The influence of extracellular Ca2+ on the time course of isovolumetric pressure development in the guinea-pig urinary bladder.

Isovolumetric contractions were induced electrically in five guinea-pig bladders in a 1.8 mM Ca2+ buffer solution at two different volumes. The rate of pressure development was studied by analysis of phase-plots, which are plots of the rate of change of intravesical pressure as a function of pressure. At relatively large volumes the rate of pressure development showed a biphasic nature, suggesting that two different processes were involved in different phases of contraction. Extracellular Ca2+ was manipulated to identify these processes. Removal of extracellular Ca2+ led to an 85% reduction of the maximum isovolumetric pressure in subsequent stimulations, but pressure developed faster. The biphasic nature of pressure development at large volumes tended to disappear. It was concluded that at large bladder volumes pressure development in this type of smooth muscle is determined by the release of intracellularly stored Ca2+ in the early phase of contraction. In the final phase the rate of pressure development is limited by the extracellular Ca2+ influx. At small bladder volumes the Ca2+ influx takes place at a higher rate, so that the contribution of intracellular Ca2+ release to pressure development could not be identified at these volumes.