A novel experimental model of orthopedic trauma with acute kidney injury in obese Zucker rats

Abstract

Obesity is associated with an increased risk of acute kidney injury (AKI) after blunt traumatic injury in humans. Because limitations exist in studying trauma in human patients, animal models are necessary to elucidate mechanisms of remote organ injury after trauma. We developed a model of severe orthopedic trauma in lean (LZ) and obese (OZ) Zucker rats, in which OZ develop greater kidney dysfunction after trauma than LZ. Orthopedic trauma was inflicted via bilateral hindlimb soft tissue injury, fibula fracture, and injection of homogenized bone components. Mean arterial pressure (MAP) and heart rate (HR) were measured for 6 h after trauma, and again at 24 h after trauma. Urine was collected for 24 h before and after trauma to measure urine albumin excretion. Glomerular filtration rate (GFR), renal plasma flow (RPF), plasma interleukin-6 (IL-6), and renal macrophage infiltration (ED-1 [CD68 Antibody] immunostaining) were measured in animals with and without trauma. MAP and HR were similar between LZ and OZ throughout the study, with the exception that OZ had a 18 mmHg lower pressure 24 h posttrauma. GFR and RPF were decreased significantly (∼50%), while urine albumin excretion, plasma IL-6, and renal ED-1-positive cells were increased in OZ 24 h after trauma compared to both OZ without trauma and LZ after trauma. In conclusion, these data are consistent with studies in humans that show that AKI develops more frequently in obese than in lean individuals. This model will be an important experimental tool to better understand the underlying mechanisms of poor outcomes after trauma in obese patients.

Keywords: Acute kidney injury; glomerular filtration rate; obese; orthopedic; trauma.

Figure 1

Glomerular filtration rate (GFR) and renal plasma flow (RPF) are decreased at 24 h after trauma in obese rats, while urine albumin excretion is increased. GFR was measured using the inulin plasma clearance method in LZ- and OZ-Control animals and in those which had trauma. RPF was measured with the plasma clearance method using para-aminohippuric acid (PAH). In LZ, neither GFR (A) nor RPF (B) was different between control and trauma animals. However, in OZ, GFR and RPF decreased significantly after trauma. LZ rats showed no difference in total albumin excretion (C) after trauma as compared to pretrauma. OZ albumin excretion was significantly increased after trauma (OZ-Trauma) as compared to pretrauma (OZ-Control). n = 6 or more for all groups. *P < 0.05 versus OZ-Control, +P < 0.05 versus LZ-Trauma.

Figure 2

Mean arterial pressure (MAP) and heart rate (HR) responses for six consecutive hours after trauma, and at 24 h after trauma. Average MAP (A) and HR (B) were measured via carotid catheters for six consecutive hours after trauma in LZ and OZ, and again at 24 h after trauma. There was no significant MAP change from baseline in either the LZ or OZ group for 6 h after trauma, but the day after trauma the MAP of OZ had decreased significantly both compared to baseline OZ levels, and to LZ-Trauma. HR gradually increased over a period of 6 h after trauma, and remained high at 24 h after trauma in both groups of animals. There were no significant differences between LZ and OZ animals at any time points. n = 8 (OZ), 6 (LZ). *P < 0.05 versus baseline, +P < 0.05 versus LZ at 24 h.

Figure 3

Plasma renin activity (PRA) is increased similarly in obese and lean rats 24 h after trauma. PRA was measured via radioimmunoassay in control and trauma LZ and OZ. Both the LZ and OZ PRA was significantly increased after trauma as compared to control values, and no differences were seen between LZ-Trauma and OZ-Trauma animals. n = 6 for all groups. *P < 0.05 versus OZ- and LZ-Control.

Figure 4

Systemic inflammatory response to trauma is more severe in obese than in lean rats. Plasma IL-6 levels (measured via ELISA) were increased significantly in both LZ- and OZ-Trauma animals as compared to control animals. OZ-Trauma rats had IL-6 levels that were significantly higher than in LZ-Trauma rats. n = 6 for all groups. *P < 0.05 versus Control, +P < 0.05 versus LZ-Trauma.

Figure 5

Renal (ED-1-positive cells) inflammatory response to trauma is more severe in obese than in lean rats. ED-1-positive cells (marker of macrophage infiltration) were counted in 20 randomly selected glomeruli in mid-hilar kidney sections from control and trauma LZ and OZ. No differences in ED-1-positive cell numbers were seen between control and trauma LZ, while the numbers were significantly increased in OZ after trauma (A). Representative histological sections can be seen for LZ-Control (B), LZ-Trauma (C), OZ-Control (D), and OZ-Trauma (E) animals. n = 5 for all groups. *P < 0.05 versus Control, +P < 0.05 versus LZ-Trauma.