Increase in hepatic mitochondrial alpha-glycerophosphate dehydrogenase activity after surgical stress in hyperthyroid rats

Regulation of tissue iodothyronine deiodinase activity in a model of prolonged critical illness

BACKGROUND

The low plasma triiodothyronine (T3) observed during prolonged critical illness can be explained in part by suppressed hepatic deiodinase type I (D1) and increased D3 activity. Infusion of thyrotropin-releasing hormone (TRH) can restore D1 and D3 activity in critically ill rabbits, but it remains unknown whether this is a direct effect of TRH or the TRH-induced rise in circulating thyroxine (T4) and T3.

METHODS

To answer this specific question, burn-injured rabbits randomly received a 4-day treatment with saline, T4, T3, T4+T3, or TRH, started on day 4 of the illness. Plasma iodothyronine concentrations, D1 and D3 activity, and T3-responsive gene expression were quantified in liver and kidney.

RESULTS

Infusion of T4, T3, or TRH increased circulating T3 levels and hepatic D1 activity. Co-infusion of T3 with T4 enhanced T4 to T3 conversion as demonstrated by lower T4, higher T3, and lower reverse T3 (rT3) levels and tended to further increase hepatic D1 activity. Hepatic D1 activity correlated positively with circulating T3 and the T3/rT3 ratio, but not with T4, rT3, or thyroid-stimulating hormone.

CONCLUSIONS

During prolonged critical illness, D1 activity is primarily regulated via changes in circulating T3, suggesting that the low plasma T3 concentrations may be important in sustaining low D1 activity in this condition.

Intestinal mitochondrial dysfunction in surgical stress

BACKGROUND

Surgical stress is associated with altered intestinal function. Our earlier study using a rat model indicated that oxidative stress plays an important role in this process. Since mitochondria are crucial to cellular function and survival and are both a target as well as a source of reactive oxygen species, the present study looks at the changes in enterocyte mitochondria during surgical stress.

METHODS

Surgical stress was induced by opening the abdominal wall and handling the intestine as done during laparotomy. Mitochondria were prepared from the isolated enterocytes at different time periods after surgical stress. The effect of surgical stress on enterocyte mitochondrial ultrastructure, respiration, anti-oxidant enzyme activity, thiol redox status, calcium flux, permeability, and matrix enzymes was then studied.

RESULTS

Surgical stress resulted in alterations in mitochondrial respiration and thiol redox status. It was also associated with altered mitochondrial matrix enzyme activity, decreased superoxide dismutase activity, induction of mitochondrial permeability transition, and swelling, as well as impairment of mitochondrial calcium flux. These alterations were seen at a maximum of 60 min following surgical stress and were reversed by 24 h.

CONCLUSIONS

Laparotomy and mild intestinal handling itself results in enterocyte mitochondrial damage. Since mitochondria are important cellular organelles, this damage can probably lead to compromised intestinal function.

The effects of chronic exposure to supraphysiological concentrations of 3, 5, 3' triiodo-L-thyronine (T3) on cultured GC cells

We have characterized the effects of supraphysiological concentrations of T3 on GC cells, a cultured cell line in which physiologic concentrations of T3 regulate cell growth, protein content, and growth hormone (GH) production. GC cells were exposed to 3 times (1.0 nM) and 80 times (25.0 nM) the physiologic concentration of T3 (0.3 nM) for either 4 d or for greater than 3 months. Both short and prolonged exposure to supranormal T3 concentrations supported maximal cell growth rate and induced significant increases in total protein (P less than 0.025) and GH production (P less than 0.01) per cell when compared to measurements in control GC cells. In addition, exposure to 1.0 nM and 25.0 nM T3 for greater than 3 months enhanced the toxicity of heat shock in a manner similar to previously described effects on GC cells due to T3 exposure of shorter duration. Thus, initial responses to raised T3 concentrations in cultured GC cells persisted without alteration when hormone exposure was prolonged for greater than 3 months.