On the tropomyosin-paramyosin system in relation to the viscous tone of lamellibranch ‘catch’ muscle

Abstract

Some lamellibranch muscles are able to maintain tension for prolonged periods with very little energy expenditure, possibly by means of a special ‘catch' mechanism. Whilst actomyosin seems to be associated with active contraction, the 'catch' might well be linked with tropomyosin A ( TM A ) which is more abundant in these muscles than actomyosin ( AM ) and located in special filaments known as paramyosin. There is in fact a correlation of tropo­myosin content and ‘tonic’ properties in three different portions of Pecten maximus adductor muscle, the striated adductor ( AM/TM = 9:1), the translucent portion of the smooth adductor ( AM/TM = 2:1), and the opaque portion of the latter ( AM/TM = 1:2). A typical ‘catch’ can be demonstrated in the glycerinated fibres of the tropomyosin-rich but not in the tropomyosin-poor type by measuring fibre rigidity in presence of adenosine triphosphate ( ATP ) and Mg ions. Quick release and stretch experiments have shown that, unlike active contraction, rigidity in presence of ATP is not dependent on a functional AM system, which can be inactivated with Salyrgan, denatured with ethanol or plasticized with pyrophosphate. Instead, a change in fibre rigidity is always found to be associated with a change in the colloidal state of TM A when the latter is selectively modified in situ , as, for example, by varying the pH between 6⋅5 and 8 or the ratio of the Mg- ATP chelate to 'free' ATP . The latter increases the solubility and the electrophoretic mobility of TM A and it plasticizes artificial tropomyosin threads or the tropomyosin system in situ . Its action is best explained in terms of a decrease in protein-protein interaction caused by an increase in net charge due to binding, an effect which it counteracted by divalent cations which combine with ATP . Extra­-polating these findings to the behaviour of living muscle one might tentatively suggest that the high rigidity of glycerinated tonic adductor muscle fibres in presence of Mg- ATP corresponds to the high muscle ‘viscosity’ of tonically contracted living molluscan muscle and that free ATP acts as a physiological plasticizer, inducing changes in muscle ‘viscosity’ by affecting the electrostatic tropomyosin-tropomyosin interaction either within or between the para­myosin (tropomyosin) filaments.