Two different electrophysiological responses to ryanodine: evidence for two populations of muscles isolated from the...

We have observed that small trabeculae isolated from the rabbit right ventricle can be grouped into two populations on the basis of action potential configuration and electrophysiological response to ryanodine. These two populations have been designated Groups I and II. Action potentials in Group I muscles were characterized by a long action potential, a fast early phase of repolarization and shortened when exposed to 0.1 microM ryanodine (from 210 +/- 22 (6) ms (Mean +/- S.E.M. (n)) to 176 +/- 20 (6). Group II muscles showed a shorter action potential with a higher plateau phase and was prolonged by 0.1 microM ryanodine (from 175 +/- 7 (9) ms to 236 +/- 18 (9) ms). No significant difference in the mechanical response to ryanodine was observed between the two groups. Two populations of muscles, isolated from the rabbit right ventricle, have been previously described and distinguished on the basis of the presence or absence of transverse (T) tubules. We have attempted to correlate the electrophysiological observations with the presence or absence of (T) tubules. Electron microscopic examination, however, revealed that T-tubules were clearly present in both groups of trabeculae and therefore the two populations cannot be distinguished simply on the basis of the presence or absence of T-tubules. It was considered possible that the difference in the two populations may be related to the existence, and/or predominance, of two transient outward currents (one activated by SR Calcium release and the other that is 4-aminopyridine (4-AP) sensitive). The relative importance of these two currents to repolarization may be different in the two groups. This was investigated by adding ryanodine in the presence of 4-AP. The differential response to ryanodine seen in the two groups was, however, still observed in the presence of 4-AP. In these experiments, three out of seven muscles showed a decrease in action potential duration and four out of seven showed an increase. Other possible explanations for the differences in the two groups of muscles, and their response to ryanodine, are discussed.