Potassium and ionic strength effects on the isometric force of skinned twitch muscle fibres of the rat and toad.

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Abstract
A study was carried out to investigate the effects of ionic strength and monovalent cations on isometric, Ca2+‐activated force and rigor responses in mechanically skinned muscle fibres. Three types of skeletal muscle fibres were used: rat fast‐ and slow‐twitch fibres and toad twitch fibres. The contractile apparatus of rat slow‐twitch fibres was affected differently from that of rat fast‐twitch and amphibian twitch fibres when changing the ionic strength (expressed either in terms of ionic equivalents as I or formally as gamma/2) and [K+]. Thus, the apparent sensitivity to Ca2+ decreased substantially more in slow‐twitch fibres (by a factor of 20) than in the other fibre types (by a factor of 12) when I and [K+] were increased from 94 to 354 mM and from 56 to 316 mM respectively. Maximum Ca2+‐activated force, however, declined only by a factor of 2.2 in slow‐twitch fibres compared with 4.2 in the other fibre types, when I was increased from 154 to 354 mM. In slow‐twitch fibres the force oscillations of myofibrillar origin were found to increase substantially in amplitude, duration and frequency at low values of I and almost disappeared at high ionic strength. At low values of I, it was also discovered that ca. 50% of the fast‐twitch fibres responded with myofibrillar force oscillations when submaximally activated. The characteristics of these oscillations were different from those of slow‐twitch fibres. Rigor force levels were found to decline markedly with increasing iota and [K+] in all fibre types. Unexpectedly, once rigor force was established in a certain ionic environment, the level of force was stable regardless of further changes in ionic strength and monovalent cation concentration. These results indicate that the rigor cross‐bridges can be formed in different stable positions and that the probability of attachment in certain positions (rather than the total number of cross‐bridges that can be formed) is influenced by the ionic conditions. Further experimental evidence provided in this study shows that the increase in [K+] is mainly responsible for the decrease of the Ca2+‐sensitivity of the contractile apparatus and that ionic strength (expressed as I rather than gamma/2) influences markedly the maximal Ca2+‐activated force, the maximum steepness of the pCa‐force relations and the oscillatory processes of myofibrillar origin.