Authors:
R. Pelc
University Laboratory of Physiology, Sherrington/Parks Road, Oxford OX1 3PT, United Kingdom and Academy of Sciences, Institutes of Microbiology and Physiology, K Vy´zkumny´m u´stavům (Vi´deňska´) 1083, CZ-14220 Praha 4-Krč, Czech Republic

N. Ishii
University Laboratory of Physiology, Sherrington/Parks Road, Oxford OX1 3PT, United Kingdom

C. C. Ashley
University Laboratory of Physiology, Sherrington/Parks Road, Oxford OX1 3PT, United Kingdom

Relaxations brought about by laser flash photolysis of diazo-2, a caged calcium chelator, were recorded in the anterior byssus retractor muscle (ABRM) of Mytilus edulis (a molluscan “catch” muscle). The relaxation rate was characterized by half-time (T_1/2) and one-fifth-time (T_1/5). We show that, unlike, e.g., in the mammalian cardiac muscle, the relaxation rate increases with the extent of relaxation (the latter being expressed as a percentage of an immediately preceding activation by Ca²⁺), in a manner similar to the Ca²⁺ activation process, as shown in other muscles [Lea and Ashley, J. Physiol. (Lond.) 427, 435–453 (1990); Saeki et al., J. Mol. Cell. Cardiol. 36, 371–380 (2004)]. This increase (i.e., decrease in T_1/2 and T_1/5) levels off at extent of relaxation ∼50%. The present paper highlights the need to take into account the differences in the extent of smooth muscle (especially the catch or “latch” type) relaxation. The data may be used in modeling the mechanism underlying the calcium-dependent (phasic) smooth muscle relaxations.