Ca²⁺ signalling in cultured smooth muscle cells from human bladder

It has been suggested that the isolation and culture of human detrusor smooth muscle cells may provide useful insights into the physiology of the intact detrusor muscle. In the present paper, data are presented from cultured human bladder smooth muscle cells isolated from small, routinely available biopsies. Since the initiation of contractions involves a rise in intracellular Ca²⁺, this study has focused on the mechanisms involved in the rise of Ca²⁺ in cultured cells. Exposure of cells to bathing solutions with elevated K⁺ concentrations resulted in an increase in Ca²⁺ consistent with the presence of voltage-activated Ca²⁺ channels. Agonists, including carbachol, histamine and ATP, also activated repetitive transient increases in Ca²⁺ in the presence and absence of external Ca²⁺. Spontaneous Ca²⁺ transients were recorded in 31% of cells isolated from normal bladders. Such spontaneous and agonist-induced oscillations were not abolished in depolarized cells, suggesting that the mechanisms underlying the oscillations are not dependent on the cyclical operation of voltage-operated Ca²⁺ channels. However, the spontaneous activity was inhibited by the Ca²⁺ blocker verapamil, pointing to the presence of Ca²⁺ channels. The operation of an IF, sensitive Ca²⁺ release mechanism was examined using saponin-permeabilized cells, which demonstrated that IF, increased the rate of ⁴⁵Ca²⁺ efflux. The conclusion from this study is that many of the mechanisms described in the intact tissue are operational in cultured cells.