of plasma interleukin-1f3to single and recurrent endotoxin challenges

In a parallel study in 10 individual rats, three time series of plasma concentrations of ACTH, corticosterone (CORT), and interleukin-1� (IL-1�) were measured before (time 0) and at intervals between 15 and 480 mm following intra-arterial (i.a.) infusions of 25�tg/kg lipopolysaccharide (LPS). All LPS injections were given at 9 AM. The first time series was performed on naive rats (day 1 ). A se- quence of six daily injections (days 3-8) of the same dose of LPS followed. The post-LPS time course of the plasma ACTH, CORT and IL-1�3 levels were studied on days 3 (second injection) and 8 (seventh injection). The first LPS injection induced a rapid (30 min) eightfold rise in plasma ACTH and CORT, culminating in concentrations 30 times the baseline at 60 nlin (ACTH) and 15 times baseline at 120 min (CORT). Both hormones receded back to the initial basal level at 480 miii. On the other hand, IL-1�3 increased slowly to peak at 13 times baseline 120 mm before declining to minimal seven- to ninefold basal levels, 480 mm and even 48 h post-LPS. During the second phase of the experiment starting 48 h after the initial LPS priming sequence, the ACTH and CORT responses to daily recurrent LPS injections again differed from those of IL-1�. The post-LPS time courses of the ACTH and CORT reaction dis- played a typical pattern of a progressive attenuation studied at days 3 and 8. The peak amplitudes at days 3 and 8 were reduced to 60 and 10%, respectively, for ACTH, and to 85 and 45% for CORT of those observed at the first LPS test. The duration of the response (both) was also shortened from 480 mm (first LPS test) to 300 mm at days 3 and 8. The post-LPS patterns of the IL- 1 � responses were char- acterized, first by basal levels seven to nine times higher than the initial baseline values (day 1), and by a rapid suppression of the post-LPS response, with only a slight (30%) increase at day 3 and no increase at day 8. Thus, after both acute and recurrent LPS administration, ACTH/CORT and IL-1� reacted dif- ferently to the endotoxin challenge. The two LPS reactive systems were not correlated. This is incon- sistent with the often proposed role of increased plasma IL- 1 � release as an intermediary factor in the LPS-induced recruitment of the corticotropic axis in general infections. J. Leukoc. Biol. 59: 341-346; 1996.