Role of endocrine pancreas in temperature acclimation

Life Sciences, Vol. 30, pp. 2253-2259 Printed in the U.S.A.

Pergamon Press

ROLE OF ENDOCRINEPANCREAS IN TEMPERATUREACCLIMATION Katsuhiko Doi, Tomie Ohno* and Akihiro Kuroshima Department o f Physiology, Asahikawa Medical College, Nishikagura, Asahikawa, 078-11 Japan and Division of Nutrition Physiology, Hokkaido Educational College, 9 Chome, Hokumoncho, Asahikawa, 070 Japan (Received in final form April 15, 1982) Stmmary Role of endocrine pancreas in temperature acclimation in rats was investigated. Plasma glucagon level increased and insulin level decreased in cold-acclimated rats (CA). The reverse was observed in heat-acclimated rats (HA). In the pancreas there were no changes in glucagon and insulin in CA, but a decrease in glucagon and an increase in insulin were found in HA. Plasma insulin/glucagon molar ratio (I/G) declined in CA and rose in HA. Pancreatic I/G rose in HA. Acute cold exposure elevated plasma glucagon, but did not affect plasma insulin. Pancreatic glucagon, insulin and I/G were not influenced by acute cold exposure, while plasma I/G decreased. Plasma I/G was inversely correlated with both blood free fatty acids and glucose levels. These results suggest that endocrine pancreas is closely associated with metabolic acclimation to cold and heat through its regulation of the metabolic direction to catabolic phase in cold acclimation and to anabolic phase in heat acclimation. A series of recent reports from our laboratory (1-6) has suggested that, in addition to norepinephrine, glucagon is closely involved in the metabolic acclimation to cold and heat, possibly through its action in the regulation of lipid and carbohydrate metabolism in brown adipose tissue. We observed the following changes in plasma glucagon levels in temperature acclimation; the plasma glucagon level increases in cold-acclimated rats, while it decreases in heat-acclimated ones (1,2). On the other hand, insulin, which is diametrically opposed to the action of glucagon, was reported to decrease in the plasma and pancreas during the prolonged exposure of rats to cold (7,8). It has been also reported that both basal and glucose-induced insulin release from the pancreas of cold-acclimated rats exhibited a significant reduction as compared with those from controls (9). It has been well established that the endocrine pancreas functions as an ~ - ~ cell couple and regulates body fuels such as glucose and free fatty acids in order to meet the energy requirement (i0). Therefore, it seems to be pertinent to observe simultaneous changes in glucagon and insulin secretions in order to investigate the role of the endocrine pancreas in temperature acclimation. The present study concerned the function of the endocrine pancreas, especially in terms of changes in the insulin/glucagon molar ratio, in temperature acclimation and thermal stress. 0024-3205/82/262253-07S03.00/0 Copyright (c) 1982 Pergamon Press Ltd.

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Pancreatic Hormones in Cold and Heat

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Materials and Methods Male rats of the Wistar strain weighing 195-200 g were used throughout the experiments. ]~e rats were kept under artificial illumination from 7:00 to 19:00 and given cor~nercial rat chow (Oriental MF, Oriental Yeast Co., Ltd., Tokyo) and tap water ad libitum. They were acclimated to 25 + 1 C at 50 % relative hun~dity (warm control rats, WC), 5 + 1 C (cold-acclimated rats, CA) and 33 + 1 C at 40 to 45 % relative humidity ~heat-acclimated rats, HA) for 4 to 5 wee-ks. CA and HA were transferred to 25 C, 18 hours bsfore the experiments. An acute cold exposure experiment was performed at -B C for 150 min using WC. Blood samples were obtained by decapitation into heparinized beakers. One ml of blood for glucagon assay was put into a polyvinyl tube containing 0.i ml of Trasyrol solution (Bayer, i0,000 IU/ml) and 1 mg Na2EDTA. Pl~sma was immediately separated at 20,000 rev/min for 2 min, and then kept at -30vC until the determination of glucagon and insulin levels. Pancreas was carefully dissected so as to be free of lymph nodes and adipose tissue, then divided into small pieces and put into 5 ~imes its weight of extraction fluid, 0.01 N HCI-saline, and homogenized at 4 C. After centrifugation the supernatant of i: i0, 000 dilution was assayed for glucagon and insulin. Glucagon was determined in duplicate by radioimmunoassay (ii) by the use of anti-pork-beef glucagon serum 30 K. Polyethylene glycoal was used to separate free glucagon from antibodybound glucagon. Insulin was measured in duplicate by radioin~nunoassay according to Yalow and Berson (12) with minor modifications by the use of rabbit antiser~n to bovine insulin. Plain charcoal was used to separate free insulin from antibody-bound insulin. Blood free fatty acids (FFA) were measured according to the method by Itaya and Ui (13) and blood glucose according to the method by Roe (14). The significance of the differences was tested by analysis of variance (15) and Student's t test. Results Body and pancreas weights of experimental animals (Table i) Increments in the body weight were significantly less in CA (p ~ 0.01) and HA (p (0.001) than in WC, as reported previously (2). Pancreas weights did not differ between WC and CA, but were significantly less in HA than in WC. A significant increase in the pancreas weight in HA was also observed when expressed on the basis of body weight (p < 0.05). Effects of cold and heat acclimation~ and acute cold exposure on the plasma and pancreatic levels of pancreatic hormones Plasma glucagon level was significantly increased in CA (p(0.05) and decreased in HA (p ~ 0.001) as compared with the controls (WC) (Table ii). The glucagon content of the pancreas did not change in CA, but was markedly lessen ed in HA (p < 0.01). Similar results were obtained in the glucagon content per unit weight (p ( 0.05). Acute cold exposure caused a significant increase in the plasma glucagon level (p (0.001), but did not affect the glucagon content of the pancreas (Table IIi). Plasma insulin level was significantly decreased in CA (p (0.01) and increased in HA (p < 0.05). The insulin content of the pancreas was not affected in CA, but increased in HA per unit weight (p (0.01). (Table ii). Plasma I/G was significantly smaller in CA (p < 0.01), but greater in HA (p <0.001) than in WC. Pancreatic I/G did not differ between WC and CA, but

200 + 13.0

193 +

CA (9)

HA (9)

9.1

260 +

5.9

285 + 13.7

335 +

At experiment

0.92 + 0.087

1.50 + 0.124

1.37 + 0.072

(g)

0.31 +_ 0.033

0.50 _+ 0.026

0.41 + 0.025

(g/100~)

Pancreas weight (g)

141 + 20.4

444 + 35.4

345 + 30.2

0aEq/l)

Blood FFA

222 + 22.5

102 ±

CA (9)

HA (9)

3.8 + 0.25

8.3 + 0.88

6.4 + 0.47

(~g)

Pancreas

23.9 + 0.94

16.2 + 0.93

20.9 + 0.83

(pU/ml)

Plasma

(U)

3.7 + 0.13

3.5 + 0.14

4.0 ± 0.54

2.5 _+ 0.21

2.3 + 0.13

(U/g)

Pancreas

3.0 + 0.23

Insulin

I/G: insulin/glucagon molar ratio.

3.5 + 0.31

5.5 _+ 0.27

4.8 + 0.33

(pglg)

Legends same as in Table I.

9.3

158 + 15.6

WC (6)

(pg/ml)

Plasma

Glucagon

Glucagon, Insulin and I/G Ratio in Temperature-acclimated Rats.

TABLE I I

5.~

2.C

3.~

P]

Each value represents the mean + S.E. Number in parenthesis indicates the number o~ *, ** and ***', significantly different from WC, p (0.05, 0.01 and 0.001, respective] WC: warm control rats, CA: cold-acclimated rats, HA: heat-acclimated rats. FFA: frc

9.6

195 + i0.i

WC (6)

Initial

Body weight (g)

Body and Pancreas Weights, and Blood FFA in Temperature-acclimated Rats.

TABLE I

356 + 23.5

<0.001

-5°C (i0) (150 rain)

p vs 25°C

N.S.

5.8 ± 0.60

5.7 + 0.37

N.S.

N.S.

2.4 ± 0.16

2.7 • 0.27

(U/g)

Pancreas

~ 0.001

1.6 + 0.14

3.9 • 0.51

Plasma

I/G

N.S.

11.9 ± 1.45

11.6 ± 0.77

Pancreas

<

607

351

(p

25°C: warm control rats, -5°C: acute cold expos

21.3 • 0.72

22.0 ± 0.73

(pU/ml)

Plasma

Insulin

Legends same as in Table I and If.

168 a 24.9

~ug/g)

(pg/ml)

25°C (I0)

Pancreas

Plasma

Glucagon

Glucagon, Insulin and I/G Ratio, and Blood Metabolites in Acutely Cold-expose(

TABLE III

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Pancreatic Hormones in Cold and Heat

2257

was significantly greater in HA (p (0.01) than in WC. Acute cold exposure affected neither plasma insulin nor pancreatic insulin (Table III). Plasma I/G was decreased (p <0.001), while pancreatic I/G was unchanged after exposure to cold. Effects of cold and heat acclimation, and acute cold exposure on blood metabolites Blood FFA level was significantly increased in CA (p (0.05) and decreased in HA (p (0.001) as compared with WC (Table i). Plasma I/G was inversely correlated to blood FFA levels as a whole in temperature-acclimated rats with a regression line of Y (plasma I/G) = - 0.0212 X (blood FFA) + 12.8833 (r= 0.92, p (0.001). Acute cold exposure caused a marked increase in blood FFA (p(0.001) and a slight but significant increase in blood glucose levels (p < 0.05) in WC (Table iii). Inverse correlations were also observed between plasma I/G and blood FFA levels with a regression line of Y (plasma I/G) = - 0.0081 X (blood FFA) + 8.2960 (r= 0.63, p (0.01), and between plasma I/G and blood glucose levels with a regression line of Y (plasma I/G) = - 0.0569 X (blood glucose ) + 9. 7697 (r= 0.40, p<0.05). Discussion The present study has shown that the endocrine pancreas is affected by temperature acclimation and acute thermal stress. Judging from the changes in pancreatic hormones in the plasma and pancreas due to ten~perature acclimation, it may be inferred that cold acclimation causes an increased release and synthesis of glucagon with a decreased release and synthesis of insulin. Heat acclimation causes a decreased release and synthesis of glucagon with an increased release and synthesis of insulin. Acute cold exposure may also stimulate glucagon release and synthesis, but does not affect insulin secretion. However, some reservations about this interpretation should be made, since the hormonal content of the pancreas might be influenced not only by the synthesis rate of hormones, but also by the degradation rate. It is well known that glucagon is physiologically a potent glycogenolytic and lipolytic agent (16) and that insulin acts the opposite direction. It is also well established that the relative concentration of these two hormones, the I/G ratio, plays a major role in determining the direction of metabolism of energy fuels such as glucose and FFA (I0). The present study showed that I/G declines in the cold concurrently with a rise in blood glucose and FFA levels and increases in the heat concurrently with a marked decrease in blood FFA levels. Plasma I/G is inversely correlated to blood FFA levels in temperature-acclimated animals. Inverse correlations are also observed between blood FFA and plasma I/G and between blood glucose and plasma I/G in WC and acute cold-exposed rats. Thus, it would appear that the endocrine pancreas is closely associated with metabolic acclimation and responds to cold and heat, partly through its action of regulating metabolism to a catabolic direction in the cold and to an anabolic direction in the heat. Both s)mpathetic nervous system and catecholamines may be involved in the modified secretion of insulin and glucagon in temperature acclimation. It has been reported that catecholamines exert a strong stimulatory effect on glucagon release (17,18) and an inhibitory effect on insulin release (19). An increased level of plasma catecholamines has been observed in cold acclimation (20,21) as

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Pancreatic Hormones in Cold and Heat

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well as in acute cold exposure (22). Therefore, the increased plasma level of catecholmnines could be responsible for the decreased insulin and the increased glucagon concentrations in CA. Furthermore, the elevated glucagon release associated with the stimulation of the sympathetic nerve of the pancreas (23) shows the existence of a direct autonomic control of glucagon secretion (24). However, the persistence of an exercised-induced rise in glucagon in adrenalectomized rats suggests that adrenal catecholamines do not play a major role in the secretion of this hormone, at least in exercise (24). The effect of cold acclimation on glucagon secretion could not be explained solely by hypoglycemia or a fall in blood FFA, both of which are strong stimuli to the secretion of this hormone (25,26), since cold acclimation did not cause a significant decrease in blood glucose (i) and caused a significant increase in blood FFA level (Table i) (i). In the case of insulin, it has been suggested that the sympathetic terminals of the pancreatic islet which lie close to the cells (27) may be of greater significance in modulating insulin release than blood catecholamines, whose level is shown to increase in cold acclimation (28). Therefore, the present findings on glucagon and insulin secretion in the cold could be explained by a combination of temperature-induced modifications of neural inputs with plasma and cellular catecholamines in the pancreas. It has been reported that sympathetic nerve activity is suppressed in heat-acclimated animals (29). Accordingly, it is likely that the increased plasma insulin and decreased plasma glucagon levels in heat-acclimated rats are due to the reduced secretion of catecholamines. References i. 2. 3. 4. 5.

A. A. A. A. A. 933

6. 7. 8. 9. i0. ii. 12. 13. 14. IS. 16. 17.

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