Effect of partial outlet obstruction on the biphasic response to field stimulation at different concentrations of calcium.

The contractile response of the rabbit urinary bladder smooth muscle to field stimulation is biphasic in nature consisting of an initial phasic contraction followed by a prolonged tonic contraction. The phasic contraction is mediated by a rapid rise in intracellular free calcium concentration which in turn is achieved by a combination of the entry of extracellular calcium into the cell and by the mobilization and release of calcium from intracellular storage sites. Partial bladder outlet obstruction in rabbits is associated with a marked depression of contractile function. The current experiments were designed to: (1) investigate the role of extracellular calcium in mediating the phasic and tonic components of the contractile response to field stimulation and (2) determine the effect of partial outlet obstruction on this relationship. The results can be summarized as follows: in the control bladder, increasing the extracellular calcium concentration from 0.6 to 5.4 mmol/l enhanced the phasic contractile response to a 2-Hz stimulation to a significantly greater degree than the response to a 16-Hz stimulation. Increasing the extracellular calcium concentration had no effect on the plateau phase of the contractile response. Partial outlet obstruction significantly reduced the phasic and tonic responses to all frequencies of stimulation. In general, there was a greater reduction in the tonic response to field stimulation (relative to the phasic response).