Effects of osmolality and ionic strength on the mechanism of Ca2+ release in skinned skeletal muscle fibres of the toad.

Abstract

SUMMARY1. The effects of increased osmolality and ionic strength on the mechanism of Ca2+ release were examined in mechanically skinned skeletal muscle fibres of the toad at 23°C. Ca2+ release was induced by depolarizing the transverse tubular (T-) system by ionic substitution.2. Increasing the osmolality of the 'myoplasmic' solution about four times (to 955 mosmol/kg), by addition of 700 mm sucrose to the standard potassium (K-)HDTA solution (HDTA: hexamethylenediamine-tetraacetate), only depressed the depolarization-induced response by about 46 %. Much of this decrease could be attributed to a reduction in the Ca2+-sensitivity of the contractile proteins at this high osmolality.3. Addition of > 400 mm sucrose itself often induced substantial Ca2+ release and a transient tension response. This 'spontaneous' release was (a) greatly enhanced when the sarcoplasmic reticulum (SR) had been heavily loaded with Ca2 , (b) little affected by inactivation of the voltage sensors by prolonged or permanent depolarization of the T-system and (c) blocked by Ruthenium Red (10 ,lM).4. When both the osmolality and ionic strength were increased, by increasing the K-HDTA concentration, the depolarization-induced force was greatly reduced (to 35% at 818 mosmol/kg and 5% at 1095 mosmol/kg). Most of this reduction could be directly attributed to the substantially reduced maximum force and Caa2+ sensitivity of the contractile apparatus.5. The small amount of releasable Ca2' remaining in the SR after a single depolarization in a high-HDTA solution with 1 mm EGTA (to chelate the released Ca2+), indicated that depolarization could still elicit massive Ca2`release at high ionic strength and osmolality (at 1 mm free Mg2+).6. In contrast, when the total Mg2+ and ATP concentrations were raised about threefold (free [Mg2+] increased 2-7-fold) along with the osmolality and ionic strength, the ability of depolarization to elicit Ca2+ release was greatly hindered.7. Osmotic compression of the skinned fibres to their in situ diameter by addition of 4 % polyvinylpyrrolidone (PVP-40), substantially potentiated the depolarization-* To whom correspondence should be addressed.

L (+)-lactate does not affect twitch and tetanic responses in mechanically skinned mammalian muscle fibres

“…In the final solution containing 30 mM lactate the osmolality was increased by less than 8% and the ionic strength decreased by less than 3.2% [8]. The net effect of such small changes in osmolality and ionic strength on Ca 2+ -activated force and depolarization-induced responses are negligible (less than 2% difference [20]) and subsequently twitch and tetanic force responses in this study would be similarly unaffected. To examine the properties of the contractile apparatus, solutions with 50 mM EGTA and 50 mM Ca-EGTA were made that were similar in composition to the K-HDTA solution above, except that all the HDTA was replaced with EGTA and the total Mg 2+ was 10.3 mM and 8.12 mM respectively, to keep the free [Mg 2+ ] at 1 mM.…”

The effects of chloride ions in excitation-contraction coupling and sarcoplasmic reticulum calcium release in twitch muscle fibre

“…2A). This increase in resting tension could also be due to an osmotic effect (Gordon & Godt, 1970;Lamb et al, 1993).…”

Raised intracellular [Ca2+] abolishes excitation‐contraction coupling in skeletal muscle fibres of rat and toad.

“…High myoplasmic [Ca2+] disrupts E-C coupling E-C coupling in skinned fibres is quite robust and unaffected by large changes in intracellular pH (Lamb & Depolarization Stephenson, 1994) or ionic strength (Lamb, Stephenson & Stienen, 1993 Fig. 2) and because Ca2+-induced Ca2+ release (Endo, 1985), although greatly dampened, is not completely blocked at 1 mM Mg2+ (Lamb & Stephenson, 1990 b Ca2" abolishes I of raised [Ca2P] in the absence of ATP, as this both stops active Ca2+ uptake and further suppresses Ca2+-induced Ca2+ release (Endo, 1985).…”