Endocrine Abstracts

Background: Acidosis activates the hypothalamic-pituitary-adrenal (HPA) axis and induces glucocorticoid-mediated atrophy of skeletal muscle. The enzyme 11beta-hydroxysteroid dehydrogenase type 1 (11β-HSD1) converts inactive cortisone to active cortisol and modulates glucocorticoid signalling locally within skeletal muscle. Here, we address a gap in knowledge how acidosis affects 11β-HSD1 activity in human skeletal muscle cells.

Methods: Quadriceps muscle tissues were acquired from consented adult patients undergoing elective joint replacement for osteoarthritis. Myoblasts were isolated, cultured in vitro and differentiatedto form multinuclear myotubes. Myotubes were incubated in media with added HCl or NaOH for pH adjustment (pH range 6.8 - 7.6) for 48 hours. 11β-HSD1 enzymatic activity was measured directly using radiolabelled cortisone and thin-layer chromatography, and normalised to total protein. Furthermore, effects of acidosis on cortisone-induced gene expression were assessed by rtPCR. All experiments were repeated in cells from 3-4 different muscle tissue donors.

Results: Muscle tissue donors were 57 – 84 years old and 2/6 were female. There was a significant trend for decreasing 11β-HSD1 activity with decreasing pH over a range from 6.8 to 7.6 (P < 0.01). Acidosis at pH 7.1 caused no significant change in gene expression of 11β-HSD1 or the co-factor enzyme H6PD compared to the control condition at pH 7.4. Interestingly, cortisone-induced mRNA expression of the catabolic genes FOXO1 and TRIM63 was diminished in the presence of acidosis compared to the control condition (FOXO1: P < 0.05; TRIM63: P < 0.01).

Conclusion: Acidosis reduced glucocorticoid activation by 11β-HSD1 in human skeletal muscle cells. Furthermore, acidosis diminished cortisone-induced activation of catabolic genes, a downstream effect of glucocorticoid activation by 11β-HSD1 function. These results suggest that reduced 11β-HSD1 activity locally in skeletal muscle may counteract effects of systemic HPA axis activation in acidotic conditions. The mechanism how acidosis changes 11β-HSD1 activity requires further investigation.