sci. 56:297-299. Duke, G. E., and T. E. Kostuch, 1975. The use of strain-gauge transducers to study gastroduodenal motility in turkeys. Poultry Sci. 54:1472—1478. Duke, G. E.,T. E. Kostuch,and O. A. Evanson, 1975a. Gastroduodenal electrical activity in turkeys. Amer. J. Dig. Dis. 20:1047-1058. Duke, G. E..T. E. Kostuch, andO. A. Evanson, 1975b.

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