Origin and propagation of spontaneous excitation in smooth muscle of the guinea‐pig urinary bladder

1 The origin and propagation of waves of spontaneous excitation in bundles of smooth muscle of the guinea‐pig bladder were examined using intracellular recording techniques and visualization of the changes in the intracellular calcium concentration ([Ca 2+ ] i ). 2 Bladder smooth muscle cells exhibited spontaneous transient increases in [Ca 2+ ] i which originated along a boundary of each smooth muscle bundle and then spread to the other boundary with a conduction velocity of 2.0 mm s −1 . 3 Spontaneous increases in [Ca 2+ ] i were always preceded by action potentials. Nifedipine (10 μM) abolished increases in both [Ca 2+ ] i and action potentials. Caffeine (10 mM), ryanodine (50 μM) and cyclopiazonic acid (10 μM) reduced the amplitude of the associated increases in [Ca 2+ ] i without preventing the generation of action potentials. 4 Spontaneous action potentials had conduction velocities of 40 mm s −1 in the axial direction and 1.3 mm s −1 in the transverse direction. The electrical length constants of the bundles of muscle were 425 μm in the axial direction and 12.5 μm in the transverse direction. 5 Neurobiotin, injected into an impaled smooth muscle cell, spread more readily to neighbouring cells located in the axial direction than those located in the transverse direction. The spread of neurobiotin was inhibited by 18β‐glycyrrhetinic acid (18β‐GA, 40 μM), a gap junction blocker. 6 Immunohistochemistry for Connexin 43 showed abundant punctate staining on the smooth muscle cell membranes. 7 These results suggested that spontaneous action potentials and associated calcium waves occur almost simultaneously along the boundary of bladder smooth muscle bundles and then propagate to the other boundary probably through gap junctions.