Nitric oxide (NO) has been proposed as an intracellular signal in the thyroid. The NO effect on function and morphology of bovine thyroid follicles in culture was analyzed by using the NO donors sodium nitroprusside (SNP) and S-nitrosoglutathione (GSNO). Both NO donors induced a concentration-dependent NO release measured by the nitrite accumulation in the culture medium. The SNP (10 to 500 micromol/L) treatment for 24 hours significantly inhibited the uptake, organification and transport of iodide in a concentration-dependent manner. When SNP (50 micromol/L) was withdrawn from the culture medium after 24 hours' incubation, iodide uptake and organification were partially recovered at 24 hours and reached the control value at 48 hours, indicating a reversible effect of SNP. A possible involvement of cyanide in the SNP inhibitory effect was excluded because incubation of follicles with potassium cyanide (KCN) at concentrations estimated to be present in the medium (40 and 80 micromol/L) for 24 hours did not modify iodide uptake and organification. The GSNO (10 to 500 micromol/L) treatment for 24 hours also reduced the iodide uptake, organification and transport in a concentration-dependent manner. A significant inhibition of iodide organification was induced after incubation with 1000 micromol/L of N2, 2'-O-dibutyrylguanosine 3':5'-cyclic monophosphate ([Bu]2cGMP). Morphological evaluation by light microscopy revealed that the incubation with NPS or GSNO (500 micromol/L) produced cellular dispersion with loss of follicular cell aggregates that was evident at 96 hours exposure. Cell viability was not altered by 10-500 micromol/L SNP or GSNO (80% to 85%). We concluded that long-term NO exposure induces functional and morphological modifications compatible with a loss of differentiation in thyroid follicles. These observations further support a role of NO in the regulation of the thyroid function.

These findings favor a long-term inhibitory role of the NO/cGMP pathway on parameters of thyroid hormone biosynthesis. A novel property of NO to inhibit TPO and TG mRNA expression is supported. The NO action on iodide uptake could involve cGK mediation. The long-term inhibition of steps of thyroid hormonogenesis by NO could be of interest in thyroid pathophysiology.

Triiodothyronine (T3) is involved in the regulation of the growth hormone-insulin-like growth factor I (GH-IGF-I) axis. In this study we investigated the effect of GH and IGF-I on the metabolic response of T3 in target tissues by evaluating the activity of two T3-dependent liver enzymes: mitochondrial alpha-glycerophosphate dehydrogenase (alpha-GPD) and cytosolic malic enzyme (ME) in rat hepatocytes in primary culture. Growth hormone (35 nmol/l) as well as IGF-I (0.5 mumol/l) reduced alpha-GPD and ME activities (p < 0.01) compared to the control group. Timecourse studies indicated that IGF-I (1.5 mumol/l) significantly decreased alpha-GPD and ME activities (P < 0.01) after 24 h, whereas the effect of GH (35 nmol/l) was recorded only after 36 h (p < 0.01). This delayed effect of GH compared to IGF-I suggested the possibility that the effect of GH could be mediated by IGF-I synthesis. To test this hypothesis, the effect of GH on the two enzyme activities was studied in the presence of anti-IGF-I antibodies. A gradual recovery of alpha-GPD and ME activities (p < 0.01) was observed in the presence of GH (35 nmol/l) plus increasing concentrations of anti-IGF-I antiserum. The maximal alpha-GPD and ME activities attained after the incubation of the liver cells with 1 mumol/l T3, a concentration high enough to fully saturate the nuclear T3 receptors for 24 h, were lowered significantly by 1.0 mumol/l IGF-I (p < 0.01). This finding suggests that the IGF-I effect might be independent of the saturation of the nuclear T3 receptors. In conclusion, in cultured rat hepatocytes, GH and IGF-I reduced the metabolic response of T3 evaluated by two liver T3-dependent enzyme activities. The effect of GH was mediated at least in part by IGF-I.

The integration of innate and adaptive immune responses is required for efficient control of Candida albicans. The present work aimed to assess, at the local site of the infection, the immunocompetence of macrophages in rats infected intraperitoneally with C. albicans and exposed simultaneously to stress during 3 days (CaS group). We studied the 1) ability to remove and kill C. albicans, 2) tumor necrosis factor-alpha (TNF-alpha) release, 3) balance of the inducible enzymes NO synthase (iNOS) and arginase, and 4) expression of interleukin (IL)-1beta and IL-1 receptor antagonist (ra) mRNA. Compared with only infected animals (Ca group), the number of colony-forming units was significantly higher in CaS rats (P < 0.01), and the macrophage candidicidal activity was approximately 2.5-fold lower (P < 0.01). Release of TNF-alpha was diminished in both unstimulated and heat-killed C. albicans restimulated macrophages of the CaS group (Ca vs. CaS, P < 0.03 and P < 0.05, respectively). In Ca- and CaS-group rats, the rates for both the arginase activity and the NO synthesis were significantly enhanced. However, the stress exposure downregulated the activity of both enzymes (CaS vs. Ca, P < 0.05). After in vitro restimulation, the IL-1ra/IL-1beta ratio was significantly diminished in CaS-group rats (P < 0.05). Our results indicate that a correlation exists between early impairment of macrophage function and stress exposure.