FLUORESCENT PROPERTIES OF RAT CARDIAC TRABECULAE MICROINJECTED WITH FURA-2 SALT

We have measured force, sarcomere length, and Ca2+ during twitches in rat cardiac trabeculae. To avoid the difficulties associated with fura -2/acetoxymethyl ester (AM), fura-2 salt was iontophoretically microin jected into the preparation using a single impalement site; this is po ssible because fura-2 diffuses through the gap junctions between cells . By use of this method, the estimated peak of the [Ca2+] transient du ring a twitch was not statistically different at different sarcomere l engths: 875 +/- 92 nM at a sarcomere length of 2.15 mum vs. 905 +/- 67 nM at a sarcomere length of 1.65 mum (means +/- SD, n = 10). When tra beculae were loaded using fura-2/AM, the estimated peak of the [Ca2+] transient at a sarcomere length of 2.15 mum was 540 +/- 180 nM (means +/- SD, n = 5). The time course of the Ca2+ transients at different sa rcomere lengths is qualitatively similar, but small systematic differe nces were observed during the relaxation period. On the other hand, th e duration of twitch force increases dramatically as the muscle length is increased. As a result, when the trabeculae were held at short mus cle lengths,the temporal relationship between force and the Ca2+ trans ient resembled the relationship between cell shortening and the Ca2+ t ransient measured in isolated myocytes. At longer lengths the temporal relationship between force and the Ca2+ transient more closely resemb les that obtained in papillary muscles using aequorin.