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 microinjected into the preparation using a single impalement site; this is possible because fura-2 diffuses through the gap junctions between cells. By use of this method, the estimated peak of the [Ca2+] transient during a twitch was not statistically different at different sarcomere lengths: 875 +/- 92 nM at a sarcomere length of 2.15 microns vs. 905 +/- 67 nM at a sarcomere length of 1.65 microns (means +/- SD, n = 10). When trabeculae were loaded using fura-2/AM, the estimated peak of the [Ca2+] transient at a sarcomere length of 2.15 microns was 540 +/- 180 nM (means +/- SD, n = 5). The time course of the Ca2+ transients at different sarcomere lengths is qualitatively similar, but small systematic differences were observed during the relaxation period. On the other hand, the duration of twitch force increases dramatically as the muscle length is increased. As a result, when the trabeculae were held at short muscle lengths, the temporal relationship between force and the Ca2+ transient resembled the relationship between cell shortening and the Ca2+ transient measured in isolated myocytes. At longer lengths the temporal relationship between force and the Ca2+ transient more closely resembles that obtained in papillary muscles using aequorin.