A quantitative analysis of triglyceride droplet structural changes in hamster brown adipose tissue during cold exposure and starvation

Abstract

Stereological procedures have been used to follow the effects of cold exposure and starvation on the structure of the triglyceride stores of brown adipose tissue in adult hamster. By a mathematical unscrambling technique, measurements of triglyceride droplet profiles were converted into true particle sizes and numbers. This enabled the calculation of the amount of stored triglycerides and the interfacial area of each droplet size class per unit of tissue volume. As a result of cold exposure, tissue triglyceride content changed between 45 and 18%. Simultaneously there was a redistribution of triglyceride into smaller, more numerous droplets. During the acute phase of cold exposure, the smallest class increased up to 15-fold in abundance and, though it maximally represented only 14% of the triglyceride store, it formed 49% of the total triglyceride interface (0.37 m 2 /cm 3 tissue). During chronic cold exposure, the middle and smallest classes predominated numerically and in terms of interfacial area, while the middle classes contained the greatest volume of triglyceride. Starvation at normal ambient temperature did not cause large changes in the tissue triglyceride content or its distribution among the droplet classes. In combination with cold exposure, starvation lowered the triglyceride content of brown fat to 3%; 68% of this was in the two smallest size classes, which accounted for 88% (0.58 m 2 /cm 3 tissue) of the total triglyceride interfacial area. In all these conditions, the total triglyceride interfacial area varied between 0.35 m 2 /cm 3 tissue (control) and 0.78 m 2 /cm 3 , which appeared to be a limiting value. Only when the tissue triglyceride content was lowered by about 90% was the increased locularity of the triglyceride stores incapable of maintaining this limiting interfacial area during intense thermogenic demand. Thus, under a variety of physiological conditions in which the balance between triglyceride depletion and accumulation was altered in several ways, an increased triglyceride metabolism was accompanied by an increased multilocularity of the triglyceride stores. In this way, a certain interfacial area was maintained in the face of even massive triglyceride depletion. This suggests that the triglyceride interfacial area must be kept above this level for efficient utilization of the thermogenic capacity of the tissue.