SARCOMERE DYNAMICS IN A SPONTANEOUS CONTRACTION WAVE AND ITS EFFECT ON THE FOLLOWING, ELECTRICALLY TRIGGERED TWITCH IN RAT MYOCYTE - COMPARISON WITH THE RESTED STATE TWITCH

A spontaneous contraction (SC) wave propagates among sarcomeres in hea rt muscle by the mechanism of Ca++-induced release of Ca++ from sarcop lasmic reticulum (SR). In the present study, some characteristics of u nloaded shortening during the SC and its effect on a subsequent, elect rically triggered twitch (Tsc) were examined at a sarcomere level in i solated rat myocytes. The results were compared with those of a rested state twitch (RS), which was accompanied by an action potential. Aver age shortening velocity from onset to peak of shortening was 3.74 +/- 1.25 (mean +/- SD, n = 18) and 5.35 +/- 2.30 mu m/s per sarcomere (n = 54) in SC and RS, respectively. That the former was smaller than the latter (P < 0.01, t test) suggests that Ca++ are released from the SR more slowly in the SC than the RS. There were no differences in either the extent or area of shortening between SC and RS. The extent of sho rtening increased significantly as shortening velocity increased in al l the SC (P < 0.05), RS, Tsc, and triggered twitch (Trs) after the RS (P < 0.001 in the last three). The slope of the line for the regressio n of the extent upon the velocity of shortening in the SC was similar to 1.5 times greater than the other three. This suggests that the SC h as a different time course of change of myoplasmic [Ca++] and therefor e a different mode of the causal SR Ca++ release from the electrically triggered twitches (RS, Trs, Tsc). There were positive correlations b etween the extent and the area of shortening in each of the RS (P < 0. 01), the Trs (P < 0.05), and the Tsc (P < 0.001), but not in SC. The s lope of the line for the regression of the extent upon the area of sho rtening in the Tsc was about three times greater than those in the RS and the Trs, suggesting characteristics of the Tsc from different thos e of the RS and the Trs. An SC inhibited a Tsc in an interval-dependen t manner. The shortening velocity in the Tsc recovered fully at a test interval of similar to 0.6 s between the onsets of the two successive contractions. The velocity increased further with further increasing the test interval (up to 0.9 s). At a test interval of 0.8-0.9 s, the shortening velocity in the Tsc was greater than those in the preceding SC and the corresponding Trs by 1.17- and 1.80-fold, respectively, as compared in the same five sarcomeres. This may be taken to indicate t hat the SC accelerates the SR Ca++ release in the subsequent, Tsc. The re tvas little difference in either the extent or area of shortening b etween the Tse and the Trs, aside from the shortening area at a test i nterval of 0.3-0.4 s. At such short intervals, the shortening area was 17.6 +/- 7.2 (n = 5) and 30.4 +/- 9.1 mu m.ms (n = 14) in the Tsc and the Trs, respectively. The difference was significant (P < 0.05). Thi s result is in agreement with the idea that a part of Ca++ entering th e myocyte during an action potential is sucked up by the SR and used f or the next twitch. In conclusion, the SC has different effects from t he RS on the subsequent action potential-mediated twitch.