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

Abstract

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 ([Ca2+]i). Bladder smooth muscle cells exhibited spontaneous transient increases in [Ca2+]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. Spontaneous increases in [Ca2+]i were always preceded by action potentials. Nifedipine (10 μM) abolished increases in both [Ca2+]i and action potentials. Caffeine (10 mM), ryanodine (50 μM) and cyclopiazonic acid (10 μM) reduced the amplitude of the associated increases in [Ca2+]i without preventing the generation of action potentials. 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. 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. Immunohistochemistry for Connexin 43 showed abundant punctate staining on the smooth muscle cell membranes. 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.

Spontaneous electrical rhythmicity and the role of the sarcoplasmic reticulum in the excitability of guinea pig gallbladder smooth muscle cells

“…These effects were associated with membrane depolarization and an increase in contractile activity. These results are consistent with previous observations in GBSM cells (24,29) and other visceral smooth muscle cells (9,42). The time-dependent decreases of these events caused by prolonged exposure (not significant) are similar to the observations on slow waves in the small intestine (4,19); however, unlike in the gallbladder, SERCA pump inhibitors did not cause a transient augmentation of slow wave action potential in the small intestine (17) and inhibited slow waves and spontaneous transient depolarizations in the guinea pig gastric pylorus (36,37).…”

Gap junction channel activity in short-term cultured human detrusor myocyte cell pairs: gating and unitary conductances

“…This observation is a clear indication of the presence and potential physiological relevance of the intercellular pathway. Consistent with these data, intercellular diffusion of hydrophilic dyes (i.e., Lucifer yellow) also has been observed between detrusor myocytes in situ, and gap junction-mediated intercellular calcium waves have been documented among detrusor myocytes in culture (37,38) and in situ (27,28). Furthermore, electron microscopy, immunogold labeling, confocal immunofluorescence, and Western blot techniques have all confirmed the presence of small junctional plaques composed of Cx43, and also Cx45, between human detrusor myocytes (25,32,36,37).…”

Macroscopic electrical propagation in the guinea pig urinary bladder

“…4, these propagations are by no means secure and may easily break down into smaller components, reverting back to microscopic propagation in one or a few bundles. We don't know what induces these spontaneous electrical collaborations involving many bundles, but it may be interesting to note that the interstitial cells that have been shown to occur in the wall of the bladder appear to be preferentially located along the edges of the bundles (6,25,26,37). If there were a possibility of signaling between adjacent bundles, for example through paracrine pathways, then electrical propagation across more bundles might then be promoted.…”

“…Hashitani et al (25) have meticulously studied the propagation of action potentials and of calcium waves along layers of bundles isolated from the guinea pig bladder and have shown that the electrical length constants were much higher in the longitudinal or axial direction compared with the transverse direction (425 vs. 12.5 m, respectively). In addition, neurobiotin injected into one cell would readily spread to neighboring cells located in the axial direction but not to those located in the transverse direction (25). Recording from two microelectrodes simultaneously, in pig and human bladder, revealed a similar anisotropy with the major axis in the longitudinal direction (24).…”