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

Abstract

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.