THE INFLUENCE OF EXTRACELLULAR CA2-COURSE OF ISOVOLUMETRIC PRESSURE DEVELOPMENT IN THE GUINEAPIG URINARY-BLADDER( ON THE TIME)

Isovolumetric contractions were induced electrically in live guinea-pi g bladders in a 1.8 mM Ca2+ buffer solution al two different volumes. The rate of pressure development was studied by analysis of phase-plot s, which are plots of the rate of change of intravesical pressure as a function of pressure. At relatively large volumes the rate of pressur e development showed a biphasic nature, suggesting that two different processes were involved in different phases of contraction. Extracellu lar Ca2+ was manipulated to identify these processes. Removal of extra cellular Ca2+ led to an 85% reduction of the maximum isovolumetric pre ssure in subsequent stimulations, bur pressure developed faster. The b iphasic nature of pressure development at large volumes tended to disa ppear. It was concluded that at large bladder volumes pressure develop ment in this type of smooth muscle is determined by the release oi int racellularly stored Ca2+ in the early phase oi contraction. In the fin al phase the rate oi pressure development is limited by the extracellu lar Ca2+ influx. At small bladder volumes the Ca2+ influx takes place at a higher rate, so that the contribution of intracellular Ca2+ relea se to pressure development could not be identified al these volumes.