Table 2.

Overview of the effects of P₁ and pH on ATPase activity and force development

30 mM P1	Control (0 P1)(%)	30 mM P1(%)	Relative change(%)
Total ATPase activity	100	82 ± 3	−18 ± 3*
AM ATPase activity	58 ± 2	46 ± 3	−21 ± 6*
SR ATPase activity	42 ± 2	38 ± 4	−9 ± 5 (P = 0.06)
Isometric force	100	38 ± 2	−62 ± 2*

pH 6.2	Control (pH 7.1) (%)	pH 6.2(%)	Relative change(%)
Total ATPase activity	100	72 ± 2	−28 ± 2*
AM ATPase activity	65 ± 2	59 ± 4	−10 ± 4 (P = 0.07)
SR ATPase activity	35 ± 2	14 ± 2	−61 ± 5*
Isometric force	100	79 ± 5	−21 ± 5*

30 mM P1, pH 6.2			Relative change
	Control(0 P1, pH 7.1) (%)	30 mM P1 pH 6.2(%)	Experimental(%)	Calculated(%)
Total ATPase activity	100	47 ± 2	−53 ± 2*	−41 ± 3†
AM ATPase activity	59 ± 2	40 ± 3	−32 ± 7*	−29 ± 3
SR ATPase activity	41 ± 2	8 ± 2	−80 ± 6*	−64 ± 5†
Isometric force	100	37 ± 3	−63 ± 3*	−70 ± 3†

The mean values for the total ATPase activity per volume, and force per cross-sectional area were, respectively, 0.26 ± 0.01 mM s⁻¹ and 59 ± 3 kN m⁻¹ (n = 44). The AM and SR ATPase activities are expressed relative to the total ATPase activity without BDM and added P₁. The relative changes were calculated from individual values relative to the bracketing control values. Values are means ± S.E.M. from 10 (30 mM P₁), 6 (pH 6.2) and 8 (30 mM P₁, pH 6.2) different preparations. The calculated relative change with 30 mM P₁ at pH6.2 was obtained from the product of the effects of 30 mM P₁ and pH 6.2 alone.

^*P < 0.05 compared with control

^†no overlap between experimental and calculated values ± s.e.m.