Table 1.
Effect of [Pi] on contractile properties of fast (psoas) and slow (soleus) muscle myofibrils and results of the hyperbolic fitting of force-[Pi] relations at different temperatures
| Psoas myofibrils | Soleus myofibrils | ||||
|---|---|---|---|---|---|
| 5°C | 15°C | 15°C | 20°C | ||
| Tension (mN mm−2) | Pi-free | 265 ± 14 (n = 36) | 356 ± 62 (n = 29) | 220 ± 30 (n = 36) | 310 ± 40 (n = 28) |
| kTR (s−1) | Pi-free | 1.9 ± 0.1 (n = 36) | 6.6 ± 1.5 (n = 29) | 2.0 ± 0.1 (n = 36) | 3.2 ± 0.2 (n = 28) |
| 5mM Pi | 6.4 ± 2.4 (n = 7) | 10.0 ± 2.1 (n = 5) | 2.1 ± 0.3 (n = 5) | 3.0 ± 0.4 (n = 5) | |
| kPi(+) (s−1) | 5 mM Pi | 18.2 ± 2.0 (n = 13) | — | 3.2 ± 0.7 (n = 6) | 11.3 ± 0.7 (n = 12) |
| Force- [Pi] curve | Kd,app (mM) | 4.15 ± 0.50 | 4.33 ± 0.36 | 4.37 ± 1.85 | 4.80 ± 1.70 |
| Asymptoteapp | 0.05 ± 0.02 | 0.07 ± 0.02 | 0.44 ± 0.06 | 0.46 ± 0.04 | |
Values of tension, kTR and kPi(+) are means ±s.e.m. obtained in psoas and soleus myofibrils. Hyperbolic fitting of experimental force-[Pi] data: Kd,app and Asymptoteapp obtained from [Pi] giving half the maximal effect and extrapolated value of relative tension for [Pi]→∞, respectively. Asymptoteapp: P < 0.0001 in psoas an soleus myofibrils at the different temperatures tested. Control Pi-free: [Pi] < 5 μM.