Resting tension participates in the modulation of active tension in isolated guinea pig ventricular myocytes

We studied active and passive properties of intact isolated guinea-pig ventricular myocytes in auxotonic conditions. Cells were attached using carbon fibres. The passive properties of the myocytes, in the presence of the stretch-activated channel blocker streptomycin sulphate, could be separated into two groups: stiff cells (stiffness slope = 2.88 +/- 0.93 nN/micron3, n = 63 cells) and compliant cells (stiffness slope = 0.91 +/- 0.35 nN/micron3, n = 52 cells). The study and the localization of the different kind of cells indicated that endocardium is mainly constituted of stiff cells (80%) while the epicardium contained more compliant cells (60%). When a longitudinal strain was applied to compliant cells, an increase in resting tension, diastolic sarcomere length and active tension were observed. On the other hand, in stiff cells, it induced an increase in resting tension and active tension with little change of diastolic sarcomere length. In both kinds of cells, strain had no effect on Ca2+ transient amplitude and shape. Plotting active tension v diastolic sarcomere length also clearly showed two separated populations of cells, corresponding to stiff and compliant cells. The results of the two groups of cells when plotting active tension v resting tension could not be distinguished. We conclude that resting tension is an important factor in the modulation of active tension by stretch in addition to interfilament lattice spacing or sarcomere length.