Figure 3.

Contractile properties, action potentials and Ca²⁺ transients in control and β₁-null tissues. (a) Electrical field stimulation induced contractions in forelimb from control fetus. Contraction is seen in response to each stimulus. (b) as in a except from β₁-null fetus. Stimulus fails to produce contraction. (c and d) Caffeine (25 mM) addition to the bath solution induced contraction in control (c) and β₁-null (d) tissue. The response in the β₁-null tissue is 20-fold lower than the control tissue. (e and f) Action potentials can be induced in both control (e) and β₁-null (f) myotubes and are tetrodotoxin sensitive. (g and h) Action potentials (Inset) induce a Ca²⁺ transient in control myotubes (g), but fail to induce a Ca²⁺ transient in myotubes from β₁-null (h) fetuses.