Next, the average surface area of a... | Rockefeller University Press

Next, the average surface area of a single t-tubule from this unit volume was calculated using the t-tubule perimeter measurements from electron micrographs (see Fig. 3). The inner t-tubule perimeter, p_i, was defined as the measured perimeter (see Table 2) minus an outer margin of 5 nm to account for the inner leaflet thickness (Andersson-Cedergren, 1959). The average surface area of a single t-tubule in the unit volume, with side length,

l,

of 10 µm, was equal to

p_{i} l .

From this value, surface area of the transverse t-tubule elements (TTS_trans) per unit volume was calculated as

Table 2.

Junctional t-tubule ultrastructure from FDB and soleus skeletal muscle fibers

	Perimeter (nm)	Maximum diameter (nm)	Minimum diameter (nm)	Mean diameter (nm)	Cross-sectional area (nm²)	Major to minor t-tubule axes ratio
Control FDB	247.2 ± 6.1	107.7 ± 3.1	24.5 ± 0.7	52.4 ± 0.9	2,498.1 ± 86.5	2.2 ± 0.1
R6/2 FDB	210.8 ± 6.2	94.6 ± 3.1	21.2 ± 0.8	46.1 ± 0.9	1,876.1 ± 88.0	2.2 ± 0.1
P value^a	0.0004	0.0253	0.0226	0.0007	2.52 × 10⁻⁵	0.99
Control soleus	250.2 ± 7.6	107.1 ± 3.9	27.1 ± 0.9	54.2 ± 1.2	2,719.4 ± 107.5	1.9 ± 0.1
Control soleus versus FDB P value^a	0.99	0.99	0.047	0.652	0.347	0.064
Control soleus versus R6/2 FDB P value^a	0.0006	0.067	0.004	0.0002	7.23 × 10⁻⁷	0.125

	Perimeter (nm)	Maximum diameter (nm)	Minimum diameter (nm)	Mean diameter (nm)	Cross-sectional area (nm²)	Major to minor t-tubule axes ratio
Control FDB	247.2 ± 6.1	107.7 ± 3.1	24.5 ± 0.7	52.4 ± 0.9	2,498.1 ± 86.5	2.2 ± 0.1
R6/2 FDB	210.8 ± 6.2	94.6 ± 3.1	21.2 ± 0.8	46.1 ± 0.9	1,876.1 ± 88.0	2.2 ± 0.1
P value^a	0.0004	0.0253	0.0226	0.0007	2.52 × 10⁻⁵	0.99
Control soleus	250.2 ± 7.6	107.1 ± 3.9	27.1 ± 0.9	54.2 ± 1.2	2,719.4 ± 107.5	1.9 ± 0.1
Control soleus versus FDB P value^a	0.99	0.99	0.047	0.652	0.347	0.064
Control soleus versus R6/2 FDB P value^a	0.0006	0.067	0.004	0.0002	7.23 × 10⁻⁷	0.125

Summary of the t-tubule ultrastructure in FDB control, FDB R6/2, snd Soleus control muscle fibers. Values shown as mean ± standard error, n = FDB Control (six mice, 18 fibers, and 765 t-tubules), R6/2 (six mice, 18 fibers, and 872 t-tubules), and soleus control (four mice, 12 fibers, and 441 t-tubules).

Significance measured with linear mixed-effect model.

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\frac{T T S_{t r a n s}}{u n i t v o l} = n p_{i} l .

(2)

The total surface area of the TTS (SA_TTS) per unit volume is found by dividing the surface area of the TTS_trans per unit volume by the fraction of the transverse pixel elements from Table 1.

\frac{S A_{T T S}}{u n i t v o l} = (\frac{\frac{T T S_{t r a n s}}{u n i t v o l}}{\frac{% t r a n s v e r s e p i x e l s}{% t o t a l p i x e l s}}) .

(3)

The surface area of the longitudinal t-tubule elements is the difference of the SA_TTS and the TTS_trans: