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Department of Orthopedics, University of Maryland School of Medicine, Baltimore, Maryland 21201
In a series of experiments,
cultured myotubes were exposed to passive stretch or pharmacological
agents that block contractile activation. Under these experimental
conditions, the formation of Z lines and A bands (morphological
structures, resulting from the specific structural alignment of
sarcomeric proteins, necessary for contraction) was assessed by
immunofluorescence. The addition of an antagonist of the voltage-gated
Na+ channels [tetrodotoxin (TTX)] for 2 days in
developing rat myotube cultures led to a nearly total absence of Z
lines and A bands. When contractile activation was allowed to resume
for 2 days, the Z lines and A bands reappeared in a significant way.
The appearance of Z lines or A bands could not be inhibited nor
facilitated by the application of a uniaxial passive stretch.
Electrical stimulation of the cultures increased sarcomere assembly
significantly. Antagonists of L-type Ca2+ channels
(verapamil, nifedipine) combined with electrical stimulation led to the
absence of Z lines and A bands to the same degree as the TTX treatment.
Western blot analysis did not show a major change in the amount of
sarcomeric 
-actinin nor a shift in myosin heavy chain phenotype as a
result of a 2-day passive stretch or TTX treatment. Results of
experiments suggest that temporal Ca2+ transients play an
important factor in the assembly and maintenance of sarcomeric
structures during muscle fiber development.
muscle; development; Z lines; A bands
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