Effects of exercise on muscle activation and metabolism in multiple sclerosis

We investigated the role of metabolism in muscle fatigue during voluntary exercise in persons with mild multiple sclerosis (MS). Six MS and 8 healthy control subjects performed intermittent, progressive, isometric contractions of the ankle dorsiflexors, during which we measured maximum voluntary force (MVC), inorganic phosphate (Pi), phosphocreatine (PCr), and pH. During exercise. MVC fell sooner in MS, but by the end of exercise the relative decrease in MVC was similar in both groups. In contrast, at the end of exercise Pi/PCr increased to 1.86 ± 0.22 in controls but to only 0.66 ± 0.04 in MS (P < 0.01); likewise, pH was 6.75±0.04 in controls and unchanged (7.06 ± 0.04) in MS (P <0.01). The smaller metabolic change at the same relative exercise intensity suggests a failure of muscle activation that is present even in mild MS. Neurophsyiologic measures of activation indicated some central activation failure and no neuromuscular junction impairment in MS, and suggested that activation failure beyond the muscle membrane(excitation–contraction coupling) may be important in MS. We conclude that metabolic factors do not play a significant role in the development of muscle fatigue during voluntary exercise in mild MS. © 1994 John Wiley & Sons, Inc.     

Isolated human hepatocytes in culture display markedly different gene expression patterns depending on attachment status

In vitro human hepatocyte cultures are a key tool in the investigation of xenobiotic toxicity and metabolism. In most in vitro hepatocyte studies, the cells are allowed to adhere to an extracellular matrix, such as collagen. Unfortunately, the ability of freshly isolated hepatocytes to adhere to collagen varies from donor to donor. We used microarray analysis to determine what gene expression differences exist between hepatocytes in suspension and hepatocytes attached to collagen. Results from different donors showed a considerable difference in gene expression patterns between the two hepatocyte populations. In addition, we also compared the gene expression profiles of hepatocytes in culture with liver tissue. The results showed that both hepatocytes in suspension and hepatocytes attached to collagen display significant gene expression differences compared with liver tissue. Finally, we show that both populations of hepatocytes are responsive to dexamethasone and regulate some of the same genes. Overall, our results suggest that either significant gene expression changes occur in isolated hepatocytes or that suspended and attached cells represent different populations of hepatocytes found in intact livers.     

CNP-induced changes in pHi, cGMP/cAMP and mRNA expression of natriuretic peptide receptors in human trabecular meshwork cells.

It has been demonstrated that natriuretic peptides lower intraocular pressure, however, the underlying cellular mechanism(s) mediating this response remain(s) to be determined. The purpose of this study was to investigate the effects of C-type natriuretic peptide (CNP) on pH(i), cGMP/cAMP and expression of atrial natriuretic peptide receptor (NPR-A), brain natriuretic peptide receptor (NPR-B) and C-type natriuretic peptide receptor (NPR-C), in HTM cells. At concentrations of 10(-7) M, CNP caused an acidification of pH(i). In addition, CNP caused a dose-dependent increase in cGMP formation and inhibition of forskolin-stimulated cAMP accumulation. These changes were not significantly altered in the absence of 10(-3) M isobutylmethylxanthine (IBMX). Treatment with the NPR-A antagonist, anantin, produced no influence on basal cGMP/cAMP levels, the CNP-stimulated cGMP accumulation and CNP-induced inhibition of forskolin-stimulated cAMP accumulation. However, CNP-induced reduction of forskolin-stimulated cAMP accumulation was inhibited by pretreatment with pertussis toxin (PTX). Furthermore, NPRB receptors were predominantly expressed and pretreatment with CNP (10(-7) M, 24hr) enhanced all NPR mRNAs expression which was not altered by higher concentrations or longer incubation. Results demonstrate that NPR-A, NPR-B and NPR-C receptors' expression can be up-regulated by CNP treatment. CNP activates NPR-B receptors preferentially to increase cGMP accumulation and acts through the PTX-sensitive cAMP-signaling pathway leading to a decrease in pH(i).