Length-tension relationships of subepicardial and sub-endocardial single ventricular myocytes from rat and ferret hearts

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 rang es of the same or differing length-tension curves. We have compared the SL- tension relationship of intact, single ventricular EPI and ENDO myocytes fr om rat and ferret hearts. Cells were attached to carbon fibres of known com pliance in order to stretch them and Co record force at rest (passive tensi on) 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 signi ficantly steeper SL-active tension relationships than EPI cells; rat cells tended to behave similarly but no significant regional differences in activ e properties were observed. There were no interspecies differences in the a ctive 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 ill vivo, ferret EPI and ENDO myocytes will function over different ranges of different SL-tension curves. There is a close relations hip between SL and active tension (the Frank-Starling law of the heart), an d our observations suggest that regional differences in the response to ven tricular dilation will depend on both the change in SL and differing region al slopes of the SL-active tension curves. (C) 2000 Academic Press.