Effect of 50% external sodium in solutions of normal and twice normal tonicity on internal sodium activity in frog skeletal muscle

Free calcium in sheep cardiac tissue and frog skeletal muscle measured with Ca2+-selective microelectrodes

Microelectrodes filled with neutral carrier selective to Ca2+ were used to measure the free intracellular Ca2+ concentration [( Ca2+]i) in sheep cardiac tissue and frog skeletal muscle. Calibration of the electrodes was performed in the presence of a solution resembling the cationic composition of the cytoplasm. [Ca2+]i at rest in normal physiological saline (20-22 degrees C) was 240 nM in Purkinje fibres, 270 nM in ventricular muscle, and 52 nM in skeletal muscle. In Purkinje fibres, elevation of [Ca2+]o from 1.8 mM to 5.4 mM produced a 1.7-fold increase in [Ca2+]i. Elevation of [Ca2+]o from 1.8 mM to 18 mM induced a 2.6-fold increase in [Ca2+]i. Exposure to Na+-free solution (Li+-substituted) gave rise to elevation of [Ca2+]i by factors of 5.8 and 14 in ventricular muscle and Purkinje fibres, respectively. These latter changes in [Ca2+]i were associated with the development of contractures which reached 34% and 172% of the corresponding twitch tension.

Effects of tonicity on tension and intracellular sodium and calcium activities in sheep heart

We have measured the effects of changing tonicity of the bathing solution on intracellular sodium and calcium activities and tension of sheep cardiac Purkinje strands and ventricular muscle. For Purkinje strands in solutions of normal tonicity, resting membrane potential was -77.4 +/- 0.4 mV (mean +/- SE), sodium activity was 7.9 +/- 0.4 mM, and calcium activity was 98 +/- 9 nM. For ventricular muscle in solutions of normal tonicity, resting membrane potential was -86.4 +/- 1.2 mV, sodium activity was 6.9 +/- 0.5 mM, and calcium activity was 70 +/- 4 nM. Reduction of tonicity to 75% of normal in both tissues produced depolarization of a few millivolts, and sodium activity fell almost to the level predicted for simple osmotic dilution. In Purkinje strands, calcium activity fell much more than that predicted for simple osmotic dilution. Twitch contraction was reduced in the hypotonic solution. Increase of tonicity to 150% and 200% caused the resting membrane potential to become more negative. In both tissues, sodium activity increased somewhat less than predicted from simple water movement, and calcium activity increased proportionately much more than sodium activity. The much larger change of calcium activity in both hypo- and hypertonic solutions could be explained by water movement plus the effect of sodium-calcium exchange. In hypertonic solutions, tonic tension was increased, along with the rise in calcium activity; however, the twitch tension was reduced. This reduction of twitch tension may be due to a direct effect of hypertonicity on cross-bridge behavior, as has been reported for skeletal muscle.

Effects of hypertonic solutions on contraction of frog tonic muscle fibers

The influence of solution hypertonicity on contraction was studied in small bundles of tonic muscle fibers from the iliofibularis muscle of the frog Rana pipiens. Muscles were activated with high-K+ solutions that had osmolalities which were increased with tris(hydroxymethyl)aminomethanepropionate. Peak potassium contracture force decreased monotonically with tonicity and was zero in solutions with 2.5 or 3 times the osmolality of control Ringer. Contracture force at all tonicities studied (less than or equal to 3 X Ringer) was increased by increasing Ca2+ in the media 10-fold (to 20 mM) and/or by adding caffeine (10-20 mM). Nevertheless, this potentiated force also was diminished as tonicity increased. Force of single, mechanically skinned tonic fibers taken from these bundles was activated by Ca2+ over the same concentration range as that of twitch fibers. Moreover, maximal Ca2+-activated force, normalized per cross-sectional area, was similar in skinned tonic and twitch fibers. As was shown previously in twitch fibers, maximal Ca2+-activated force was decreased when ionic strength was increased. These data suggest that, as with twitch fibers, increased tonicity depresses contraction of tonic fibers by increasing the intracellular ionic strength, which in turn inhibits the ability of the contractile apparatus to generate force. Unlike twitch fibers, however, disruption of the excitation-contraction coupling process probably plays a more significant role in the action of hypertonicity on tonic fibers.

Influence of sarcomere length, tonicity, and external sodium concentration on conduction velocity in frog muscle fibres

1. Using an optical technique, conduction velocity in isolated frog muscle fibres has been measured at different sarcomere lengths and in solutions of altered tonicity and Na content. 2. Conduction velocity (in m/s) in normal Ringer solution is found to be independent of sarcomere length in the range of 2-5 microns. 3. Fibre cross-section appears to become circular with stretch to sarcomere lengths exceeding 4 microns. The data on fibre diameter and length are in agreement with the assumption that constant fibre volume is maintained during passive length changes. 4. In Na-deficient solutions, conduction velocity is reduced, in agreement with predictions based on action potential parameters. 5. In solutions of half or twice the normal tonicity, the conduction velocity is proportional to the square root of the measured fibre diameter. After correcting the bias involved in estimating fibre cross-section from only one measurement of fibre diameter, the data suggest an increase in specific internal resistance (Ri) by about 8% in twice hypertonic solution and a decrease by about 5% in half normal tonicity. 6. Releasing and stretching a fibre in hypertonic solution has no effect on conduction velocity as long as the initial sarcomere length is not exceeded. Stretching the fibre beyond the sarcomere length at which it was transferred to hypertonic solution reversibly increases conduction velocity.