Laser flash photolysis of diazo-2, a caged calcium chelator: The relationship between the extent and rate of smooth muscle relaxation

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 (T1/2) and one-fifth-time (T1/5). 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 T1/2 and T1/5) 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.