CALCIUM AND THE CONTRACTION OF MAIA MUSCLE FIBRES.

Abstract

In this work the cannulated single muscle fibre preparation from the crab Maia sguinado (Caldwell & Walster 1961,1963) has been used. In experiments done in collaboration with Portzehl & Rüegg (1964) the contractions produced by the injection of various calcium buffer solutions have been studied, the micro-injector used being similar to that devised by Hodgkin & Keynes (1956). Previous work on frog muscle fibres (Heilbrunn & Wiercinski 1947; Niedergerke 1955) and on Maia muscle fibres (Caldwell & Walster 1961, 1963) had shown that the introduction of calcium ions (Ca 2+ ) into the sarcoplasm caused contraction, but the minimum concentration needed had not been determined. The compound EGTA (ethylene glycol bis (β-amino ethyl ether)- N , N' -tetra-acetate) has a much greater tendency to form complexes with calcium than with magnesium (association constant for (Ca EGTA ) 2- , 10 11 ; association constant for (Mg EGTA ) 2- , 10 5.21 ; from Bjerrum, Schwarzenbach & Sillén 1957). Mixtures of the calcium complex of EGTA and of free EGTA contain very low stabilized Ca 2+ concentrations and act as calcium buffers which are unlikely to be greatly affected by magnesium at the concentrations present in muscle fibres. It was found that the injection of a calcium buffer containing ~0.3μM Ca 2+ (69 mM Ca EGTA ; 31 mM EGTA ) caused a contraction of only one out of six Maia fibres. On the other hand, a calcium buffer containing ~0.7 μM Ca 2+ (84 mM Ca EGTA ; 16 mM EGTA ) caused contraction of nine of the eleven fibres into which it was injected. It can be argued that since the buffers are thought to cause contraction by increasing the Ca 2+ concentration in the fibres, the Ca 2+ concentration in resting fibres must be below 0.3 to 0.7 μM. Calculations based on this upper limit suggest that in the absence of magnesium the Ca 2+ concentration of the buffers would be virtually unaffected by dilution in the fibres. The buffer Ca 2+ concentrations would however rise from 0.3 to 0.6 μM and from 0.7 to 1.5 μM if all the fibre magnesium was ionized and able to react with the buffers.