Effects of Ba2+ on the mechanical properties of glycerinated heart muscle

At a muscle length, L0 (just taut), isometric tension and tension transients in response to rapid step stretches in length (mostly less than 1.2% of L0 within 2 ms) were measured at constant levels of Ba2+ activation of varying magnitude in glycerinated cat right-ventricular papillary muscles (2-3 mm long, 130-200 micron in diameter). The majority of the experiments were carried out at room temperature (26-27 degrees C) and at a pH of 6.8. The steady isometric tension increased as Ba2+ was varied from slightly below pBa 6 to about pBa 4. The concentration range of Ba2+-activated muscle was roughly 10 times higher than that of Ca2+-activated muscle. Maximum Ba2+-activated isometric tension was 79.0 +/- 4.2% (mean +/- SD, n = 8) of that activated with Ca2+. The tension transients in Ba2+-activated muscle were characterized by at least three distinct phases; an immediate tension increase coincident with the stretch, a rapid exponential tension decrease (time constant, tau 1 = 7.3 +/- 1.0 ms, n = 5) and a delayed exponential tension rise (tau 2 = 104 +/- 5.7 ms, n = 5). The profile of tension response was quite similar to that of Ca2+-activated muscle. These results suggest that in Ba2+-activated glycerinated heart muscle the cross-bridge turnover is taking place as in the Ca2+-activated muscle, but the number of active cross-bridges at maximally activated state is smaller than that of Ca2+-activated muscle.