ENDOCARDIAL ENDOTHELIUM SELECTIVELY MODIFIES RELAXATION IN RAT PAPILLARY-MUSCLE

The selective removal of endocardial endothelium of rat left ventricul ar 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/mm(2) at L(max) p > 0.05) with no reduction in maximal rate of tension development (+dT/d t(max): 136 +/- 21 vs 137 +/- 18 mN/mm(2)/s p > 0.05). In contrast, th ere was a marked increase in maximal rate of tension decline (-dT/dt(m ax)) from 71 +/- 14 to 92 +/- 15 mN/mm(2)/s (p < 0.05), so that the ra tio between +dT/dt(max) and -dT/dt(max) fell from 1.98 +/- 0.27 to 1.5 1 +/- 0.13 (p < 0.01). Removal of endocardial endothelium led to a sig nificant shortening of isometric twitch contractions. Time to peak ten sion 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/dt(max) was also shortened from 31 +/- 6 to 44 +/- 9 ms (p < 0.05 ) and time to -dT/dt(max) from 90 +/- 12 to 62 +/- 10 ms (p < 0.01). T hese effects were not influenced by alterations in stimulation frequen cy or muscle length. The early onset of relaxation and abbreviated dur ation of relaxation together with an increased rate of decline in tens ion led to a shorter total twitch which may explain the slightly lower peak tension once the endocardial endothelium was removed. Our findin gs confirm that endocardial endothelium modulates myocardial contracti on, with a predominant influence on relaxation.