The double knockout mouse for utrophin and dystrophin (utr(-/-)/mdx) has been proposed to be a better model of Duchenne Muscular Dystrophy (DMD) than the mdx mouse because the former displays more similar muscle pathology to that of the DMD patients. In this paper the properties of action potentials (APs) and Ca(2+) transients elicited by single and repetitive stimulation were studied to understand the excitation-contraction (EC) coupling alterations observed in muscle fibers from mdx and utr(-/-)/mdx mice. Based on the comparison of the AP durations with those of fibers from wild-type (WT) mice, fibers from both mdx and utr(-/-)/mdx mice could be divided in two groups: fibers with WT-like APs (group 1) and fibers with significantly longer APs (group 2). Although the proportion of fibers in group 2 was larger in utr(-/-)/mdx (36%) than in mdx mice (27%), the Ca(2+) release elicited by single stimulation was found to be similarly depressed (32-38%) in utr(-/-)/mdx and mdx fibers compared with WT counterparts regardless of the fiber's group. Stimulation at 100 Hz revealed that, with the exception of those from utr(-/-)/mdx mice, group 1 fibers were able to sustain Ca(2+) release for longer than group 2 fibers, which displayed an abrupt limitation even at the onset of the train. The differences in behavior between fibers in groups 1 and 2 became almost unnoticeable at 50 Hz stimulation. In general, fibers from utr(-/-)/mdx mice seem to display more persistent alterations in the EC coupling than those observed in the mdx model.
