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Effect of temperature on the rates of digestion, amino acid absorption and assimilation in the alligator
0300-9629/X6 $3.00 + 0.00
c‘,~mp. Eiochm~. P/Iw~I/. Vol. X3A. No. 3. pp. 5X5-5X8. 1986 Prmted in Great Britain
t’
19X6 Pergamon Press Ltd
EFFECT OF TEMPERATURE ON THE RATES OF DIGESTION, AMINO ACID ABSORPTION AND ASSIMILATION IN THE ALLIGATOR ROLAND
Department
of Biochemistry.
A.
Louisiana
COULSON and
THOMAS D. COULSON
State University Medical Center. Telephone: (504) 943-7567 (Rwriwd
New Orleans,
LA 70119, USA.
28 June 1985)
Abstract-l. Small alligators were fed lean meat (50 g/kg of body weight). The rates of digestion, amino acid absorption, and assimilation were determined at 25 , 28 and 31 C. 2. At 25 C. digestion was slow and incomplete. 3. At 28 C. digestion proceeded about half as fast as at 31 C. 4. In all experiments. there was an immediate rise in plasma alanine and glycine and a delayed and prolonged rise in plasma glutamine. 5. At all temperatures, the rate of absorption and amino acid disappearance seemed to match the rate of digestion, suggesting that the rates of all were controlled by a common factor. 6. It was postulated that the common factor was blood flow and that the reason the rate of digestion etc. was affected by temperature was that it affected the rate of blood flow.
INTRODUCTION To
provide an environment suitable for an alligator (Alligator mississipiensis) to grow at the maximum rate it is necessary to know the proper diet, how much and how often to feed, and the optimum temperature. Although these factors have been studied empirically bj observing weight gain over several months, the experimental conditions were not well controlled (Coulson rt al., 1973; Joanen and McNease, 1976; Coulson and Hernandez, 1983). Growth studies on sufficient alligators for statistical evaluation are essential but they are expensive and quite timeconsuming and short-term experimental solutions are desirable. Determination of the optimum temperature is possible under laboratory conditions if it is accepted that the temperature permitting digestion to proceed at a maximal rate would be that required for maximum growth in the feeding pens. Some information on the relationship between temperature and the rate of digestion was available. Diefenbach (1975a.b) studied the length of time food remained in the stomach of force-fed caimans (Cairnan crocodilus) at several temperatures, and also the influence of temperature of gastric motility. The feeding experiments of Joanen and McNease (1976), and Coulson er a/. (1973) had indicated that the optimum appeared to be above 28”‘C, but by exactly how much was unknown. Considering that an increase in body temperature from 28 to 29°C increases oxygen consumption in an alligator by 30% (Coulson and Hernandez, 1964), finding the proper temperature to within I C is of importance. It is possible to determine the rate of protein digestion and amino acid absorption after feeding by observing the rate at which plasma amino acids increase, and the length of time they remain elevated (Coulson and Hernandez, 1970; Herbert and Coulson. 1976). As protein is digested, free amino acids are released, absorbed as fast as they appear, and
small increases are detectable in the plasma. The excess plasma amino acids disappear by the routes of catabolism and protein synthesis at rates proportional to the metabolic rate. The amount of the increase in a particular amino acid in the plasma may not reflect exactly its relative contribution to the ingested protein since it may have provided carbon or nitrogen or both for the synthesis of glycine, alanine and glutamine in the intestinal mucosa or in the liver (Coulson and Hernandez. 1983). However. the length of time the essentials remain elevated appears to be a useful index of the rate of absorption and digestion. [In the case of a protein deficient in most essential amino acids (such as gelatin), post-prandial plasma amino acids are in about the same ratio as in the ingested protein. Failure to absorb a mixture of amino acids in the proper ratio for body protein synthesis results in a prolonged high plasma amino acid concentration.] By following the gain in various plasma amino acids in fed alligators at different temperatures it is possible to determine not only their rates of absorption but also whether temperature is a factor in determining which of several possible catabolic pathways is favored for each compound. One problem encountered, in either long-term feeding experiments in pens, or short-term ones in the laboratory, is the degree of individual variation. To reduce it, the same three alligators were used in feeding experiments at 25, 28 and 31-C. The results indicated that 31’ C must be close to the optimum temperature for digestion. Evidence is presented to indicate that balance between digestion, absorption and assimilation rates at any temperature is achieved in all animals by the regulatory action of blood flow.
METHODS
Three alligators weighing from 780 to lO8Og were obtained from the Louisiana Department of Wildlife and Fisheries. In the first experiment they were fasted for over 585
586
ROLAND
A.
COULSON
a week in an outside pen at ambient temperatures varying from 15 to 30 C. They were removed from the pen and kept in a constant temperature room at 28 C without food but with water for three days. On the fourth day they were force-fed lean nutria muscle (50 g/kg) by pushing pieces down the esophagous with the aid of a lead pencil. During the following week, tail blood was withdrawn at intervals. the plasma was drawn off after centrifuging, and the change in its content of free amino acids was determined on proteinfree filtrates by means of an amino acid analyzer (Coulson and Hernandez. 1968). The temperature of the environmental chamber was kept at 28 + 0.3 C. At the beginning of each day they were hydrated by placing them in a tank of water at 28 C for about 30 min and then returned to the dry constant temperature room. At the conclusion of this experiment the temperature was raised to 31 C, the alligators were allowed to adjust to the temperature change for two days and the feeding experiment was repealed. Using the same procedure each time. the next experiment was conducted at 28 C, then one at 31 C. and finally two at 25 C. In summary. the same three alligators were used in two feeding experiments at 25, two at 28, and two at 31 C.
RESULTS
The averages of the results of the changes in the plasma amino acid contents at different temperatures and times were graphed (Fig. 1). As there were three alligators in each experiment and two experiments at each temperature, each point is the average derived from six chromatograms. Fine perpendicular lines represent standard error of the mean (SEM). Although random variation was considerable, the amino acid curves at the three temperatures differed greatly. At 28 C increases in total plasma amino acids
t
1.”
I
j”~.y~-__:::\_ 40
60
80
109
I20
D. COULSON
reached their peak in 55-72 hr and in 30-38 hr in those kept at 31 C. About 98% of the excess amino acids were 31 C. The
gone
in 144 hr at 28 C and
DISCUSSION
Alligators fed and kept at 25 C (77 F) seemed to digest so slowly and incompletely as to cast doubt on their ability to thrive at this constant temperature. After the completion of these experiments, J. D. Herbert (personal communication) fed three more and kept them at 25 ‘C. Some days later two of them vomited a considerable fraction of the meat which
GLUTAMI’dE
,.l...
ijo i
IFtO
in 96 hr at
peak for the essential amino acids was between 24 and 30 hr at 31 C and between 48 and 55 hr at 28 C, and the time at which 90% of the excess essential amino acids had disappeared was about 72 hr at 31 C and 144 hr in the 28 C experiment. Not only was the peak reached more quickly in the 31 C experiments, but the initial rate of rise in the totals. essentials, and in alanine was much greater also. Evidently, digestion, absorption. and removal of the absorbed amino acids was about twice as fast at 31 C as at 28’C. The results of the 25 C experiments differed from those at 28 and 31’ C in several respects. The shape of the curves from these experiments seemed to indicate that digestion was slow to begin and that it was also slow to complete. Beginning on the fourth day. feces were obtained which appeared to contain undigested muscle fibers. The quantity was small. semiliquid, and scattered thinly over the containers and over the alligators, preventing analysis of the contents.
: *. I.0
ALANINE
20
THOMAS
and
20
60
80
IO0
I20
140
Fig. 1. Changes in plasma amino acids after feeding lean meat (50 g/kg) to alligators kept at 25, 28 or 31 C. There were three alligators and all were used twice at each temperature. Each point therefore represents changes seen in six amino acid chromatograms. “Essential amino acids” represents the sum of the changes in threonine, valine. methionine. leucine, isoleucine. phenylalanine. lysine. histidine and arginine. “Total amino acids” represents the essentials plus aspartate, serine, glutamine, glutamate,
glycine. alanine and tyrosine.
160
Digestion.
absorption.
appeared to be almost unaffected. Perhaps these results should be viewed with caution as failure to digest may have been the result of a type of temperature shock. Ambient temperatures in the outside tanks in the week preceding the 25 C experiment had reached as high at 35 C each day. making the temperature differential great. Failure to digest well at a constant tcmperaturc of 25 C in the laboratory does not necessarily mean that wild alligators would be handicapped to an equal degree on a day with an average temperature of 25 C. Basking in the sun can keep the body temperature well above 30 C for many hours and digestion could proceed fairly rapidly during part of the day. The accelerated rates of digestion, absorption etc. at 31 C are in accord with the increase in metabolic rate when the temperature was raised above 28 C in the early experiments (Coulson and Hernandez. 1964). Whether a further increase would be beneficial is questionable as there is a narrow gap between the temperature at which metabolic rate is maximal and that which is injurious to the alligator. Any statement to the effect that “an alligator can digest 50g of meat/kg in three days at a body temperature of 31 C” is largely devoid of meaning. Not only is temperature important, but the size of the alligator is of equal or even greater importance. The largest male alligator is 20.000 times the size of the smallest. and, not surprisingly, their metabolic rates diirer greatly. Very large alligators will digest slowly and very small ones. rapidly. As an example of the exceedingly rapid digestion in the smallest ones it was observed that they can eat enough ad lib to grow at a rate of 3%/day for a period of up to a month (Coulson and Hernandez. 1983). Giving a 50% rate of conversion of food to increased body mass (Coulson et al.. 1973) they are then eating voluntarily an amount equal to 6% of their body weight every day. Growth (protein synthesis) costs energy. and the more metabolic energy produced per unit time, the greater the potential for growth, and as a corollary, the greater the possible rate of digestion. The cost of bonding one amino acid to the next during protein synthesis in the alligator was estimated to be about 4 moles of ATP/“mole” of bonds (Herbert and Coulson. 1975). On a kg basis the smallest alligators were eating about 60 g of ground meat and bones/day containing about 9g of protein of which half (4.5 g) went into body tissue. Conversion of 4.5 g (39 mmoles of amino acids) into macromolecules (growth) in these alligators would cost 156 mmoles of ATP/kg/day compared with a total production (at rest) of about 630mmoles/kg at 28 C. (Feeding increases the metabolic rate enough to supply the required extra energy.) The metabolic cost/kg of an equivalent amount of protein synthesis would be the same in one weighing 700 kg as in one just out of the egg. but where the small one can produce energy in excess of the required rate, the big one cannot. At 28 C a resting grown male alligator produces only about 23 mmoles of ATP/kg/day. or 15% of the amount needed to match the rate of protein synthesis in the small alligator even if all the ATP produced were used for growth and none for the ordinary life processes. A reasonable estimate would be that if a I kg alligator can digest 50 g of meat/kg in three days,
assimilation
m alligator
587
one of 40 g could do it in one day while it would take one of 700 kg over 20 days. It may seem that the rates of digestion, absorption and protein synthesis have been treated as though they were all equal, and indeed it is true that they are. Even when the rate of digesfion was at its maximum, only traces of free amino acids were detected in the gut lumens of fed caimans (Coulson and Hernandez, 1970) and at no time did the essential amino acids in body fluids exceed I % of the total present initially in the food given. In the present study. increasing the temperature from 28 to 31 C increased digestion, absorption and assimilation rates equally. This remarkable balance is common to all vertebrates, coldor warm-blooded. as witnessed by the fact that during digestion there is only a very small rise in essential amino acids in the plasma in a man. a mouse or a I kg alligator, even though the rates of each of the three processes are 100 times as fast in the mouse as in the alligator. It is reasonable to believe that some common factor must be responsible for controlling the rate of digestion, absorption and assimilation, and in maintaining the nearly perfect balance among the three processes. Protein hydrolysis in the gut. amino acid catabolism and protein synthesis are chemical reactions and something governs their velocities in ~!ipa. It cannot be “metabolic rate”, which is simply an expression of reaction velocity, nor can it be the area of the skin/unit body weight or body weight to any exponential power, for there is no direct way these parameters can affect enzyme reactions within a cell. However, since reaction rates are proportional to surface area/g in animals of different size within any one species, the unknown factor must vary in concord. The most probable regulator is blood flow which is proportional to metabolic rate in all resting vertebrates, and to surface area/g within any closely related group of species. A theory has been proposed to account for differences in metabolic rate among the vertebrates. In this “Flow Theory” blood flow in I/kg/hr is determined by the usual factors such as the size of the heart, the cross-sectional area of the aorta. heart rate, pressure in the aorta etc., and of even greater importance, by the average distance the blood must go to complete one circulatory revolution. The larger the animal, the longer the vessels. and the longer the vessels the greater the peripheral resistance, the slower the flow and the longer the circulation time. In the theory. the more blood that passes through a kg of tissue/hr. the greater the reaction velocities within that tissue (Coulson et al., 1977; Coulson and Herbert. 1982; Coulson and Hernandez, 1983; Coulson, 1983; Coulson and Herbert, 1984). According to this theory, digestion, absorption and assimilation would always be in balance in every animal. If each liter of blood passing through the absorptive area of the small intestines of each species picks up the same fraction of the small molecules released during digestion, the greater the flow through the gut, the greater the rate of absorption. Metabolic removal of the absorbed molecules would also be proportional to the rate of flow through the tissues, and that flow would be related to the flow through the intestine. Presumably, the actual rate of digestion would be
ROLAND A. COULSON and THOMAS D. COULSON
588
determined by the rate at which each molecule released during hydrolysis was removed. Flow would therefore control not only the rate of absorption and removal from the plasma. but the rate of digestion as well. Large alligators have low blood flows and therefore digestion and absorption are slow. Flow would also affect the rate of synthesis of hydrolytic enzymes in the pancreas, intestinal wall. etc. Temperature is important because it affects the rate of blood flow through the intestine and the rest of the body (Campos, 1964). Acknowledgement-We thank the Louisiana Department of Wildlife and Fisheries for supplying the alligators, for financial support, and for supplying food for the alligators.
REFERENCES
Campos V. (1964) Efecto de las cambios de temperatura sobre las frecuencias cardiaca y respiratoria de1 lagarto (Alligator mississipiensis). Reora. Biol. rrop. 12, 49951. Coulson R. A. and Herbert J. D. (1982) Reaction velocities in ciro: Standardization of kinetic “constants” by correction for blood flow. Comp. Biochem. Physiol. 72A, 125-132. Coulson R. A. (1983) Relationship between fluid flow and 0: demand in tissues in cit’o and in vitro. Perspect. Biol. Med. 21, 12lLl26. Coulson R. A. and Herbert J. D. (1984) Energy production per circulatory cycle: A constant in resting land vertebrates? Comp. Biochem. Physiol. 79A, 133-136. Coulson R. A. and Hernandez T. (1964) Biochemistry qf
the Alliguror. .4 Study of Meruholism in S/OH Motion. Louisiana State University Press. Baton Rouge. Coulson R. A. and Hernandez T. (1968) Amino acid metabolism in chameleons. Camp. B&hem. Physiol. 25. 861-872. Coulson R. A. and Hernandez T. (1970) Protein digestion and amino acid absorption in the caiman. J. Nurr. 100, 810.-826. Coulson R. A. and Hernandez T. (1983) AI/igu/or Meluholism, Studies on Chemicul Reucrion.r In Viva. Pergamon Press, Oxford. Coulson T. D., Coulson R. A. and Hernandez T. (1973) Some observations on the growth of captive alligators. Zoologica 58, 47-52. Coulson R. A., Hernandez T. and Herbert J. D. (1977) Metabolic rate: Enzyme kinetics in riw. C’omp. Biochem. Ph_vsiol. 56A, 25 1-262. Diefenbach C. 0. da C. (1975a) Gastric function in Caiman crocodilus (Crocodilia: Reptiliu) 1. Rate of gastric digestion and gastric motility as a function of temperature. Camp. Biochem. Physiol. SlA, 259-265. Diefenbach C. 0. da C. (1975b) Gastric function in Caiman crocodilus (Crocodilia: Repliliu). II. Effects of temperature on pH and proteolysis. Camp. Biochem. Physiol. SlA, 267-274. Herbert J. D. and Coulson R. A. (1975) Free amino acids in crocodilians fed proteins of different biological value. J. Nutr. 105, 6166623. Herbert J. D. and Coulson R. A. (1976) Plasma amino acids in reptiles after feeding protein or amino acids, and after injecting amino acids. J. Nutr. 106, 1097-1101. Joanen T. and McNease L. (1976) Culture of immature American alligators in controlled environmental chambers. Pro<. 7th Workshop World Mariculture Society 7, 201-21 I.
c‘,~mp. Eiochm~. P/Iw~I/. Vol. X3A. No. 3. pp. 5X5-5X8. 1986 Prmted in Great Britain
t’
19X6 Pergamon Press Ltd
EFFECT OF TEMPERATURE ON THE RATES OF DIGESTION, AMINO ACID ABSORPTION AND ASSIMILATION IN THE ALLIGATOR ROLAND
Department
of Biochemistry.
A.
Louisiana
COULSON and
THOMAS D. COULSON
State University Medical Center. Telephone: (504) 943-7567 (Rwriwd
New Orleans,
LA 70119, USA.
28 June 1985)
Abstract-l. Small alligators were fed lean meat (50 g/kg of body weight). The rates of digestion, amino acid absorption, and assimilation were determined at 25 , 28 and 31 C. 2. At 25 C. digestion was slow and incomplete. 3. At 28 C. digestion proceeded about half as fast as at 31 C. 4. In all experiments. there was an immediate rise in plasma alanine and glycine and a delayed and prolonged rise in plasma glutamine. 5. At all temperatures, the rate of absorption and amino acid disappearance seemed to match the rate of digestion, suggesting that the rates of all were controlled by a common factor. 6. It was postulated that the common factor was blood flow and that the reason the rate of digestion etc. was affected by temperature was that it affected the rate of blood flow.
INTRODUCTION To
provide an environment suitable for an alligator (Alligator mississipiensis) to grow at the maximum rate it is necessary to know the proper diet, how much and how often to feed, and the optimum temperature. Although these factors have been studied empirically bj observing weight gain over several months, the experimental conditions were not well controlled (Coulson rt al., 1973; Joanen and McNease, 1976; Coulson and Hernandez, 1983). Growth studies on sufficient alligators for statistical evaluation are essential but they are expensive and quite timeconsuming and short-term experimental solutions are desirable. Determination of the optimum temperature is possible under laboratory conditions if it is accepted that the temperature permitting digestion to proceed at a maximal rate would be that required for maximum growth in the feeding pens. Some information on the relationship between temperature and the rate of digestion was available. Diefenbach (1975a.b) studied the length of time food remained in the stomach of force-fed caimans (Cairnan crocodilus) at several temperatures, and also the influence of temperature of gastric motility. The feeding experiments of Joanen and McNease (1976), and Coulson er a/. (1973) had indicated that the optimum appeared to be above 28”‘C, but by exactly how much was unknown. Considering that an increase in body temperature from 28 to 29°C increases oxygen consumption in an alligator by 30% (Coulson and Hernandez, 1964), finding the proper temperature to within I C is of importance. It is possible to determine the rate of protein digestion and amino acid absorption after feeding by observing the rate at which plasma amino acids increase, and the length of time they remain elevated (Coulson and Hernandez, 1970; Herbert and Coulson. 1976). As protein is digested, free amino acids are released, absorbed as fast as they appear, and
small increases are detectable in the plasma. The excess plasma amino acids disappear by the routes of catabolism and protein synthesis at rates proportional to the metabolic rate. The amount of the increase in a particular amino acid in the plasma may not reflect exactly its relative contribution to the ingested protein since it may have provided carbon or nitrogen or both for the synthesis of glycine, alanine and glutamine in the intestinal mucosa or in the liver (Coulson and Hernandez. 1983). However. the length of time the essentials remain elevated appears to be a useful index of the rate of absorption and digestion. [In the case of a protein deficient in most essential amino acids (such as gelatin), post-prandial plasma amino acids are in about the same ratio as in the ingested protein. Failure to absorb a mixture of amino acids in the proper ratio for body protein synthesis results in a prolonged high plasma amino acid concentration.] By following the gain in various plasma amino acids in fed alligators at different temperatures it is possible to determine not only their rates of absorption but also whether temperature is a factor in determining which of several possible catabolic pathways is favored for each compound. One problem encountered, in either long-term feeding experiments in pens, or short-term ones in the laboratory, is the degree of individual variation. To reduce it, the same three alligators were used in feeding experiments at 25, 28 and 31-C. The results indicated that 31’ C must be close to the optimum temperature for digestion. Evidence is presented to indicate that balance between digestion, absorption and assimilation rates at any temperature is achieved in all animals by the regulatory action of blood flow.
METHODS
Three alligators weighing from 780 to lO8Og were obtained from the Louisiana Department of Wildlife and Fisheries. In the first experiment they were fasted for over 585
586
ROLAND
A.
COULSON
a week in an outside pen at ambient temperatures varying from 15 to 30 C. They were removed from the pen and kept in a constant temperature room at 28 C without food but with water for three days. On the fourth day they were force-fed lean nutria muscle (50 g/kg) by pushing pieces down the esophagous with the aid of a lead pencil. During the following week, tail blood was withdrawn at intervals. the plasma was drawn off after centrifuging, and the change in its content of free amino acids was determined on proteinfree filtrates by means of an amino acid analyzer (Coulson and Hernandez. 1968). The temperature of the environmental chamber was kept at 28 + 0.3 C. At the beginning of each day they were hydrated by placing them in a tank of water at 28 C for about 30 min and then returned to the dry constant temperature room. At the conclusion of this experiment the temperature was raised to 31 C, the alligators were allowed to adjust to the temperature change for two days and the feeding experiment was repealed. Using the same procedure each time. the next experiment was conducted at 28 C, then one at 31 C. and finally two at 25 C. In summary. the same three alligators were used in two feeding experiments at 25, two at 28, and two at 31 C.
RESULTS
The averages of the results of the changes in the plasma amino acid contents at different temperatures and times were graphed (Fig. 1). As there were three alligators in each experiment and two experiments at each temperature, each point is the average derived from six chromatograms. Fine perpendicular lines represent standard error of the mean (SEM). Although random variation was considerable, the amino acid curves at the three temperatures differed greatly. At 28 C increases in total plasma amino acids
t
1.”
I
j”~.y~-__:::\_ 40
60
80
109
I20
D. COULSON
reached their peak in 55-72 hr and in 30-38 hr in those kept at 31 C. About 98% of the excess amino acids were 31 C. The
gone
in 144 hr at 28 C and
DISCUSSION
Alligators fed and kept at 25 C (77 F) seemed to digest so slowly and incompletely as to cast doubt on their ability to thrive at this constant temperature. After the completion of these experiments, J. D. Herbert (personal communication) fed three more and kept them at 25 ‘C. Some days later two of them vomited a considerable fraction of the meat which
GLUTAMI’dE
,.l...
ijo i
IFtO
in 96 hr at
peak for the essential amino acids was between 24 and 30 hr at 31 C and between 48 and 55 hr at 28 C, and the time at which 90% of the excess essential amino acids had disappeared was about 72 hr at 31 C and 144 hr in the 28 C experiment. Not only was the peak reached more quickly in the 31 C experiments, but the initial rate of rise in the totals. essentials, and in alanine was much greater also. Evidently, digestion, absorption. and removal of the absorbed amino acids was about twice as fast at 31 C as at 28’C. The results of the 25 C experiments differed from those at 28 and 31’ C in several respects. The shape of the curves from these experiments seemed to indicate that digestion was slow to begin and that it was also slow to complete. Beginning on the fourth day. feces were obtained which appeared to contain undigested muscle fibers. The quantity was small. semiliquid, and scattered thinly over the containers and over the alligators, preventing analysis of the contents.
: *. I.0
ALANINE
20
THOMAS
and
20
60
80
IO0
I20
140
Fig. 1. Changes in plasma amino acids after feeding lean meat (50 g/kg) to alligators kept at 25, 28 or 31 C. There were three alligators and all were used twice at each temperature. Each point therefore represents changes seen in six amino acid chromatograms. “Essential amino acids” represents the sum of the changes in threonine, valine. methionine. leucine, isoleucine. phenylalanine. lysine. histidine and arginine. “Total amino acids” represents the essentials plus aspartate, serine, glutamine, glutamate,
glycine. alanine and tyrosine.
160
Digestion.
absorption.
appeared to be almost unaffected. Perhaps these results should be viewed with caution as failure to digest may have been the result of a type of temperature shock. Ambient temperatures in the outside tanks in the week preceding the 25 C experiment had reached as high at 35 C each day. making the temperature differential great. Failure to digest well at a constant tcmperaturc of 25 C in the laboratory does not necessarily mean that wild alligators would be handicapped to an equal degree on a day with an average temperature of 25 C. Basking in the sun can keep the body temperature well above 30 C for many hours and digestion could proceed fairly rapidly during part of the day. The accelerated rates of digestion, absorption etc. at 31 C are in accord with the increase in metabolic rate when the temperature was raised above 28 C in the early experiments (Coulson and Hernandez. 1964). Whether a further increase would be beneficial is questionable as there is a narrow gap between the temperature at which metabolic rate is maximal and that which is injurious to the alligator. Any statement to the effect that “an alligator can digest 50g of meat/kg in three days at a body temperature of 31 C” is largely devoid of meaning. Not only is temperature important, but the size of the alligator is of equal or even greater importance. The largest male alligator is 20.000 times the size of the smallest. and, not surprisingly, their metabolic rates diirer greatly. Very large alligators will digest slowly and very small ones. rapidly. As an example of the exceedingly rapid digestion in the smallest ones it was observed that they can eat enough ad lib to grow at a rate of 3%/day for a period of up to a month (Coulson and Hernandez. 1983). Giving a 50% rate of conversion of food to increased body mass (Coulson et al.. 1973) they are then eating voluntarily an amount equal to 6% of their body weight every day. Growth (protein synthesis) costs energy. and the more metabolic energy produced per unit time, the greater the potential for growth, and as a corollary, the greater the possible rate of digestion. The cost of bonding one amino acid to the next during protein synthesis in the alligator was estimated to be about 4 moles of ATP/“mole” of bonds (Herbert and Coulson. 1975). On a kg basis the smallest alligators were eating about 60 g of ground meat and bones/day containing about 9g of protein of which half (4.5 g) went into body tissue. Conversion of 4.5 g (39 mmoles of amino acids) into macromolecules (growth) in these alligators would cost 156 mmoles of ATP/kg/day compared with a total production (at rest) of about 630mmoles/kg at 28 C. (Feeding increases the metabolic rate enough to supply the required extra energy.) The metabolic cost/kg of an equivalent amount of protein synthesis would be the same in one weighing 700 kg as in one just out of the egg. but where the small one can produce energy in excess of the required rate, the big one cannot. At 28 C a resting grown male alligator produces only about 23 mmoles of ATP/kg/day. or 15% of the amount needed to match the rate of protein synthesis in the small alligator even if all the ATP produced were used for growth and none for the ordinary life processes. A reasonable estimate would be that if a I kg alligator can digest 50 g of meat/kg in three days,
assimilation
m alligator
587
one of 40 g could do it in one day while it would take one of 700 kg over 20 days. It may seem that the rates of digestion, absorption and protein synthesis have been treated as though they were all equal, and indeed it is true that they are. Even when the rate of digesfion was at its maximum, only traces of free amino acids were detected in the gut lumens of fed caimans (Coulson and Hernandez, 1970) and at no time did the essential amino acids in body fluids exceed I % of the total present initially in the food given. In the present study. increasing the temperature from 28 to 31 C increased digestion, absorption and assimilation rates equally. This remarkable balance is common to all vertebrates, coldor warm-blooded. as witnessed by the fact that during digestion there is only a very small rise in essential amino acids in the plasma in a man. a mouse or a I kg alligator, even though the rates of each of the three processes are 100 times as fast in the mouse as in the alligator. It is reasonable to believe that some common factor must be responsible for controlling the rate of digestion, absorption and assimilation, and in maintaining the nearly perfect balance among the three processes. Protein hydrolysis in the gut. amino acid catabolism and protein synthesis are chemical reactions and something governs their velocities in ~!ipa. It cannot be “metabolic rate”, which is simply an expression of reaction velocity, nor can it be the area of the skin/unit body weight or body weight to any exponential power, for there is no direct way these parameters can affect enzyme reactions within a cell. However, since reaction rates are proportional to surface area/g in animals of different size within any one species, the unknown factor must vary in concord. The most probable regulator is blood flow which is proportional to metabolic rate in all resting vertebrates, and to surface area/g within any closely related group of species. A theory has been proposed to account for differences in metabolic rate among the vertebrates. In this “Flow Theory” blood flow in I/kg/hr is determined by the usual factors such as the size of the heart, the cross-sectional area of the aorta. heart rate, pressure in the aorta etc., and of even greater importance, by the average distance the blood must go to complete one circulatory revolution. The larger the animal, the longer the vessels. and the longer the vessels the greater the peripheral resistance, the slower the flow and the longer the circulation time. In the theory. the more blood that passes through a kg of tissue/hr. the greater the reaction velocities within that tissue (Coulson et al., 1977; Coulson and Herbert. 1982; Coulson and Hernandez, 1983; Coulson, 1983; Coulson and Herbert, 1984). According to this theory, digestion, absorption and assimilation would always be in balance in every animal. If each liter of blood passing through the absorptive area of the small intestines of each species picks up the same fraction of the small molecules released during digestion, the greater the flow through the gut, the greater the rate of absorption. Metabolic removal of the absorbed molecules would also be proportional to the rate of flow through the tissues, and that flow would be related to the flow through the intestine. Presumably, the actual rate of digestion would be
ROLAND A. COULSON and THOMAS D. COULSON
588
determined by the rate at which each molecule released during hydrolysis was removed. Flow would therefore control not only the rate of absorption and removal from the plasma. but the rate of digestion as well. Large alligators have low blood flows and therefore digestion and absorption are slow. Flow would also affect the rate of synthesis of hydrolytic enzymes in the pancreas, intestinal wall. etc. Temperature is important because it affects the rate of blood flow through the intestine and the rest of the body (Campos, 1964). Acknowledgement-We thank the Louisiana Department of Wildlife and Fisheries for supplying the alligators, for financial support, and for supplying food for the alligators.
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