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 degrees, 25 degrees, and 30 degrees C) and maximum velocity of sarcomere shortening (Vo; range 15 degrees-35 degrees C) was studied in trabeculae from rat heart. Sarcomere length and Vo 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 microns. Isovelocity release techniques were used to obtain sarcomere velocity and Vo directly. Sarcomere velocity was measured at SL = 1.9-2.0 microns for elimination of contributions of parallel elastic force and restoring force to the external load of the sarcomeres. Peak twitch force development (Fo) was maximal (Fo-max) at 25 degrees C at [Ca2+]o = 1.5 mM. Lowering of the temperature below 25 degrees C led to development of spontaneous sarcomere activity and depression of Fo; both responses could be prevented by the addition of 0.5 mM procaine. Increase of temperature above 25 degrees C reduced twitch duration and Fo. Hill's rectangular hyperbola fitted the force-velocity data if the load during shortening was less than 70% of Fo. Vo appeared to be independent of the level of activation at all temperatures when Fo was maintained above 90% of Fo-max, either by an increase of [Ca2+]o (to 3.0 mM) or by paired pulse stimulation. Vo increased with increasing temperature; the parameter a, calculated from force-velocity relations measured at 20 degrees, 25 degrees, and 30 degrees C, decreased with increasing temperature. The Arrhenius plot of Vo was studied in detail over a wider temperature range (15 degrees-35 degrees C) and in smaller temperature increments. The relation was linear between 18 degrees and 33 degrees C; the observed Q10, defined as the ratio of Vo measured at temperature (T) over Vo at T-10 degrees C, was 4.6 A Q10 of 4.6 for Vo 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.