Biochemistry of Starlings

The Starling law describes an enhancement of contractile function of the heart resulting from the increased end-diastolic ventricular volume and consequently from an increase in muscle length. The Starling law also applies to skeletal muscle but in cardiac muscle it is much more pronounced. The simplest explanation of the phenomenon is based on the constant volume principle, i.e. the increased muscle length requires a decrease in the interfilament space in order to keep the volume of the muscle cell constant. X-ray diffraction studies verified this assumption (Irving et al., 2000). Accurate measurements of interfilament spacing, by synchrotron X-ray diffraction, as a function of sarcomere length, using skinned and intact rat trabecules demonstrated that the lattice spacing was decreased as sarcomere length increased. Thus, the Starling law can be explained by the enhanced actin and myosin interaction as the result of moving the actin and myosin filaments closer to each other at the longer muscle length.

The Starling law is also characterized by an increase in Ca2+ sensitivity of the cardiac myofilaments as the sarcomere length increases (Fig, H8.).

Fig. H8. Relation between Ca2+ concentration and tension generation of cardiac myofilaments at long and short sarcomere length(SL) (From Solaro, 1999).

pM Ca2+

At longer sarcomere length the distance between thick and thin filaments decreases and hence the length of the diffusion pathway for Ca2+, in the interfilament space, to reach TN-C decreases. One can assume that saturation of TN-C with Ca2+ and subsequent activation of the thin filaments is easier and faster at smaller interfilament spacing than at the larger spacing. However, the explanation is more complex because tension development induced by Ca is not a linear but a cooperative activation of the actin-myosin combination.

Fig. H9 illustrates a current method to measure force-length relation in cardiac muscle. A small strip of cardiac muscle is mounted in an experimental setup. Sarcomere length is measured by

Fig. H8. Relation between Ca2+ concentration and tension generation of cardiac myofilaments at long and short sarcomere length(SL) (From Solaro, 1999).

laser diffraction techniques. Force is measured by a sensitive force transducer. Muscle length is controlled by a high speed device.

Fig.H9.

Force-length measurement with small heart strips (Courtesy of Dr. Pieter de Tombe).

Fig.H9.

Force-length measurement with small heart strips (Courtesy of Dr. Pieter de Tombe).

Was this article helpful?

0 0

Post a comment