Table 1.

Anatomical parameters and myofibrillar function

Anatomical parameters	3 days	7 days	21 days	42 days	63 days
Number of animals	N= 11	N= 15	N= 8	N= 5	N= 8
Body weight (g)	2.0 ± 0.1***	4.6 ± 0.1***	11.1 ± 0.1***	18.1 ± 0.3***	24.4 ± 0.8
Heart weight (mg)	12.7 ± 0.3***	29.8 ± 0.7***	75.2 ± 1.2***	110 ± 3***	144 ± 4
Heart weight/body weight (mg g−1)	6.3 ± 0.1	6.5 ± 0.1*	6.8 ± 0.1***	6.1 ± 0.2	5.9 ± 0.1
Myofibrillar function	3 days	7 days	21 days	42 days	63 days
Number of fibres	n= 4	n= 10	n= 10	n= 10	n= 9
Resting tension (mN mm−2)	2.5 ± 0.6	3.3 ± 0.4	3.0 ± 0.4	4.3 ± 0.5	4.2 ± 0.4
Active tension (mN mm−2)	9.4 ± 0.7***	16.8 ± 1.2**	26.3 ± 1.9	29.5 ± 2.1	27.7 ± 1.9
pCa50	5.97 ± 0.03	5.86 ± 0.03	5.91 ± 0.01	5.89 ± 0.01	5.92 ± 0.02
Hill coefficient	2.64 ± 0.5**	2.82 ± 0.3**	4.0 ± 0.3	4.3 ± 0.4	4.7 ± 0.4

Resting tension was recorded in the absence of calcium at a sarcomere length of 2.1 μm (20% stretch over the slack length). Active tension was recorded at maximal calcium concentration (pCa 4.5). pCa–tension relations were determined under isometric conditions by briefly placing each fibre into solutions of increasing calcium concentration until maximal tension was reached. Data were fitted using linearization of the Hill equation for relative tensions. pCa for half-maximal activation (pCa₅₀) and the Hill coefficient were calculated for each fibre by means of linear regression analysis.

^*P < 0.05;

^**P < 0.01;

^***P < 0.001 versus adult (63 days).