EFFECTS OF OSMOLALITY AND IONIC-STRENGTH ON THE MECHANISM OF CA2-MUSCLE FIBERS OF THE TOAD( RELEASE IN SKINNED SKELETAL)

1. The effects of increased osmolality and ionic strength on the mecha nism of Ca2+ release were examined in mechanically skinned skeletal mu scle fibres of the toad at 23-degrees-C. Ca2+ release was induced by d epolarizing the transverse tubular (T-) system by ionic substitution. 2. Increasing the osmolality of the 'myoplasmic' solution about four t imes (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%. Muc h of this decrease could be attributed to a reduction in the Ca2+-sens itivity of the contractile proteins at this high osmolality. 3. Additi on of > 400 mm sucrose itself often induced substantial Ca2+ release a nd a transient tension response. This 'spontaneous' release was (a) gr eatly enhanced when the sarcoplasmic reticulum (SR) had been heavily l oaded with Ca2+, (b) little affected by inactivation of the voltage se nsors by prolonged or permanent depolarization of the T-system and (c) blocked by Ruthenium Red (10 mum). 4. When both the osmolality and io nic strength were increased, by increasing the K-HDTA concentration, t he depolarization-induced force was greatly reduced (to 35 % at 818 mo smol/kg and 5 % at 1095 mosmol/kg). Most of this reduction could be di rectly attributed to the substantially reduced maximum force and Ca2sensitivity of the contractile apparatus. 5. The small amount of relea sable Ca2+ remaining in the SR after a single depolarization in a high -HDTA solution with 1 mm EGTA (to chelate the released Ca2+), indicate d that depolarization could still elicit massive Ca2+ release at high ionic strength and osmolality (at 1 mm free Mg2+). 6. In contrast, whe n the total Mg2+ and ATP concentrations were raised about threefold (f ree [Mg2+] increased 2-7-fold) along with the osmolality and ionic str ength, the ability of depolarization to elicit Ca2+ release was greatl y hindered. 7. Osmotic compression of the skinned fibres to their in s itu diameter by addition of 4 % polyvinylpyrrolidone (PVP-40), substan tially potentiated the depolarization-induced force responses, due par tly to an increase in the Ca2+-sensitivity of the contractile apparatu s. 8. These results indicate how increased intracellular osmolality, i onic strength and [Mg2+] produce the transient contraction and subsequ ent inhibition of tetanic tension in intact muscle fibres exposed to h ypertonic solutions.