Endocardial endothelium selectively modifies relaxation in rat papillary muscle

The selective removal of endocardial endothelium of rat left ventricular papillary muscles by 1-second immersion in 0.5% Triton X-100 showed little influence on resting tension and only a small decrease in peak isometric tension (8.3 +/- 1.4 vs 9.6 +/- 2.4 mN/mm2 at Lmax, p > 0.05) with no reduction in maximal rate of tension development (+dT/dtmax; 136 +/- 21 vs 137 +/- 18 mN/mm2/s, p > 0.05). In contrast, there was a marked increase in maximal rate of tension decline (-dT/dtmax) from 71 +/- 14 to 92 +/- 15 mN/mm2/s (p < 0.05), so that the ratio between +dT/dtmax and -dT/dtmax fell from 1.98 +/- 0.27 to 1.51 +/- 0.13 (p < 0.01). Removal of endocardial endothelium led to a significant shortening of isometric twitch contractions. Time to peak tension was abbreviated from 111 +/- 20 to 84 +/- 8 ms (p < 0.05) and the half relaxation time from 92 +/- 9 to 68 +/- 8 ms (p < 0.01). Time to +dT/dtmax was also shortened from 31 +/- 6 to 44 +/- 9 ms (p < 0.05) and time to -dT/dtmax from 90 +/- 12 to 62 +/- 10 ms (p < 0.01). These effects were not influenced by alterations in stimulation frequency or muscle length. The early onset of relaxation and abbreviated duration of relaxation together with an increased rate of decline in tension led to a shorter total twitch which may explain the slightly lower peak tension once the endocardial endothelium was removed. Our findings confirm that endocardial endothelium modulates myocardial contraction, with a predominant influence on relaxation.