The aim of this thesis was to examine the molecular mechanism underlying the
force response to stretch in the heart. The properties of cardiac muscle were
studied in rat trabeculae activated by laser-flash photolysis of NPE-caged ATP at
20°C. The rate of ATP hydrolysis was determined, on a millisecond time scale, using
a fluorescently labelled phosphate binding protein to measure the rate of inorganic
phosphate release.
The results show that an increase in sarcomere length causes an increase in
isometric force with a similar response seen upon increasing thin filament
activation. ATPase rate increases with thin filament activation, however, the
increase in force with sarcomere length is not accompanied by a corresponding
increase in ATPase rate. At a lower activation level a more substantial increase in
isometric force is seen with increasing sarcomere length, compared to at a higher
activation level. The results show that at a longer sarcomere length less ATP is used
per unit of force produced.
Interestingly, stretch of an active muscle causes a substantial and instantaneous
reduction in cross-bridge ATPase activity, even though force remains high.
Conversely, active shortening resulted in an increase in the ATPase rate, above that
of the isometric level. The stretch applied to the active trabeculae was greater than
the reach of the cross-bridges therefore cross-bridge detachment must occur,
however as force remains high this must be followed by rapid reattachment. As Pi
release is low during the stretch this detachment and reattachment cannot occur
via the classical view of the cross-bridge cycle where attachment is preceded by ATP
hydrolysis. To explain these results a branched cross-bridge cycle is suggested
whereby rapid detachment and reattachment can occur without additional Pi
release and ATP hydrolysis.