The role of intracellular and extracellular calcium in mechanical and intracellular electrical activity of human urinary bladder smooth muscle.

We studied the role of extracellular and intracellular Ca2+ in human detrusor smooth muscle contraction. Simultaneous recordings of mechanical and intracellular electrical activity were made in three different Ca2+ concentrations: normal Krebs' solution (100%), 10% of the standard Ca2+ concentration and a solution in which Ca2+ was omitted from the medium (0%). Spontaneous contractions and KCl or CCh induced contractions were studied. Ryanodine and caffeine were used to manipulate the intracellular Ca2+ stores. The present results show that only a very small amount of Ca2+ in the extracellular space is sufficient to support spontaneous and induced contractions. Spike-shaped potentials and long lasting depolarisations were recorded in all three solutions. However, the prevalence of long lasting depolarisations increased when the extracellular Ca2+ concentration was reduced. The amplitude of the spike-shaped potentials and long lasting depolarisations appeared to be negatively affected by diminishing the extracellular Ca2+ concentration. Additionally, the duration of the long lasting depolarisations was reduced in 0% Ca2+. The contraction upon KCl stimulation was primarily depending on the extracellular Ca2+. Upon muscarinic receptor stimulation, a combined activation of Ca2+ mobilisation from intracellular and extracellular stores may occur; the ratio of contribution of these two sources changes in accordance with the requirements of the conditions.