Measurement of tissue volume during non-steady state high-intensity muscle contraction.

To investigate the pressures driving water into stimulated muscle, water distribution during and after muscle stimulation was studied in isolated cat muscles perfused by recirculating diluted blood. 51Cr-labeled EDTA (51Cr-EDTA) and Evans blue-labeled albumin were used to determine extracellular volume and plasma volume (PV), respectively. Change in tissue volume was calculated as -PV. Interstitial volume (IFV) was determined from the ratio of interstitial solute (51Cr-EDTA and sodium) mass and interstitial concentration. Interstitial mass was determined by mass balance, and interstitial concentration was determined from solute flux and Fick's Law. One group was stimulated at 4 Hz for 2 min, and a second was stimulated by 80-Hz trains (1 train/s, 0.1 s duration). Four Hertz stimulation increased total tissue volume by approximately 3 ml/100 g and decreased IFV by 1 ml/100 g. Train stimulation increased total tissue volume by 6 ml/100 g and decreased IFV by 4. These data indicate that water moves into cells faster than the simultaneous transcapillary flow, suggesting that intracellular osmoles provide the primary driving pressure in stimulation-induced swelling.