FORCE AND VELOCITY OF SARCOMERE SHORTENING IN TRABECULAE FROM RAT-HEART - EFFECTS OF TEMPERATURE

The effect of temperature on the force-sarcomere velocity relation (20°, 25°, and 30°C) and maximum velocity of sarcomere shortening (V(o); range 15°-35°C) was studied in trabeculae from rat heart. Sarcomere length and V(o) were measured by laser diffraction techniques. Sarcomere length and sarcomere velocity, determined from each of the first-order diffraction lines, differed by less than 4%. Slack sarcomere length in the trabeculae appeared to be 1.9 μm. Isovelocity release techniques were used to obtain sarcomere velocity and V(o) directly. Sarcomere velocity was measured at SL = 1.9-2.0 μm for elimination of contributions of parallel elastic force and restoring force to the external load of the sarcomeres. Peak twitch force development (F(o)) was maximal (F(o-max)) at 25°C at [Ca2+](o) = 1.5 mM. Lowering of the temperature below 25°C led to development of spontaneous sarcomere activity and depression of F(o); both responses could be prevented by the addition of 0.5 mM procaine. Increase of temperature above 25°C reduced twitch duration and F(o). Hill's rectangular hyperbola fitted the force-velocity data if the load during shortening was less than 70% of F(o). V(o) appeared to be independent of the level of activation at all temperatures when F(o) was maintained above 90% of F(o-max), either by an increase of [Ca2+](o) (to 3.0 mM) or by paired pulse stimulation. V(o) increased with increasing temperature; the parameter a, calculated from force-velocity relations measured at 20°, 25°, and 30°C, decreased with increasing temperature. The Arrhenius plot of V(o) was studied in detail over a wider temperature range (15°-35°C) and in smaller temperature increments. The relation was linear between 18° and 33°C; the observed Q10, defined as the ratio of V(o) measured at temperature (T) over V(o) at T-10°C, was 4.6. A Q10 of 4.6 for V(o) is consistent with the reported temperature dependence of rat cardiac actin-activated myosin ATPase, which suggests that the same reaction step may limit the activity of the enzyme in vitro and during shortening of the cardiac sarcomeres at zero external load.