CA-2 AND SR-2+ ACTIVATION PROPERTIES OF SKINNED MUSCLE-FIBERS WITH DIFFERENT REGULATORY SYSTEMS FROM CRUSTACEA AND RAT

1. The contractile activation properties of long- (sarcomere length (SL)>6 mum) and short- (SL<4 mum) sarcomere fibres from the claw muscle of the yabby (freshwater crustacean, Cherax destructor) and the fast- and slow-twitch fibres from the rat have been investigated using single skinned muscle fibres activated in solutions containing Ca 2+ or Sr2+ or both Ca2+ and Sr2+. 2. Sr 2+ could not fully activate the contractile apparatus of either the long- or the short-sarcomere yabby preparations and the force-pSr curves for both fibre types were biphasic in shape. 3. The long- and short-sarcomere fibres from the yabby differed in their Ca2+-and Sr2+-activation properties. Thus the long-sarcomere fibres required a significantly lower [Ca2+] to produce 10% maximum force, had Ca2+-activation curves which were significantly shallower, and had a significantly higher ratio between maximum Sr2+- and maximum Ca2+-activated force than the short-sarcomere fibres. 4. Simultaneous activation with Ca2+ and Sr2+ showed a synergistic effect in the rat muscle fibres where Sr2+ could fully replace Ca2+ in activation. In contrast the results with the long- and short-sarcomere fibres from the yabby showed that in some functional states Sr2+ binds but cannot (or can only partially) activate the regulatory system while in others Sr2+ can fully replace Ca2+ in activating contraction. 5. The synergistic effect between Sr2+ and Ca2+ in mammalian muscle could be quantitatively explained if all regulatory sites involved in contractile activation have a similar value for the ratio between their affinity for Ca2+ and Sr2+. 6. Three distinguishable functional states (based on the fibre's ability to be activated by Sr2+ and Ca2+) were identified in the long-sarcomere fibres from the yabby: one where both Ca2+ and Sr2+ were able to activate contraction and had a relatively high sensitivity to Sr2+; one where both Ca2+ and Sr2+ were able to activate contraction but where the sensitivity to Sr2+ was lower and was more sensitive to a decrease in ionic strength; and one where Sr2+ binding to the regulatory system could not activate contraction. Equivalent states of the three described for the long-sarcomere fibres were also found in the short-sarcomere preparations. However, the short-sarcomere fibres had, in addition, a fourth state which was characterized by the ability of Sr2+ to activate contraction at reduced ionic strength but not at standard ionic strength. 7. The decrease in the concentration of ionic equivalents (I) from 154 to 94 mmol/l markedly increased the ratio between maximum Sr2+-activated and maximum Ca2+-activated force in the short-sarcomere but not in the long-sarcomere fibres of the yabby. This suggests that in the former fibre type, electrostatic type of interactions are involved in transmitting the signal following binding of divalent ions to the regulatory system. 8. Results from this study show that the regulatory system of the yabby claw muscle fibres exists in several functional states and thus makes a very interesting preparation with which to study contractile activation.