Résumé
In vivo the sub-epicardial myocardium (EPI) and sub-endocardial myocardium (ENDO) operate over different ranges of sarcomere length (SL). However, it has not been previously shown whether EPI and ENDO work upon different ranges of the same or differing length–tension curves. We have compared the SL–tension relationship of intact, single ventricular EPI and ENDO myocytes from rat and ferret hearts. Cells were attached to carbon fibres of known compliance in order to stretch them and to record force at rest (passive tension) and during contractions (active tension). In both species, ENDO cells were significantly stiffer (i.e. had steeper SL–passive tension relationships) than EPI cells. Ferret ENDO cells had significantly steeper SL–active tension relationships than EPI cells; rat cells tended to behave similarly but no significant regional differences in active properties were observed. There were no inter-species differences in the active and passive properties of EPI cells, but ferret ENDO cells displayed significantly steeper passive and active SL–tension relationships than rat ENDO. We conclude that in vivo, ferret EPI and ENDO myocytes will function over different ranges of different SL–tension curves. There is a close relationship between SL and active tension (the Frank–Starling law of the heart), and our observations suggest that regional differences in the response to ventricular dilation will depend on both the change in SL and differing regional slopes of the SL–active tension curves.