Role of interstitial cells and gap junctions in the transmission of spontaneous Ca²⁺ signals in detrusor smooth muscles of the guinea‐pig urinary bladder

Abstract

To investigate mechanisms underlying the transmission of spontaneous Ca 2+ signals in the bladder, changes in intracellular concentrations of Ca 2+ ([Ca 2+ ] i ) were visualized in isolated detrusor smooth muscle bundles of the guinea‐pig urinary bladder loaded with a fluorescent Ca 2+ indicator, fura‐PE3 or fluo‐4. Spontaneous increases in [Ca 2+ ] i (Ca 2+ transients) preferentially originated along the boundary of muscle bundles and then spread to the other boundary (Ca 2+ waves). The synchronicity of Ca 2+ waves across the bundles was disrupted by 18β‐glycyrrhetinic acid (18β‐GA, 40 μ m ), carbenoxolone (30 μ m ) or 2‐aminoethoxydiphenylborate (2‐APB, 50–100 μ m ), while CPA (10 μ m ), ryanodine (100 μ m ), xestospongin C (3 μ m ) and U‐73122 (10 μ m ) had no effect. Intracellular recordings using two independent microelectrodes demonstrated that 2‐APB (100 μ m ) blocked electrical coupling between detrusor smooth muscle cells. Nifedipine (10 μ m ) but not nominal Ca 2+ ‐free solution diminished the synchronicity of Ca 2+ waves before preventing their generation. Staining for c‐kit identified interstitial cells (IC) located along both boundaries of muscle bundles. IC were also scattered amongst smooth muscle cells and were more dominantly distributed in connective tissue between muscle bundles. IC generated nifedipine‐resistant spontaneous Ca 2+ transients, which occurred independently of those of smooth muscles. In conclusion, the propagation of Ca 2+ transients in the bladder appears to be exclusively mediated by the spread of action potentials through gap junctions being facilitated by the regenerative nature of L‐type Ca 2+ channels, without significant contribution of intracellular Ca 2+ stores. IC in the bladder may modulate the transmission of Ca 2+ transients originating from smooth muscle cells rather than being the pacemaker of spontaneous activity.