Reduced cardiac muscle power with low ATP simulating heart failure

Figure 1. Mechanical protocols. (A) Measurement of force-velocity relation. Activated muscle with length set to 10% above the baseline length was rapidly shortened by 2%–6% (upper panel), which resulted in a rapid drop of force (lower panel). Muscle length was then shortened at constant velocity, and the corresponding muscle force was assessed when the muscle length shortened to the baseline length (arrowheads on force traces). (B) Measurement of the time constant of tension redevelopment (ktr). Activated muscle at the baseline length (Lo) was rapidly shortened to 0.8 Lo for 20 ms, briefly stretched to 5% above Lo, and then reset to Lo (upper panel). This protocol reduced force to close zero (middle panel). Muscle force redeveloped exponentially with a time constant ktr that was determined by fitting the data to Eq. 2 (red trace in lower panel).

Figure 2. Representative example force-velocity data from individual trabeculae. (A) Shortening velocity versus muscle force measured during isovelocity contractions with MgATP levels set to 8 mM, 4 mM, or 2 mM. Lines show fits to the Hill equation. (B) The same data from (A) transformed to contraction power (force multiplied by velocity) versus shortening velocity. Lines are based on the fits shown in (A).

Figure 3. Low MgATP reduced contraction power. Maximum power (W/L) was significantly reduced by low MgATP level in paired experiments comparing 8 mM with 4 mM (left panel) or 8 mM with 2 mM (middle panel). Values in each pair are connected by a line, and for each pair the first measured condition is indicated by an open symbol (∗p < 0.05, ∗∗p < 0.01, paired t-test). The mean value per group (± SE, n = 6) at each ATP level is also shown. For all experiments (right panel), there was a significant relationship between contraction power and MgATP level.

Figure 4. Low MgATP increased the curvature of the force-velocity relation. Low MgATP resulted in greater curvature of the force-velocity relation (lower values of a/Po) in paired experiments comparing 8 mM with 4 mM (left panel) or 8 mM with 2 mM (middle panel) (∗∗p < 0.01, paired t-test). The mean value per group (± SE, n = 6) at each ATP level is also shown. For all experiments (right panel), there was a significant relationship between a/Po and MgATP level.

Figure 5. Low MgATP did not reduce maximum shortening velocity. Maximum shortening velocity (muscle lengths s−1) was unchanged by low MgATP level in paired experiments comparing 8 mM with 4 mM (left panel) or 8 mM with 2 mM (middle panel). The mean value per group (± SE, n = 6) at each ATP level is also shown. For all experiments (right panel), there was no significant relationship between maximum shortening velocity and MgATP level.

Figure 6. Effect of low MgATP on maximum force. Maximum force developed per muscle area (stress, kPa) was unchanged in paired experiments comparing 8 mM with 4 mM MgATP (left panel) but increased in paired experiments comparing 8 mM with 2 mM MgATP (right panel) (∗p < 0.05, paired t-test). The mean value per group (± SE, n = 6) at each ATP level is also shown.

Figure 7. Low MgATP slowed force development kinetics. (A) Example traces of tension redevelopment after mechanical disruption of cross-bridges. Low MgATP level slowed the time course of tension redevelopment (decreased the rate constant of tension redevelopment, ktr). (B) ktr was significantly reduced by low MgATP level in paired experiments comparing 8 mM with 4 mM (left panel) or 8 mM with 2 mM (middle panel) (∗p < 0.05, ∗∗p < 0.01, paired t-test). The mean value per group (± SE, n = 6) at each ATP level is also shown. For all experiments (right panel), there was a significant relationship between ktr and MgATP level.

Figure 8. Pooled data for all experiments. (A) Low MgATP reduced contraction power. For all experiments, shortening velocity versus muscle force was measured during isovelocity contractions with MgATP levels set to 8 mM (n = 12), 4 mM (n = 6), or 2 mM (6) (left panel). Lines show fits to the Hill equation. Right panel shows the same data transformed to contraction power (force multiplied by velocity) versus shortening velocity. Lines are based on the fits shown in the left panel. Error bars indicate SE (when the SE exceeds the symbol size). (B) Low MgATP slowed cross-bridge kinetics. Low MgATP level reduced both maximum power and ktr. There was a significant relationship between maximum power and ktr.

Figure 9. Simulation of muscle dynamics. (A) Schematic of cross-bridge dynamics model. This model was adapted from Marzban and Beard (21), which integrates elements of the model of Tewari et al. (5,22) with the model of transition between super-relaxed (SRX) and non-super-relaxed (non-SRX) states of Campbell et al. (23). The state USR represents myosin heads in the SRX state. The variable P represents the proportion of myosin heads that are in the permissive states (adjacent to calcium-activated myosin-binding site). The states A1, A2, and A3 represent loosely bound, strongly bound (pre-ratcheted), and post-ratcheted states. For skinned-fiber simulations considered here we assume saturating calcium conditions, and thus no non-permissive (non-calcium bound) state is included in the model formulation. (B) Fit of steady-state model predictions of force versus sliding velocity to data obtained at [MgATP] = 8, 4, and 2 mM.