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

The relaxing effects of 5-hydroxytryptamine (5-HT) have been tested on glycerol-extracted and chemically treated fibres (EDTA and Triton X-100 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 fibers in conditions of ionic strength 0.28 and 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 (10-9 M) at ionic strength 0.28 and pH 7.0 is increased by the addition of 5-HT. No effect is observed in glycerol extreacted fibres. 3. Chemically treated fibres in catch-state induced at ionic strength 0.28 and pH 6.5. 4. Cyclic AMP (c-AMP) induces the same relaxing effects at 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 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.