Partial opening and subconductance gating of mechanosensitive ion channels in dystrophic skeletal muscle

Key points • Mechanosensitive (MS) channels in skeletal muscle are heteromeric proteins composed of TRPV4 and other subunits. Experiments investigated subconductance fluctuations associated with gating of independent subunits. • MS channels open by passing through subconductance states and fluctuate between at least four subconductance levels. • MS channels in mdx muscle spend ∼60% of the time at smaller subconductance levels; applying pressure to the membrane increases occupancy of the fully open state. Channels in wild‐type muscle spend more time at the open state. • Recordings show partial opening of MS channels that fail to reach the open state. Partial openings occur ∼3–4 times more frequently in mdx than wild‐type muscle. • During single activations, burst durations and charge/burst are greater in mdx than in wild‐type muscle. • Partial opening and prolonged burst duration are distinct mechanisms contributing to excess Ca2+ entry through MS channels in dystrophic muscle We recorded the activity of single mechanosensitive (MS) ion channels in skeletal muscle from the mdx mouse, a deletion mutant that lacks the cytoskeletal protein, dystrophin. Experiments were designed to examine the influence of dystrophin, a major component of skeletal muscle costameres, on the behaviour of single MS channels. In the majority of recordings from cell‐attached patches, MS channels have a conductance of ∼23 pS. Recordings from some patches, however, showed a smaller conductance channel of ∼7–14 pS. Large and small conductance channels were detected in a single patch and showed serial, non‐random gating, suggesting different opening levels of a single channel. Analysis of the distribution of current amplitudes within the open channel showed MS channels fluctuate between subconductance levels. MS channels in dystrophic muscle spend ∼60% of the time at smaller subconductance levels, often failing to reach the fully open level. Applying pressure to the membrane of mdx fibres increases in a graded manner occupancy of the fully open state, while reducing occupancy of subconductance levels. Recordings also show partial openings of MS channels in both wild‐type and mdx muscle that fail to reach the fully open state. Partial openings occur at a higher frequency in mdx muscle and reflect occupancy of subconductance levels seen during complete activations. In muscle from mdx/utrn−/− double knockout mice, MS channels also spend more time at subconductance levels than the f