CAM? pmol/ IO' ceIIs with <strong>the</strong>ophylline 2ooC ISM) I om 5M) 0 . CAMP, pmol I IO' cells wilhout <strong>the</strong>ophylline 500 4M) 300 m rZO888 n: 8 100 p
<strong>tissue</strong> was incubated in <strong>the</strong> presence <strong>of</strong> a maximally effective concentration <strong>of</strong> ISNA (Fig. 1). This indi- cates that <strong>the</strong> maximal capacity <strong>of</strong> both triglyceride lipase and adenylate cyclase is increased in large fat cells in agreement with our previous findings (21). The interrelationship <strong>between</strong> lipolytic effect, cAMP concentration, and fat cell size supports a previous investigation where indirect evidence was given for a positive correlation <strong>between</strong> ISNA-induced cAMP <strong>level</strong> and fat cell size (1 1). When Figs. 1 and 2 are compared, it is observed that ISNA is about twice as potent a stimulator <strong>of</strong> <strong>the</strong> rate <strong>of</strong> lipolysis as NA, whereas <strong>the</strong> effect <strong>of</strong> ISNA on cAMP production is approximately ten times as great as that <strong>of</strong> NA. This is in agreement with our previous observation (1 1) showing a semilogarithmic relationship <strong>between</strong> lipolysis (linear) and cAMP <strong>level</strong>s (log) in <strong>human</strong> <strong>adipose</strong> <strong>tissue</strong> incubated with ISNA. It is reported that no correlation exists <strong>between</strong> <strong>the</strong> rate <strong>of</strong> hormone-induced lipolysis and <strong>the</strong> <strong>level</strong> <strong>of</strong> cAMP <strong>of</strong> <strong>the</strong> rat adipocyte (22), whereas <strong>the</strong> present investigation speaks for a quantitative relationship <strong>between</strong> rate <strong>of</strong> lipolysis and maximal cAMP concen- trations in <strong>human</strong> fat cells. The difference <strong>between</strong> our results and previous findings (22) could be ex- plained ei<strong>the</strong>r by species differences or by <strong>the</strong> fact that we used intact <strong>adipose</strong> <strong>tissue</strong>, unlike most <strong>of</strong> <strong>the</strong> previous investigators who used isolated adipocytes. Although <strong>the</strong> experimental conditions for <strong>the</strong> cAMP and <strong>the</strong> glycerol experiments differed, some general information could be obtained from <strong>the</strong> results re- corded in Fig. 5. First, about twice as much glycerol was produced per unit <strong>of</strong> cAMP when <strong>adipose</strong> <strong>tissue</strong> was incubated with NA in comparison with <strong>the</strong> basal state. This was true for small and for large fat cells and indicates that different mechanisms exist for basal and hormonal activation <strong>of</strong> lipolysis. Second, <strong>the</strong> exchange <strong>of</strong> glycerol produced per unit <strong>of</strong> <strong>tissue</strong> cAMP increased as <strong>the</strong> fat-cell volume increased. This was observed for both basal and NA-induced lipolysis and suggests that triglyceride lipase is more readily activated by cAMP when fat cells are enlarged. This conclusion is fur<strong>the</strong>r supported by <strong>the</strong> comparison <strong>of</strong> <strong>the</strong> regression lines for basal and <strong>the</strong> NA experi- ments in Figs. 1,3, and 4. In <strong>the</strong> absence <strong>of</strong> <strong>the</strong>ophyl- line (center panels in Figs. 3 and 4) or in <strong>the</strong> presence <strong>of</strong> <strong>the</strong> phosphodiesterase inhibitor (left panels in Figs. 3 and 4) NA affects only <strong>the</strong> intercepts <strong>of</strong> <strong>the</strong> regression lines, showing that <strong>the</strong> net effect <strong>of</strong> NA on cAMP production is <strong>of</strong> <strong>the</strong> same order <strong>of</strong> magnitude in small and large fat cells. In <strong>the</strong> lipolysis experiments, how- ever, shown in <strong>the</strong> left and right panels <strong>of</strong> Fig. l, NA increased both <strong>the</strong> slope and <strong>the</strong> intercept <strong>of</strong> <strong>the</strong> regression curve. Thus, <strong>the</strong> net lipolytic effect <strong>of</strong> NA was more pronounced in larger fat cells. Several reports have suggested that <strong>the</strong> hormone- induced cAMP <strong>level</strong> would represent an unphysiologi- cal overproduction <strong>of</strong> CAMP in <strong>adipose</strong> <strong>tissue</strong> (23-29) and that cAMP is sequestered into different compart- ments with only a small pool being responsible for <strong>the</strong> transmission <strong>of</strong> <strong>the</strong> hormonal effect (25, 26, 30- 33). Our results are in contrast to those ideas, since <strong>the</strong> correlations <strong>between</strong> cAMP and lipolysis on <strong>the</strong> one hand and fat cell size on <strong>the</strong> o<strong>the</strong>r hand were observed both when <strong>the</strong> phosphodiesterase inhibitor <strong>the</strong>ophylline was added and when it was omitted in <strong>the</strong> experiments with CAMP. Thus, cAMP seems to be produced and degraded in physiological amounts corresponding to <strong>the</strong> requirement for activating lipoly- sis by <strong>human</strong> adipocytes. Fur<strong>the</strong>rmore, <strong>the</strong> catechola- mine-induced cAMP <strong>level</strong> <strong>of</strong> <strong>human</strong> <strong>adipose</strong> <strong>tissue</strong> very probably represents a single compartment <strong>of</strong> <strong>the</strong> nucleotide, as we have suggested earlier (1 l).m This investigation was supported by grants from <strong>the</strong> Swed- ish Medical Research Council, Swedish Diabetic Associa- tion, and <strong>the</strong> Loo and Hans Osterman Fund. We thank Mrs. Raija Jonsson and Mrs. Else-Britt Lundstrom for <strong>the</strong>ir excellent technical assistance. Manuscript received 29 Apll 1977 and in revised form 6 September 1977; accepted 24 January 1978. 1. 2. 3. 4. 5. 6. 7. 8. REFERENCES Faulhaber, J. D., E. N. Petruzzi, H. Eble, and H. Ditschuneit. 1969. In vitro Untersuchungen uber den Fettst<strong>of</strong>fwechsel isolierter menschlischer Fettzellen in Abhangigkeit von der Zellgrosse: Die durch Adrenalin induzierte Lipolyse. Horm. Metab. Res. 1: 80-86. Goldrick, R. B., and G. M. McLoughlin. 1970. Lipolysis and lipogenesis from glucose in <strong>human</strong> fat cells <strong>of</strong> different sizes. J. Clin. Invest. 49: 1213-1223. Gries, F. A., M. Berger, M. Neumann, H. Preiss, H. Liebermeister, C. Hesse-Wortmann, and K. Jahnke. 1972. Effects <strong>of</strong> norepinephrine, <strong>the</strong>ophylline and dibuturyl <strong>cyclic</strong> AMP on in vitro lipolysis <strong>of</strong> <strong>human</strong> <strong>adipose</strong> <strong>tissue</strong> in obesity. Diabetologia 8: 75-83. Jacobson, B., and U. Smith. 1972. Effect <strong>of</strong> cell size on lipolysis and antilipolytic action <strong>of</strong> insulin in <strong>human</strong> fat cel1s.J. Lzpid. Res. 13: 651-656. Knittle, J. L., and F. Ginsberg-Fellner. 1972. Effect <strong>of</strong> weight reduction on in vitro <strong>adipose</strong> <strong>tissue</strong> lipolysis and cellularity in obese adolescents and adults. Diabetes 21: 754-766. Lisch, H.-J., S. Sailer, F. Sandh<strong>of</strong>er, and H. Braunsteiner. 1970. Untersuchungen an isolierten menschlichen Fettzellen verschiedner Fettgewebsregionen. 11. Beziehungen zwischen Zellvoiumen und basaler Lipol- se Klan. Wochemchr 22: 1353-1356. imith, U. 1970. Effect <strong>of</strong> glucose and insulin on lipoly- sis rates in <strong>human</strong> fat cells <strong>of</strong> different sizes. FEBS Lett. 11: 8-10. Ostman, J., L. Backman, and D. Hallberg. 1975. Cell size and <strong>the</strong> antilipolytic effect <strong>of</strong> insulin in <strong>human</strong> subcutaneous <strong>adipose</strong> <strong>tissue</strong>. Diubetologia 11: 159- 164. Arner and Ostmun <strong>Relationship</strong> <strong>between</strong> <strong>cyclic</strong> AMP and fat cell size 617 Downloaded from www.jlr.org by guest, on March 28, 2013