S

Control Total

Muscle length

{Contractility Passive

Muscle length

I Contractility

I Contractility

{Contractility

Afterload (force)

FIGURE 11 The mechanical responses of cardiac muscle. Note the qualitatively similar shapes of the length-passive force, length-total force, and afterload-velocity curves to those of skeletal muscle (see Figs. 9 and 10). (A) The length-passive force curve indicates that there is significant passive force at Lo. Also, note that there is more than one total force curve, due to the presence of more than one state of contractility. Thus, at any given length, active force can vary. (B) The afterload-force curves indicate that the entire relationship, including Vmax, can be shifted by changes in contractility. Thus, for any given afterload, at any given initial length velocity can vary.

{Contractility

Afterload (force)

FIGURE 11 The mechanical responses of cardiac muscle. Note the qualitatively similar shapes of the length-passive force, length-total force, and afterload-velocity curves to those of skeletal muscle (see Figs. 9 and 10). (A) The length-passive force curve indicates that there is significant passive force at Lo. Also, note that there is more than one total force curve, due to the presence of more than one state of contractility. Thus, at any given length, active force can vary. (B) The afterload-force curves indicate that the entire relationship, including Vmax, can be shifted by changes in contractility. Thus, for any given afterload, at any given initial length velocity can vary.

Although there are many similarities in mechanical function between cardiac and skeletal muscle, there are some important quantitative and qualitative differences. Quantitatively, cardiac muscle exhibits greater passive force at all lengths. At Lo, skeletal muscle exhibits almost no passive force, whereas cardiac muscle exhibits passive force equal to approximately 20% of the total force. This difference probably is related to differences in connective tissue composition. Qualitatively, cardiac muscle can exhibit moment-to-moment differences in: (1) the isometric force developed at any given length, (2) the velocity of shortening against an afterload at any given muscle length, and (3) the maximum velocity (Vmax) of an isotonic contraction.

Such changes in Vmax, in isometric force, and in velocity of afterloaded contractions that occur at constant thick and thin filament overlap are called changes in contractility. Some changes in contractility are due to differences in the amounts of calcium that are released with each action potential. As stated above, approximately the same level of cytosolic calcium is reached in skeletal muscle in response to each action potential, and this level of calcium is more than enough to activate fully the contractile proteins. This is not the case in cardiac muscle. Both the influx and efflux of calcium during the cardiac action potential and the amount of calcium released from the cardiac SR can change from contraction to contraction. Thus, cytosolic calcium levels can fluctuate from contraction to contraction. This means that during some contractions not all the crossbridges that proximate actin binding sites will be cycling because the binding sites will still be protected by tropomyosin-troponin complexes that are not binding calcium. Alterations in the number of cycling cross-bridges will lead to differences in isometric force and in velocities of shortening against afterloads. The strength and velocity of crossbridge interaction also are influenced by phosphorylation of contractile proteins brought about through the actions of norepinephrine and epinephrine.

In summary, the force of cardiac muscle contraction is influenced by the length of the muscle cells before stimulation and by changes in contractility. Force is not influenced by variations in the number of excited cells nor by the occurrence of summation and tetanus.

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