Modeling the contractility of urinary bladder smooth muscle using isometric contractions.

In the course of developing a clinical contractility index, isometric contractions of pig urinary bladder smooth muscle were analyzed in terms of phase plots (a plot of the rate of change of a variable, in this case force, as a function of the variable itself). The straight line describing the major part of the phase plots is characterized by the two parameters Fiso (the horizontal intercept) and c (the slope) of the line. It was found that changes in the geometry of the tissue sample and changes in extracellular calcium grossly influence Fiso, but not c, whereas changes in prestimulus rest time influence c, but not Fiso. It is concluded that Fiso is related to the number of available contractile units, and c is related to the limiting rate constant in the excitation-contraction coupling, which is probably determined by the release of intracellular calcium. The existence of the calcium paradox and the interpretation of the concept of facilitation or potentiation in this type of smooth muscle are discussed.