Migrating peristaltic contractions in the mammalian upper urinary tract serve to propel urine from the kidney through the ureter to the bladder, where it is stored until micturition. In most mammals, circumferencially cut strips of the renal pelvis display spontaneous contractions which decrease in frequency as strips are taken from regions more distal of the renal calyx, the isolated ureter being quiescent in most mammals except man and pig (Constantinou et al. 1978;Constantinou, 1979). Extracellular sucrose gap recordings from the guinea-pig renal pelvis indicated that the spontaneous contractions were associated with simple oscillations in the membrane potential, analogous to the 'pacemaker' action potentials recorded in the sino-atrial node of the mammalian heart. In contrast, electrically evoked contractions in both the distal renal pelvis and ureter were associated with action potentials consisting of a rapid rising phase and a long plateau (Zawalinski et al. 1975;Constantinou et al. 1979;Santicioli & Maggi, 1997). Thus, it has often been postulated that the initiation of peristaltic contractions in the upper urinary tract involves the spontaneous generation of 'pacemaker' potentials within the proximal renal pelvis which trigger 'driven' action potentials and contraction in the usually quiescent more distal regions of the upper urinary tract (
1. Current clamp studies using two patch electrodes and morphological observations have been performed in guinea-pig mesenteric arterioles to evaluate intercellular electrical couplings.2. In electron micrographs, preparations were found to have a single layer of smooth muscle cells. Typical gap junctions were readily observed between endothelial cells only.3. While immunoreactivity to connexin 40 was strongly expressed on the membranes of endothelial cells only, that to connexin 43 was expressed on both smooth muscle and endothelial cell membranes.4. Neurobiotin injected into a smooth muscle cell diffused into several neighbouring smooth muscle cells while that injected into an endothelial cell diffused into many endothelial cells.5. Acetylcholine-induced hyperpolarizations were conducted from endothelial cells to smooth muscle cells with a relative amplitude of 80.1 %. Ba 2+-induced action potentials were conducted in the opposite direction with a relative amplitude of 92.4 %.6. An electrotonic potential produced in a smooth muscle cell by current injection diminished steeply with distance as it spread along the muscle layer, plateauing at distances beyond 25 µm. An electrotonic potential produced in an endothelial cell spread within the intima with virtually no reduction. Electrotonic potentials could conduct through myoendothelial couplings, which seemed to behave as ohmic resistors without rectification.7. The coupling resistance between adjacent smooth muscle cells was estimated to be at least 90 MΩ and that between a smooth muscle cell and the whole endothelial layer to be 0.9 GΩ.8. The results indicate that although the resistance of myoendothelial couplings is appreciable, the endothelium may be important as a low resistance path connecting many smooth muscle cells.
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.
1. Intracellular recordings were made from the parasympathetic ganglion cells that lie in the epicardium of the left atrium of guinea-pig heart near the interatrial septum. 2. Three distinct types of neurone were identified on the basis of their electrophysiological properties. In one group of neurones, S cells, somatic action potentials were followed by brief after-hyperpolarizations. In the other two sets of neurones, somatic action potentials were followed by prolonged after-hyperpolarizations.
Movement of the contents of the peripheral prostatic acini into the minor and major prostatic ducts is likely to occur via spontaneous contractions triggered by myogenic slow wave activity in the stromal wall. By analogy with the intestine and urethra prostatic interstitial cells may well act as the pacemaker for prostatic slow waves as well as form an intercellular communication network interacting with the intrinsic nerves and stromal cells.
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