259
Atherosclerosis, 33 (1979) 259-265 0 Elsevier/North-Holland Scientific
Preliminary ADIPOSE
Publishers,
Ltd.
Note CELL SIZE IN SPONTANEOUSLY
PETER SINGER,
SIEGFRIED
HYPERTENSIVE
VOIGT, VERA MORITZ and RUDOLF
Central Institute of Cardiovascular Regulation Research, Academy 1 I15 Berlin-Buch (G.D.R.)
RATS (SHR)
BAUMANN
of Sciences of the G.D.R.,
(Received 6 October, 1978) (Revised, received 8 January, 1979) (Accepted 9 January, 1979)
Summary The size of adipose cells in spontaneously hypertensive rats (SHR) and normotensive controls has been evaluated at 4,8 and 26 weeks of age. Age-matched groups showed significant differences only in &week-old rats, but this can be explained by the lower body weight of SHR. In both groups of animals fat cell size varies with body weight (r = 0.965 in SHR and r = 0.863 in normotensive rats) independent of the stage of hypertension. The regression lines are not significantly different. Thus, no evidence of enlarged adipocytes in SHR has been obtained. Key words:
Adipose cell size - Age hypertensive rats (SHR)
Body
weight - Hypertension
- Spontaneously
Introduction Fat cell size has been accepted as a relevant factor in several metabolic disturbances [3,4]. Adipocytes do not serve as an inert reservoir of depot fat but are characterized by a high triglyceride turnover rate. They are significantly enlarged in obesity, hyperlipoproteinemia and maturity-onset diabetes. On the other hand, these metabolic disorders are frequently accompanied by essential arterial hypertension [ 7,121. The pathogenetic interrelationships, however, are largely unknown. Hyperinsulinemia, impaired carbohydrate tolerance, increased lipolysis, as well as a prevalence of obesity, hypertriglyceridemia and fatty liver, have been described in essential hypertension [ 1,13,27]. Spontaneously hypertensive rats (SHR) in some respects are a suitable model of essential hypertension in man [ 21,291. High levels of serum triglycerides and free fatty acids (FFA) together with augmented liver lipids have been described
260
in these animals [ 19,221. On the other hand, lower levels of serum triglycerides in SHR compared to normotensive animals have been found by Baumann et al. [ 21, corresponding to results [ 8,231 in rats deficient in essential fatty acids (EFA). In addition, an insulin enhancement in SHR associated with a disturbed glucose tolerance comparable to the findings in patients with essential hypertension [ 1,281 has been stressed in the same study [ 21. Since larger adipose cells are less sensitive to insulin than smaller cells [25] the problem arises of whether adipose cell size might be involved in the development of hyperinsulinemia in SHR. The present study was made to evaluate fat cell size in SHR in comparison to normotensive controls. Material and Methods Male SHR of Wistar-Kyoto strain and normotensive controls (WistarSchonwalde), 4,8 and 26 weeks of age, were killed by decapitation and bleeding after an overnight fast. Both groups of animals received a commercial rat diet (pellets provided by VEB KIM, Berlin) and water ad libitum. Arterial blood pressure was measured by a tail cuff method as described earlier [ 21. In animals 4 weeks of age only systolic blood pressure was significantly elevated in SHR. In &week-old rats it was not significantly different whereas in animals 26 weeks of age the mean systolic and diastolic blood pressure as well as the mean blood pressure were significantly higher in SHR (Table 1). However, the mean body weight of the SHR was significantly lower (amounting to about 80%) compared to controls of the same age (Fig. 1).
26 weeks (n *Ii?; ~6)
-
SHR
_ _ _ Controls
Fig. 1. Adipose cell volume (mean f SD) and weight of SHR and normotensive controls in relation to we.
TO AGE
AND
b P < 0.01.
WEIGHT
ADIPOSE
0.007
r 13 a * 1.7 f 17 r 2.3 f 15 + 2.0 f 8b
0.060 f
121 16.1 75 10.0 91 12.1 37 0.014
f 18 t 2.4 f 12 f 1.6 ?r12 t 1.6 ?r 5
0.070 f
97 12.9 57 7.6 75 10.0 46
CELL
0.016 b
f 21 + 2.8 f 24 r 3.2 f 20 + 2.7 ?: z6b
0.132 t
136 18.1 70 9.3 94 12.5 114
SHR (n = 10)
AND
SHR (n = 10)
a Significant between SHR and controls, P < 0.05.
Adipose ceII volume (nI)
BODY
controls (n = 9)
PRESSURE.
8 weeks
BLOOD
4 weeks
MEAN
Systolic blood pressure (mm Hg) kPa Diastolic blood pressure (mm Hg) kPa Mean blood pressure (mm Hg) kPa Body weight (g)
DIASTOLIC
RELATION
1
SYSTOLIC,
TABLE
f * + f f + f 0.199 +
137 18.3 82 10.9 96 12.8 152 0.033
22 2.9 17 2.3 13 2.4 28
0.044
RATS
+ * t f f f + 0.316 i
135 18.0 89 11.9 104 13.9 366
0.112
27 3.6 25 3.3 24 3.2 52
controls (n = 6)
NORMOTENSIVR
+ z3b + 3.1 +25b f 3.3 r 22 b f 2.9 * 29 b 0.291 +
228 30.4 154 20.5 178 23.7 308
SHR (n = 10)
26 weeks
+ SD) IN SHR AND
controls (n = 10)
SIZE (MEAN
IN
262
At killing perirenal fat was removed from the rats 4 weeks of age because there was insufficient epididymal fat for evaluation of fat cell size in these animals, but epididymal fat pads were obtained from rats 8 and 26 weeks of age. The samples (0.2-0.4 g adipose tissue) were incubated at 37°C in Krebs-Henseleit bicarbonate buffer (pH 7.4), containing bovine albumin (3.0%), glucose (200 mg/lOO ml) and collagenase (SERVA, 2 mg, 700 MANDL-U), as described by Rodbell [ 241. Cells were counted in a “TuR” ZG 2 counter using the method of Leonhardt [ 161. The mean and standard deviation (SD), regression lines and correlation coefficients were calculated for each group. Statistical significances of the differences were assessed by Student’s t-test.
.?. o--o
WR: y5Q857x+32,45, r*6?965(%O) Co~ols : y=P?49x+g95; r=o1863fna29)”
’
Fig. 2. Adipose cell size versus body weight of SHR and normotensive controls.
263 TABLE 2 ADIPOSE CELL VOLUME (MEAN f SD) IN WEIGHT-MATCHED SHR AND NORMOTENSIVE CONTROL RATS TOGETHER WITH BODY WEIGHT AND SYSTOLIC, DIASTOLIC AND MEAN BLOOD PRESSURE (MEAN i SD) FOR COMPARISON n
Adipose ceII volume
Body weight
W)
SHR Controls
6 6
0.286 0.291
f 0.038 + 0.025
Blood pressure
Mean blood
(9)
292 f 11 291 c 12
PXfSSUlV systolic
diastolic
(mm Hg)
(mm He)
218 f 32
140 f 40
P < 0.01
P < 0.01
120*
9
85k
a
(mm Hg)
165 i 32 P < 0.01 97a! 8
Results Mean adipose cell size in SHR was lower than in normotensive controls (Fig. 1). Differences in the age-matched groups were significant only in animals 8 weeks of age because of the large standard deviations. This might be due to the lower body weight of the SHR. In order to prove the correlation between volume of adipose cells and body weight two groups of weight-matched rats 6 months of age (6 SHR and 6 normotensive controls) were selected. As can be seen in Table 2 the sizes of the adipose cells were strikingly similar. Comparison of weight and adipose cell size for all animals showed that the regression lines of both groups were not significantly different in slope (Fig. 2). The highest correlation coefficient was found in SHR. Thus, adipose cell size seems to vary with body weight. No correlation was observed between fat cell size and arterial blood pressure (not documented). Discussion The equal adipose cell size in weight-matched SHR and normotensive rats might be of interest in relation to recent results confirming lower serum triglycerides in SHR [2], though elevated FFA [22] and insulin levels [2] were observed. Since insulin is known to stimulate lipogenesis, excess circulating insulin could cause accumulation of fat in the liver as well as in the arterial wall [1,26] by increasing its deposition and inhibiting its removal. Hypotriglyceridemia as a consequence is plausible. By this mechanism it is suggested that insulin plays an important role in the pathogenesis of atherosclerosis. However, this is not obvious in adipose tissue because of the lack of increase in adipose cell size of SHR. On the contrary, the adipose cell size and body weight of age-matched groups are less in SHR in spite of hyperinsulinemia found in a previous series [ 21. A lower triglyceride uptake and synthesis as well as minor fatty acid re-esterification in small fat cells in comparison with large fat cells have been demonstrated [ 5,9]. The evidence presented here, together with that in published reports, supports the hypothesis of Baumann [ 11, that increased lipolysis stimulated by catecholamines and other lipolytic hormones plays a primary role in the biochemical alterations in hypertension and that insulin enhancement should be
264
accepted as a counter-regulation, being hardly sufficient to balance the triglyceride supply in the adipocytes. Nevertheless, hitherto existing results concerning the behaviour of FFA in SHR are controversial, for no differences could be ascertained by other authors [ 181. This is consistent with the finding that plasma catecholamines are elevated in SHR [lo] or unchanged in comparison to normotensive controls
[6,201. Hyperinsulinemia is assumed to be an adaptive mechanism to enlargement of the adipocytes [ 141. However, since insulin enhancement occurs without apparent correlation with the fat cell size in SHR, insulin resistance and hyperinsulinemia can not be explained by enlarged adipocytes alone. It could, however, be due to a stimulation of insulin secretion by high glucose levels caused by the impaired glucose tolerance in SHR, as has been confirmed by our previous study [ 21. It seems more likely that body weight is correlated with fat cell size in SHR, normotensive rats and human beings. The correlation coefficient of our control group is of striking similarity to that calculated by Hammermiiller et al. [ll] in epididymal adipose tissue of rats, evaluating the adipocyte size by cell diameter. As regards the site of adipose tissue in rats, it must be emphasized that preferably an enlargement without hyperplasia occurs in epididymal fat pads whereas in the perirenal fat adipose cell hypertrophy and hyperplasia have been observed [ 151. Such possibilities for deviation, however, have been excluded because of the choice of the same site in age-matched groups. Moreover, out of 4 different sites of rats the perirenal and epididymal fat cells were the largest, revealing the smallest differences mutually in contrast to others such as mesenteric and subcutaneous fat cells [9]. Since body weight and weight of epididymal fat pads are correlated in their turn, comparison of fat cell size and body weight seems to be allowable [ 91. Thus, from the data presented it seems unlikely that an enlargement of adipocytes is a cause of insulin enhancement in SHR as has been assumed for other metabolic disorders such as obesity, hyperlipoproteinemia and maturity onset diabetes. Other m,echanisms must be considered. Acknowledgements The authors are most grateful to Mrs. R. Sauck and Mrs. M. Boyke for their skilful technical assistance and to Dr. J. Lauter, Division of Statistics, for the statistical analyses. References Baumann. R., Kohlenhydratund Fettstoffwechselstii~ngen in den Friihstadien der essentiellen Hypertonie aIs pathogenetische Risikofaktoren der Arteriosklerose. Ber. Ges. Inn. Med., 8 (1972) 191. Baumann. R.. MorItz. V.. GGdicke, W., Postnow, J.W. and ZiegIer. M.. Insulin kinetic, gIucose tolerance, and lipid metabolism in genetics& spontaneous-hypertensive rats, Acta Biol. Med. Germ., 35 (1976) K 33. Bernstein, R.S., Grant. N. and Kipnis, D.M.. HyperinsuIInemIa and enlarged adipocytes in patients with endogenous hyperlipoproteinemia without obesity or diabetes mellitus. Diabetes, 24 (1976) 207. BiSrntorp. P. and SiiistrGm. L., Number and size of adipose tissue fat cells in relation to metabolism in human obesity. Metabolism. 20 (1971) 703. Bjiirntorp, P.. Enzi. G., Ohlson. R.. Persson. B.. Sponbergs. P. and Smith, U.. Lipoprotein lipase
265 activity and uptake (1975) 230. 6 Biihler, dopamine
H.U.,
of exogenous
Da Prada.
M.,
in man and different
triglycerides
Haefely.
in fat cells
W. and Picotti,
animal species,
J. Physiol.
of different
G.B..
Plasma
(Land.),
size.
Horm.
adrenaline.
276 (1978)
Metab.
Res..
noradrenaline
7
and
311.
7 Drazin, M.L., Glucose tolerance in hypertension and obesity. Diabetes, 2 (1953) 433. 8 Fukazawa, T., Privett. O.S. and Takahashi. Y.. Effect of essential fatty acid deficiency on release of triglycerides by the perfused rat liver. J. Lipid Res., 11 (1970) 522. 9 Di Girolamo. M., Thurman, L. and CuBen. J.. Observations on adipose tissue cellularity and development in rats and rabbits fed ad lib. In: J. Vague and J. Bayer (Eds.), The Regulation of the Adipose Tissue Mass, Excerpta Medica, Amsterdam, 1973. p. 174. 10 Grobecker. H., Reizen. M.F.. Weise. V., Saavedra, J.M. and Kopin, I.J.. Sympathoadrenal medullary activity in young. spontaneously hypertensive rats, Nature (Land.), 258 (1975) 267. 11 HammermiiBer, B., Leonhardt. W. and Hanefeld, M., Vorliiufige Mitteilung iiber die elektronische Messung der Volumenvertailungskurven isolierter menschhcher Fettzellen bei Adipositas und ihre Beziehungen zum Diabetes meIIitus, Dtsch. Ges. wesen, 25 (1970) 2020. 12 Hirata. Y., Horino, M.. Ito. M., Yamauchi. M.. Makino, N., Ishimoto. M., Sate. T. and Hososako, A., A diabetes detection study in Kynshu - The relation of diabetes to hypertension. Diabetes, 11 (1962) 44. 13 Hood, B., Brolin. I., KjeIIbo, H. and Anger&I, G., Serum lipids in renal artery stenosis and other hypertensive states, Acta Med. Stand., 179 (1966) 575; 583. 14 Joosten, H.F.P. and Van der Kroon, P.H.W., Enlargement of epididymal adipocytes in relation to hyperinsulinemia in obese hyperglycemic mice (oblob). Metabolism, 23 (1974) 59. 15 Lemonmer, D.. Alex@ A. and Lanteceume, M.-T., Effect of two dietary lipids on the cellularity of the rat adipose tissue, J. Physiol. (Paris), 66 (1973) 729. ZG 2, TeiI 1 (Gewinnung Leonhardt, W., Gr5ssenmessung menschlicher Fettzellen mit dem “TuR” und Messung der FettzeIIen), Labortechnik, 6 (1973) 29. 17 Leonhardt. W., Hanefeld, M. and Ha&r, H.. Fat cell volume in maturity onset diabetes, Lancet, 1 (1971) 221. 18 Louis, W.J., Turnover of catecholamines in experimental hypertension. Circulat. Res.. 26/27. SUPP~. 2 (1970) 49. 19 Nagaoka, A., Kikuchi. K., Kawaji, H., Matsuo, T. and Aramaki, Y.. Life-span, hematological abnormalities, thrombosis and other macroscopical lesions in the spontaneously hypertensive rats, in: K. Okamoto (Ed.), Spontaneous Hypertension - Its Pathogen&s and Complications, Igaku Shoin Ltd., 1972, P. 149. 16
20
Nagaoka, A. and Lovenberg, W.. Plasma norepinephrine and do&amine-!I-hydroxylase in genetic hypertensive rats, Life Sci., 13 (1976) 29. 21 Okamoto, K. and Aoki, K., Development of a strain of spontaneously hypertensive rats, Jap. Circulat. J.. 27 (1963) 282. 22 Grbetzova. V.. Kiprov, D. and Puchlev Al., The action of arterial hypertension on lipid and lipoprotein metabolism, Part 2 (Qualitative and quantitative alterations of blood serum, liver and aortic lipids and lipoproteins in Okamoto-Aoki rats with spontaneous hypertension). Cor Vasa, 18 (3) (1976) 221. 23
De Pury, G.G. and Collins, F.D., Very low density lipoproteins and lipoprotein lipase in serum of rats deficient in essential fatty acids, J. Lipid Res.. 13 (1972) 268. 24 RodbeII, M., Metabolism of isolated fat cells. Part 1 (Effects of hormones on glucose metabolism and li~ol~sis), J. Biol. Chem., 239 (1964) 375. 25 Salans, L.B.. J.L. KnittIe and J. Hirsch, The role of adipose ceB size and adipose tissue instdin sensitivity in the carbohydrate intolerance of human obesity, J. CIin. Invest., 47 (1968) 163. 26 Stout, R.W., The relationship of abnormal circulating insulin levels to atherosclerosis, Atherosclerosis, 27 (1977) 1. 27 Thiel, H. and Woossmann. H., Leberveriinderungen in den Friihstadien der essentiellen Hypertonic. Dtsch. Ges. wesen. 26 (1971) 1489. 28 Welborn, T.A.. Breckenridge, A., Rubinstein. A.H.. Dollery, C.T. and Fraser, T.R., Serum insulin in essential hypertension and in peripheral vascular disease, Lancet, 1 (1966) 1336. 29 Yamori. Y., Pathogenesis of spontaneous hypertension as a model for essential hypertension, Jap. CircuIat. J.. 41 (1977) 259.
Atherosclerosis, 33 (1979) 259-265 0 Elsevier/North-Holland Scientific
Preliminary ADIPOSE
Publishers,
Ltd.
Note CELL SIZE IN SPONTANEOUSLY
PETER SINGER,
SIEGFRIED
HYPERTENSIVE
VOIGT, VERA MORITZ and RUDOLF
Central Institute of Cardiovascular Regulation Research, Academy 1 I15 Berlin-Buch (G.D.R.)
RATS (SHR)
BAUMANN
of Sciences of the G.D.R.,
(Received 6 October, 1978) (Revised, received 8 January, 1979) (Accepted 9 January, 1979)
Summary The size of adipose cells in spontaneously hypertensive rats (SHR) and normotensive controls has been evaluated at 4,8 and 26 weeks of age. Age-matched groups showed significant differences only in &week-old rats, but this can be explained by the lower body weight of SHR. In both groups of animals fat cell size varies with body weight (r = 0.965 in SHR and r = 0.863 in normotensive rats) independent of the stage of hypertension. The regression lines are not significantly different. Thus, no evidence of enlarged adipocytes in SHR has been obtained. Key words:
Adipose cell size - Age hypertensive rats (SHR)
Body
weight - Hypertension
- Spontaneously
Introduction Fat cell size has been accepted as a relevant factor in several metabolic disturbances [3,4]. Adipocytes do not serve as an inert reservoir of depot fat but are characterized by a high triglyceride turnover rate. They are significantly enlarged in obesity, hyperlipoproteinemia and maturity-onset diabetes. On the other hand, these metabolic disorders are frequently accompanied by essential arterial hypertension [ 7,121. The pathogenetic interrelationships, however, are largely unknown. Hyperinsulinemia, impaired carbohydrate tolerance, increased lipolysis, as well as a prevalence of obesity, hypertriglyceridemia and fatty liver, have been described in essential hypertension [ 1,13,27]. Spontaneously hypertensive rats (SHR) in some respects are a suitable model of essential hypertension in man [ 21,291. High levels of serum triglycerides and free fatty acids (FFA) together with augmented liver lipids have been described
260
in these animals [ 19,221. On the other hand, lower levels of serum triglycerides in SHR compared to normotensive animals have been found by Baumann et al. [ 21, corresponding to results [ 8,231 in rats deficient in essential fatty acids (EFA). In addition, an insulin enhancement in SHR associated with a disturbed glucose tolerance comparable to the findings in patients with essential hypertension [ 1,281 has been stressed in the same study [ 21. Since larger adipose cells are less sensitive to insulin than smaller cells [25] the problem arises of whether adipose cell size might be involved in the development of hyperinsulinemia in SHR. The present study was made to evaluate fat cell size in SHR in comparison to normotensive controls. Material and Methods Male SHR of Wistar-Kyoto strain and normotensive controls (WistarSchonwalde), 4,8 and 26 weeks of age, were killed by decapitation and bleeding after an overnight fast. Both groups of animals received a commercial rat diet (pellets provided by VEB KIM, Berlin) and water ad libitum. Arterial blood pressure was measured by a tail cuff method as described earlier [ 21. In animals 4 weeks of age only systolic blood pressure was significantly elevated in SHR. In &week-old rats it was not significantly different whereas in animals 26 weeks of age the mean systolic and diastolic blood pressure as well as the mean blood pressure were significantly higher in SHR (Table 1). However, the mean body weight of the SHR was significantly lower (amounting to about 80%) compared to controls of the same age (Fig. 1).
26 weeks (n *Ii?; ~6)
-
SHR
_ _ _ Controls
Fig. 1. Adipose cell volume (mean f SD) and weight of SHR and normotensive controls in relation to we.
TO AGE
AND
b P < 0.01.
WEIGHT
ADIPOSE
0.007
r 13 a * 1.7 f 17 r 2.3 f 15 + 2.0 f 8b
0.060 f
121 16.1 75 10.0 91 12.1 37 0.014
f 18 t 2.4 f 12 f 1.6 ?r12 t 1.6 ?r 5
0.070 f
97 12.9 57 7.6 75 10.0 46
CELL
0.016 b
f 21 + 2.8 f 24 r 3.2 f 20 + 2.7 ?: z6b
0.132 t
136 18.1 70 9.3 94 12.5 114
SHR (n = 10)
AND
SHR (n = 10)
a Significant between SHR and controls, P < 0.05.
Adipose ceII volume (nI)
BODY
controls (n = 9)
PRESSURE.
8 weeks
BLOOD
4 weeks
MEAN
Systolic blood pressure (mm Hg) kPa Diastolic blood pressure (mm Hg) kPa Mean blood pressure (mm Hg) kPa Body weight (g)
DIASTOLIC
RELATION
1
SYSTOLIC,
TABLE
f * + f f + f 0.199 +
137 18.3 82 10.9 96 12.8 152 0.033
22 2.9 17 2.3 13 2.4 28
0.044
RATS
+ * t f f f + 0.316 i
135 18.0 89 11.9 104 13.9 366
0.112
27 3.6 25 3.3 24 3.2 52
controls (n = 6)
NORMOTENSIVR
+ z3b + 3.1 +25b f 3.3 r 22 b f 2.9 * 29 b 0.291 +
228 30.4 154 20.5 178 23.7 308
SHR (n = 10)
26 weeks
+ SD) IN SHR AND
controls (n = 10)
SIZE (MEAN
IN
262
At killing perirenal fat was removed from the rats 4 weeks of age because there was insufficient epididymal fat for evaluation of fat cell size in these animals, but epididymal fat pads were obtained from rats 8 and 26 weeks of age. The samples (0.2-0.4 g adipose tissue) were incubated at 37°C in Krebs-Henseleit bicarbonate buffer (pH 7.4), containing bovine albumin (3.0%), glucose (200 mg/lOO ml) and collagenase (SERVA, 2 mg, 700 MANDL-U), as described by Rodbell [ 241. Cells were counted in a “TuR” ZG 2 counter using the method of Leonhardt [ 161. The mean and standard deviation (SD), regression lines and correlation coefficients were calculated for each group. Statistical significances of the differences were assessed by Student’s t-test.
.?. o--o
WR: y5Q857x+32,45, r*6?965(%O) Co~ols : y=P?49x+g95; r=o1863fna29)”
’
Fig. 2. Adipose cell size versus body weight of SHR and normotensive controls.
263 TABLE 2 ADIPOSE CELL VOLUME (MEAN f SD) IN WEIGHT-MATCHED SHR AND NORMOTENSIVE CONTROL RATS TOGETHER WITH BODY WEIGHT AND SYSTOLIC, DIASTOLIC AND MEAN BLOOD PRESSURE (MEAN i SD) FOR COMPARISON n
Adipose ceII volume
Body weight
W)
SHR Controls
6 6
0.286 0.291
f 0.038 + 0.025
Blood pressure
Mean blood
(9)
292 f 11 291 c 12
PXfSSUlV systolic
diastolic
(mm Hg)
(mm He)
218 f 32
140 f 40
P < 0.01
P < 0.01
120*
9
85k
a
(mm Hg)
165 i 32 P < 0.01 97a! 8
Results Mean adipose cell size in SHR was lower than in normotensive controls (Fig. 1). Differences in the age-matched groups were significant only in animals 8 weeks of age because of the large standard deviations. This might be due to the lower body weight of the SHR. In order to prove the correlation between volume of adipose cells and body weight two groups of weight-matched rats 6 months of age (6 SHR and 6 normotensive controls) were selected. As can be seen in Table 2 the sizes of the adipose cells were strikingly similar. Comparison of weight and adipose cell size for all animals showed that the regression lines of both groups were not significantly different in slope (Fig. 2). The highest correlation coefficient was found in SHR. Thus, adipose cell size seems to vary with body weight. No correlation was observed between fat cell size and arterial blood pressure (not documented). Discussion The equal adipose cell size in weight-matched SHR and normotensive rats might be of interest in relation to recent results confirming lower serum triglycerides in SHR [2], though elevated FFA [22] and insulin levels [2] were observed. Since insulin is known to stimulate lipogenesis, excess circulating insulin could cause accumulation of fat in the liver as well as in the arterial wall [1,26] by increasing its deposition and inhibiting its removal. Hypotriglyceridemia as a consequence is plausible. By this mechanism it is suggested that insulin plays an important role in the pathogenesis of atherosclerosis. However, this is not obvious in adipose tissue because of the lack of increase in adipose cell size of SHR. On the contrary, the adipose cell size and body weight of age-matched groups are less in SHR in spite of hyperinsulinemia found in a previous series [ 21. A lower triglyceride uptake and synthesis as well as minor fatty acid re-esterification in small fat cells in comparison with large fat cells have been demonstrated [ 5,9]. The evidence presented here, together with that in published reports, supports the hypothesis of Baumann [ 11, that increased lipolysis stimulated by catecholamines and other lipolytic hormones plays a primary role in the biochemical alterations in hypertension and that insulin enhancement should be
264
accepted as a counter-regulation, being hardly sufficient to balance the triglyceride supply in the adipocytes. Nevertheless, hitherto existing results concerning the behaviour of FFA in SHR are controversial, for no differences could be ascertained by other authors [ 181. This is consistent with the finding that plasma catecholamines are elevated in SHR [lo] or unchanged in comparison to normotensive controls
[6,201. Hyperinsulinemia is assumed to be an adaptive mechanism to enlargement of the adipocytes [ 141. However, since insulin enhancement occurs without apparent correlation with the fat cell size in SHR, insulin resistance and hyperinsulinemia can not be explained by enlarged adipocytes alone. It could, however, be due to a stimulation of insulin secretion by high glucose levels caused by the impaired glucose tolerance in SHR, as has been confirmed by our previous study [ 21. It seems more likely that body weight is correlated with fat cell size in SHR, normotensive rats and human beings. The correlation coefficient of our control group is of striking similarity to that calculated by Hammermiiller et al. [ll] in epididymal adipose tissue of rats, evaluating the adipocyte size by cell diameter. As regards the site of adipose tissue in rats, it must be emphasized that preferably an enlargement without hyperplasia occurs in epididymal fat pads whereas in the perirenal fat adipose cell hypertrophy and hyperplasia have been observed [ 151. Such possibilities for deviation, however, have been excluded because of the choice of the same site in age-matched groups. Moreover, out of 4 different sites of rats the perirenal and epididymal fat cells were the largest, revealing the smallest differences mutually in contrast to others such as mesenteric and subcutaneous fat cells [9]. Since body weight and weight of epididymal fat pads are correlated in their turn, comparison of fat cell size and body weight seems to be allowable [ 91. Thus, from the data presented it seems unlikely that an enlargement of adipocytes is a cause of insulin enhancement in SHR as has been assumed for other metabolic disorders such as obesity, hyperlipoproteinemia and maturity onset diabetes. Other m,echanisms must be considered. Acknowledgements The authors are most grateful to Mrs. R. Sauck and Mrs. M. Boyke for their skilful technical assistance and to Dr. J. Lauter, Division of Statistics, for the statistical analyses. References Baumann. R., Kohlenhydratund Fettstoffwechselstii~ngen in den Friihstadien der essentiellen Hypertonie aIs pathogenetische Risikofaktoren der Arteriosklerose. Ber. Ges. Inn. Med., 8 (1972) 191. Baumann. R.. MorItz. V.. GGdicke, W., Postnow, J.W. and ZiegIer. M.. Insulin kinetic, gIucose tolerance, and lipid metabolism in genetics& spontaneous-hypertensive rats, Acta Biol. Med. Germ., 35 (1976) K 33. Bernstein, R.S., Grant. N. and Kipnis, D.M.. HyperinsuIInemIa and enlarged adipocytes in patients with endogenous hyperlipoproteinemia without obesity or diabetes mellitus. Diabetes, 24 (1976) 207. BiSrntorp. P. and SiiistrGm. L., Number and size of adipose tissue fat cells in relation to metabolism in human obesity. Metabolism. 20 (1971) 703. Bjiirntorp, P.. Enzi. G., Ohlson. R.. Persson. B.. Sponbergs. P. and Smith, U.. Lipoprotein lipase
265 activity and uptake (1975) 230. 6 Biihler, dopamine
H.U.,
of exogenous
Da Prada.
M.,
in man and different
triglycerides
Haefely.
in fat cells
W. and Picotti,
animal species,
J. Physiol.
of different
G.B..
Plasma
(Land.),
size.
Horm.
adrenaline.
276 (1978)
Metab.
Res..
noradrenaline
7
and
311.
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