The force recovery following repeated quick releases applied to pig urinary bladder smooth muscle

A method for measuring several quick-releases during one contraction of a pig urinary bladder smooth muscle preparation was developed. The force recovery following quick release in this muscle type was studied by fitting a multiexponential model to 926 responses measured during the first 700 ms after release, both in the stimulated and in the unstimulated muscle. It was concluded that the force recovery in this observation window was biexponential and that the two time constants result from two fundamentally different processes. The slower time constant in the order of 0.45 s was ascribed to crossbridge cycling, and this hypothesis was supported by the considerable dependence of the amplitude associated with this time constant on the stimulus condition of the muscle. The faster time constant in the order of 0.032 s was found to be largely independent of the degree of stimulation of the muscle and was ascribed to a passive, viscoelastic process.