The transverse tubular system (t-system) of cardiac muscle is a structure
that allows rapid propagation of excitation into the cell interior.
Using 2-photon molecular excitation microscopy and digital image-processing
methods, we have obtained a comprehensive overview of the t-system
of rat ventricular myocytes in living cells. We show that it is possible
to quantify the morphology of the t-system in terms of average local
tubule diameter, branching pattern, and local abundance of the t-system
by immersing living myocytes in a dextran-linked fluorescein solution.
Our data suggest that previous electron microscopic examinations
of t-system structure have underestimated both the geometric complexity
of the t-system morphology and the fraction of cell volume occupied
by the t-system (3.6\% in this species). About 40\% of tubules occur
between Z-lines, and the t-tubule diameter is 255+/-0.85 nm (mean+/-SEM).
The t-tubules leave the outer surface of the cell in an approximately
rectangular array; however, at some points junctions between the
t-tubules and the surface membrane are missing. In view of the complexity
of the t-system apparent from our images, we propose that the t-system
be renamed the "sarcolemmal Z rete." The methods presented here are
generally applicable to the quantification of the sarcolemmal Z rete
and other structures within cells by fluorescence microscopy in a
variety of cell types.