Frog sartorius muscles were superfused for 40 min with solutions of K-free Ringer, normal Ringer containing ouabain, or K-free Ringer containing ouabain. Changes in myoplasmic K and Na were measured with ion-selective microelectrodes; changes in total fibre K and Na were measured by means of atomic absorption spectroscopy; and changes in total fibre water content were obtained from wet and dry weights. Application of a two-compartment model permitted one to calculate (i) the K, Na, and water changes in the myofibrils and in the surrounding myoplasm (extramyofibrillar space); (ii) the changes in the transmyofibrillar Donnan potential (ED); and (iii) the changes in the ratio of the apparent association constants (kNa/kK) of the myofilament charge sites to Na and K. In the resting fibres, the K, Na, and water content of the myofibrils were calculated to be 82, 87, and 80% of total fibre content, respectively; ED was calculated as −4.5 mV; kNa/kK was calculated as 1.4. After a 40-min ouabain treatment, 12 mmol (per kg fibre water) of intrafibre K exchanged with 7.5 mmol of extrafibre Na, 6.4 mmol of myofibrillar K exchanged with an equal amount of extramyofibrillar Na, ED increased to −8.3 mV, and kNa/kK remained relatively constant. After a 40-min K-free treatment, the fibres gained 5.5 mmol of Na without any change in fibre K or water, the myofibrils shifted 9.3% of their water into the extramyofibrillar space instead of exchanging K for Na, ED increased to −10.7 mV, and kNa/kK decreased to 0.47. When ouabain and zero K were combined, an equimolar transmembrane K for Na exchange occurred, the myofibrils appeared to shift water and to exchange K for Na, ED increased to −14.3 mV, and kNa/kK decreased to 0.63. Analyses of other experiments involving Na-free superfusions suggest that the zero K-induced intrafibre water shift is dependent on an uncompensated net accumulation of Na in the extramyofibrillar myoplasm. These results indicate that the myofibrils in the intact fibre may adjust their water and electrolyte content quite differently during different perturbations, and that the myofibrillar changes need not mimic the whole fibre water and electrolyte changes.
Donnan potential of rabbit skeletal muscle myofibrils I: electrofluorochromometric detection of potential.
