To investigate mechanisms underlying the transmission of spontaneous Ca2+ signals in the bladder, changes in intracellular concentrations of Ca2+ ([Ca2+]i) were visualized in isolated detrusor smooth muscle bundles of the guinea-pig urinary bladder loaded with a fluorescent Ca2+ indicator, fura-PE3 or fluo-4. Spontaneous increases in [Ca2+]i (Ca2+ transients) preferentially originated along the boundary of muscle bundles and then spread to the other boundary (Ca2+ waves). The synchronicity of Ca2+ waves across the bundles was disrupted by 18beta-glycyrrhetinic acid (18beta-GA, 40 microm), carbenoxolone (30 microm) or 2-aminoethoxydiphenylborate (2-APB, 50-100 microm), while CPA (10 microm), ryanodine (100 microm), xestospongin C (3 microm) and U-73122 (10 microm) had no effect. Intracellular recordings using two independent microelectrodes demonstrated that 2-APB (100 microm) blocked electrical coupling between detrusor smooth muscle cells. Nifedipine (10 microm) but not nominal Ca2+-free solution diminished the synchronicity of Ca2+ 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 Ca2+ transients, which occurred independently of those of smooth muscles. In conclusion, the propagation of Ca2+ 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 Ca2+ channels, without significant contribution of intracellular Ca2+ stores. IC in the bladder may modulate the transmission of Ca2+ transients originating from smooth muscle cells rather than being the pacemaker of spontaneous activity.