FIGURE 6.

Dye and electrical coupling is absent in cx39.9 mutant slow muscle. A–D, sulforhodamine dye was loaded into a single slow muscle cell. Dye coupling was seen in wild-type (A and D) but not cx39.9 mutant (B and E) nor wild-type fibers exposed to the gap junction blocker heptanol (C and F). G and H, a voltage ramp from −50 mV to 0 mV over 1 s was generated in a slow muscle cell (command cell). Membrane currents in the command cell and an adjacent slow muscle cell were recorded in wild-type embryos (G) and cx39.9 mutants (H). No electrical coupling was detected in mutants. Note that currents are considerably smaller in mutants as a result of the higher input resistance of the mutant cells, which is also consistent with the absence of coupling in slow muscles.