title: Effects of osmolality and ionic strength on the mechanism of Ca2+ release in skinned skeletal muscle fibres of the toad.
abstract: 1. 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 degrees 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 Ca(2+)-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 microM). 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 Ca2+ 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-induced force responses, due partly to an increase in the Ca(2+)-sensitivity of the contractile apparatus. 8. These results indicate how increased intracellular osmolality, ionic strength and [Mg2+] produce the transient contraction and subsequent inhibition of tetanic tension in intact muscle fibres exposed to hypertonic solutions.
authors.affiliation: Department of Zoology, La Trobe University, Bundoora, Melbourne, Australia.
authors.forename: G D
authors.initials: GD
authors.lastname: Lamb
authors.forename: D G
authors.initials: DG
authors.lastname: Stephenson
authors.forename: G J
authors.initials: GJ
authors.lastname: Stienen
mesh_terms: D000818:Animals; D002024:Bufo marinus; D002118:Calcium / Q000378:metabolism*; D002794:Choline / Q000494:pharmacology; D004492:Edetic Acid / Q000031:analogs & derivatives; D006652:Histological Techniques; D006982:Hypertonic Solutions / Q000494:pharmacology; D007477:Ions; D008274:Magnesium / Q000378:metabolism; D009132:Muscles / Q000378:metabolism*; D009210:Myofibrils / Q000187:drug effects / Q000378:metabolism; D009994:Osmolar Concentration; D009995:Osmosis; D011205:Povidone / Q000494:pharmacology; D013395:Sucrose / Q000378:metabolism
chemical_list: D006982:Hypertonic Solutions; D007477:Ions; C026061:HDTA; D013395:Sucrose; D004492:Edetic Acid; D011205:Povidone; D008274:Magnesium; D002794:Choline; D002118:Calcium