Résumé
The study of the Frank-Starling's law in mammalian single cells has been hindered by a lack of an easily performed method of stretching cells. Some authors have succeeded in this but their methods required a great deal of technical expertise and in most cases they have not had much success. We have developed an easy method of stretching mammalian ventricular cells from slack sarcomere length (S.L.) (Lo, 1.77 +/- 0.05 microns) to about 117% of this length. Thin carbon fibers (12 microns in diameter) which can be bound electrochemically to the cell membrane surface have been used. A flexible long fiber of known compliance (80 microns/microN) was attached to one end of the cell and a stiff double fiber (4 microns/microN) to the other end. The cell attachment was relatively easy to perform and successful results were obtained in 80% of the attempts. The displacement of the flexible fiber allows the quantitative measurements of the resting tension in a group of non-stimulated cells and of auxotonic contractions developed upon stimulation in another group of cells. Increasing S.L. from Lo to 105-106% of Lo, an increase in active tension from 0.21 +/- 0.03 mN/mm to 0.26 +/- 0.01 mN/mm (n = 4) could be noticed with a stimulation frequency of 0.5 Hz. An increase in active tension was also observed at 1 Hz. Staircase kinetics were accelerated with stretching; this confirms at the single cell level the hypothesis of an effect of length-dependent activation on the staircase. Eulerian differential stiffness constant was calculated and found to be 13.5 +/- 1.2, a value which is comparable to that described in intact heart. Thus the important stiffness found in the whole heart may be due to intracellular component(s) such as myofilament and/or connectin.