PRESSURE GENERATION IN A CONTRACTING MYOCYTE

The central hypothesis of this investigation is that a shortening myoc yte generates a time-varying transmural pressure, or intracellular pre ssure. A mathematical model was formulated for a single myocyte, consi sting of a fluid-filled cylindrical shell with axially arranged contra ctile filaments, to quantitate the fiberfluid interaction. In this mod el, the intracellular pressure mediates the interaction between myofil ament force, cell shortening, and the mechanical properties of the sar colemma. Shortening of myofibrils, which are embedded in the fluid-fil led myocytes, deforms the myocyte, thereby altering its transmural flu id pressure. This increase in transmural pressure counteracts fiber sh ortening, hence constituting an. internal load to shortening. The shor tening of the myocyte is accompanied by thickening, due to the incompr essible nature of its contents. Consequently, the overall contractile performance of the cell is integrally linked to the generation of intr acellular pressure. The model manifests a positive transmural pressure during shortening, but not without shortening. The pressure in the my ocyte, therefore, is not a direct function of the force generated, but rather of shortening. Intracellular pressure was measured through a f luid-filled glass micropipette (5mu ID) employing a servo-nulling pres sure transducer in a standard micropuncture technique. Measured intrac ellular pressure in a contracting isolated skeletal myocyte of the gia nt barnacle is observed to be dynamically related to shortening, but n ot to tension without shortening. The relation between the force of co ntraction, cell shortening, and intracellular pressure was assessed du ring both isotonic and isometric contractions. The results support the prediction that isometric, or nondeforming, contractions will not dev elop intracellular pressure and identify a reason for relengthening of the myocytes during relaxation.