Table 2.

MHC and activation dependence of phase 3 delayed force development in rat myocardium

%α MHC	Activation level (P/P0)	kdf (s−1)	a	k1 (s−1)	b	k2 (s−1)
0	1.00	14.7 ± 1.0	1.00	15.5 ± 1.2	—	—
59 ± 6	1.00	25.3 ± 2.7*	1.00	26.7 ± 2.9*	—	—
100	1.00	35.4 ± 2.0**	1.00	34.7 ± 2.0**	—	—
0	0.53 ± 0.03	9.1 ± 0.8	0.85 ± 0.04	11.3 ± 0.7	0.15 ± 0.02	1.4 ± 0.3
59 ± 6	0.49 ± 0.05	15.5 ± 2.0*	0.79 ± 0.06	16.8 ± 1.5*	0.21 ± 0.04	2.2 ± 0.4*
100	0.48 ± 0.02	23.4 ± 1.7**	0.82 ± 0.03	26.9 ± 1.5**	0.18 ± 0.02	3.2 ± 0.5**
0	0.27 ± 0.02	5.1 ± 0.5	0.68 ± 0.03	8.7 ± 0.7	0.32 ± 0.03	1.0 ± 0.2
59 ± 6	0.22 ± 0.04	9.5 ± 0.9*	0.63 ± 0.05	11.8 ± 0.9*	0.37 ± 0.04	1.5 ± 0.2*
100	0.23 ± 0.02	15.4 ± 1.5**	0.66 ± 0.03	19.9 ± 1.3**	0.34 ± 0.02	2.3 ± 0.4**

Rate and amplitude data were obtained from force transients in response to stretches of 1% of muscle length at each of the indicated levels of activation (adjusted by varying free [Ca²⁺]). Data in each case are reported as means ±s.e.m. from 10 thyroidectomized and hyperthyroid preparations and seven euthyroid preparations. As described in the Methods, the apparent rate constants for delayed force recovery were obtained by fitting each record with a double exponential equation: y=a exp(−k₁×x) +b exp(−k₂×x), where a is the amplitude of the first exponential phase with rate constant k₁ and b is the amplitude of the second exponential phase with rate constant k₂.

^*Significantly different from thyroidectomized (P < 0.05)

^**significantly different from thyroidectomized and euthyroid (P < 0.05).