CHARACTERIZATION OF PERFUSED PERIAORTIC BROWN ADIPOSE-TISSUE FROM THERAT

A technique was developed for the perfusion of periaortic brown adipos e tissue (BAT) with a view to assessing vascular system involvement in BAT thermogenesis. The procedure involved cannulation of the thoracic aorta and ligation of the intercostal branches and the distal thoraci c aorta. Perfusion was conducted in a buffer-filled chamber using cons tant flow at 37 degrees C. Lactate dehydrogenase leakage was less than 2%/h, and after 30 min of perfusion the energy charge was 0.72 +/- 0. 05 (n = 4) and differed little from freshly sampled interscapular BAT (0.71 +/- 0.03 (n =7)). Periaortic BAT was indistinguishable from inte rscapular BAT in enzyme content, mitochondrial size, mitochondrial cri stae, lipid content, and cell size. Basal oxygen consumption (V-O2) wa s 64.3 +/- 7.4 mu mol . h(-1) . g(-1) wet weight, and basal perfusion pressure was 65 +/- 3 mmHg (1 mmHg = 133.3 Pa). Norepinephrine and iso proterenol each increased V-O2 of perfused periaortic BAT in a time-de pendent and reversible manner. Half-maximal stimulation of Vo(2) occur red at 12 nM norepinephrine and 8 nM isoproterenol; maximally stimulat ed tissue had a V-O2 of approximately 150 mu mol . h(-1) . g(-1) wet w eight. Norepinephrine (50 nM) had no consistent effect on perfusion pr essure, but the increase in V-O2 by this agonist was completely blocke d by 10 mu M DL-propranolol and unaffected by phentolamine (1-20 mu M) or nitroprusside (0.01-1 mM). Increasing the perfusion flow rate incr eased pressure and had no effect on basal V-O2, but increased the V-O2 response due to norepinephrine. Several observations suggest that vas cular thermogenesis and (or) vascular control of BAT thermogenesis wer e minimal in constant flow perfused periaortic BAT. These include (i) the failure of norepinephrine to consistently cause vasoconstriction o r vasodilation in association with an increase in V-O2; (ii) isoproter enol-mediated increase in V-O2; (iii) the failure of increased flow to increase basal V-O2; (iv) the failure of nitroprusside or phentolamin e to inhibit norepinephrine mediated increase in V-O2; and (v) the com plete inhibition by propranolol of the norepinephrine- or isoprotereno l-mediated increase in V-O2. Finally, the present findings provide no evidence for the presence of an alpha-adrenergic mechanism to control BAT thermogenesis directly, or indirectly by alterations in flow.