The control mechanism of relaxation in molluscan catch-muscle (ABRM)

Summary

• 1. The relaxing effects of 5-Hydroxtryptamine (5-HT) have been tested on glycerol-extracted and chemically treated fibres (EDTA and Triton X-100 solutions) of ABRM in a low Ca²⁺ solution (10⁻⁹ M) on a high Ca²⁺ solution (10⁻⁶ M): at two ionic strengths (0.06 and 0.28) and two pH's (6.5 and 7.0). The resistance to stretch of the two muscle preparations has been studied in the low Ca²⁺ medium i.e. without contractile activity in presence and in absence of 5-HT. The presence of 5-HT reduces significantly the resistance to stretch of chemically treated fibres in conditions of ionic strength 0.28 and pH 7.0, but has no effect on glycerol extracted fibres.

• 2. After the maximal tension has been developed in a high Ca²⁺ solution (10⁻⁶ M) the rate of relaxation of chemically treated fibres, induced by a low Ca²⁺ solution (10⁻⁹M) at ionic strength 0.28 and pH 7.0 is increased by the addition of 5-HT. No effect is observed in glycerol extracted fibres.

• 3. Chemically treated fibres in catch-state induced at ionic strength 0.28 and pH 7.0, relaxe after addition of 5-HT to the low Ca²⁺ solution. No relaxing effect is observed when the catch-state is induced at ionic strength 0.06 and pH 6.5.

• 4. Cyclic AMP (c-AMP) induces the same relaxing effects as 5-HT on chemically treated fibres in the same conditions of ionic strength, pH and Ca²⁺ concentration.

• 5. The results are in disagreement with the hypothesis that the catch-state in ABRM is due to a lowering of the intracellular ionic strength and pH; they suggest that the intracellular physiological conditions are rather near ionic strength 0.28 and pH 7.0 even in catch-state. We suppose that the relaxing effect of 5-HT, is not be due to a decrease of the intracellular free Ca²⁺ level but rather to an increase of the rate of Ca²⁺ release from the contractile proteins, and that it is mediated through c-AMP and intracellular relaxing mediator.