Effects of acidification and increased extracellular potassium on dynamic muscle contractions in isolated rat muscles

Abstract

Since accumulation of both H(+) and extracellular K(+) have been implicated in the reduction in dynamic contractile function during intense exercise, we investigated the effects of acidification and high K(+) on muscle power and the force-velocity relation in non-fatigued rat soleus muscles. Contractions were elicited by supramaximal electrical stimulation at 60 Hz. Force-velocity (FV) curves were obtained by fitting data on force and shortening velocity at different loads to the Hill equation. Acidification of the muscles by incubation with up to 24 mm lactic acid produced no significant changes in maximal power (P(max)) at 30 °C. More pronounced acidification, obtained by increasing CO(2) levels in the equilibration gas from 5% to 53%, markedly decreased P(max) and maximal isometric force (F(max)), increased the curvature of the FV relation, but left maximal shortening velocity (V(max)) unchanged. Increase of extracellular K(+) from 4 to 10 mm caused a depression of 58% in P(max) and 52% in F(max), but had no significant effect on V(max) or curvature of the FV curve. When muscles at 10 mM K(+) were acidified by 20 mm lactic acid, P(max) and F(max) recovered completely to the initial control level at 4 mm K(+). CO(2) acidification also induced significant recovery of dynamic contractions, but not entirely to control levels. These results demonstrate that in non-fatigued muscles severe acidification can be detrimental to dynamic contractile function, but in muscles depolarised by exposure to high extracellular [K(+)], approaching the [K(+)] level seen during intense fatiguing exercise, acidification can have positive protective effects on dynamic muscle function.

Figure 1. Contraction protocol for determination of isometric force and FV curves

The lower part shows the time course of the whole experiment. Muscles were initially incubated for at 60–70 min in standard buffer (4 mm K⁺). During the last 30–40 min (indicated by thick line) 8 brief contraction protocols using different holding forces were elicited to record data for the force–velocity relationship. Thereafter the incubation buffer was changed as indicated. The inset shows a sample force (dotted line) and length trace from one contraction protocol where the holding force was reduced to a level below initial isometric contraction. The part of the length curve that is used for determination of velocity is indicated (V).

Figure 2. Effect of 24 mm lactic acid on the force–velocity curve of rat soleus at 30°C

A, force–velocity curves fitted to data points from a representative muscle incubated at 4 mm K⁺ without lactic acid (pH 7.45, control), 4 mm K⁺ with 24 mm lactic acid (pH 6.8) and finally at 4 mm K⁺ again (pH 7.45, post control). B, same data as above but expressed as power versus velocity and fitted to power–velocity curves using the parameters obtained from the Hill fits in the top panel.

Figure 3. Effect of 24 and 53% CO₂ on the force–velocity curve of rat soleus at 30°C

A, force–velocity curves fitted to data points from representative muscles incubated at 4 mm K⁺ using 5% CO₂ in the equilibration gas (pH 7.45, control), followed by 24% CO₂ (pH 6.9, left panel) or 53% (pH 6.5, right panel) in the equilibration gas and then again using 5% CO₂ (pH 7.45, post control). B, same data as above but expressed as power versus velocity and fitted to power–velocity curves using the parameters obtained from the Hill fits in A.

Figure 4. Effects of 10 mm K⁺ on FV curves and power–velocity curves in soleus muscles before and after addition of 20 mm lactic acid

A, force–velocity curves fitted to data points from a representative muscle incubated at 4 mm K⁺ (control), 10 mm K⁺, 10 mm K⁺ with 20 mm lactic acid, and finally at 4 mm K⁺ again (post control). B, same data as above expressed as power versus velocity and fitted to power–velocity curves using the parameters obtained from the Hill fits in the top panel.

Figure 5. Effects of 10 mm K⁺ on dynamic contractile variables in soleus muscles before and after addition of 20 mm lactic acid or 24% CO₂

P_max, V_max and curvature were obtained from Hill fits of force–velocity data obtained during the indicated incubations. F_max is average isometric force measured during the indicated incubations. All values are expressed as a percentage of the control level obtained at 4 mm K⁺. Bars represent means of 13 (10 mm K⁺), 8 (10 mm K⁺+ 20 mm lactic acid) or 5 (10 mm K⁺+ 24% CO₂) muscles with error bars indicating standard error of means. ^aSignificant different from control conditions (P < 0.05); ^bsignificant different from control conditions and from 10 mm K⁺.

Figure 6. Relation between P_max and F_max in soleus muscles incubated at 9 or 10 mm K⁺

Data points from 18 individual muscles incubated at high K⁺ are plotted as a percentage of control levels obtained at 4 mm K⁺. Dotted line represents best linear fit of the data points (r= 0.98).