Fig. 1.

Fig. 1.

Absence of DHPR tetrad restoration in β₄-expressing relaxed myotubes. (A) Freeze-fracture replicas of peripheral couplings in tail myotomes of 27- to 30-hpf zebrafish. Control myotomes (Top) show arrangement of DHPR particles in tetrads (center indicated by red dots), organized in orthogonal arrays. In β₄-expressing relaxed zebrafish (Bottom) DHPR tetrads show a lack of tetrad formation. (Scale bar, 50 nm.) (B) Quantification of voltage dependence of cytoplasmic Ca²⁺ transients yielded (ΔF/F₀)_max values that are significantly lower (P < 0.001) in β₄ (n = 13)- compared to β_1a (n = 9)-expressing relaxed myotubes. ΔF/F₀ values recorded from untransfected relaxed myotubes were below detection level (n = 10). (C) Similarly, plots of voltage dependence of the integral of the ΔF/F₀ transients in response to 200-ms test depolarizations indicate a highly significant difference (P < 0.001) in the total amount of Ca²⁺ released between relaxed myotubes expressing β_1a (n = 9) or β₄ (n = 12) subunit. (Right) Representative ΔF/F₀ recordings from relaxed myotubes expressing β_1a or β₄. (Scale bars, 50 ms [horizontal], ΔF/F₀ = 1 [vertical].) Error bars indicate SEM. P determined by unpaired Student’s t test.