Activation in frog atrial trabeculae: dependence on temperature and length

Isolated frog atrial trabeculae were activated using the method of Na+ withdrawal to induce contractures of relatively steady tension. External Na+ concentration [( Na+]o) during contractures was varied between 0.25 and 45 mM. Isometric contracture tension was measured at cold (4 degrees C) and warm (20 degrees C) temperatures. In addition, rapid temperature jumps (complete in approximately 400 ms) were imposed during cold contractures, resulting in tension transients that consisted of an initial increase in tension followed by a decrease, the latter phase being greater at small and moderate reductions in [Na+]o. Peak contracture tension varied with relative muscle length. The trabeculae became more sensitive with stretch to Na+ withdrawal at 20 degrees C and generated relatively greater tensions at a given [Na+]o. The initial tension increase after a temperature jump was directly proportional to the peak contracture tension immediately preceding the increase in temperature and was therefore interpreted as reflecting an effect of the higher temperature on the attached force-generating cross bridges. The effects of cold and warm steady temperatures and temperature jumps during isometric twitches were also studied. Peak twitch tension varied inversely with temperature (stimulus frequency = 0.2 Hz). In contrast, temperature jumps imposed during the rising phase of twitches at a steady cold temperature (approximately 4 degrees C) resulted in a large initial increase in tension followed by relaxation at a rate that was characteristic of the elevated temperature. The results suggest that, at the warmer temperature (approximately 20 degrees C), activation (i.e., number of attached cross bridges) of the myocardium is significantly less than maximal during the twitch response. The dependence of the tension vs. [Na+]o curves and the tension transients resulting from the temperature jumps on relative muscle length provide evidence for a length dependency of contractile activation in intact atrial trabeculae under conditions of steady-state tension development.