Collagenase dispersal of strips of rabbit urethra yielded, in addition to normal spindle‐shaped smooth muscle cells, a small proportion of branched cells which resembled the interstitial cells of Cajal dispersed from canine colon. These were clearly distinguishable from smooth muscle in their appearance under the phase‐contrast microscope, their immunohistochemistry and their ultrastructure. They had abundant vimentin filaments but no myosin, a discontinuous basal lamina, sparse rough endoplasmic reticulum, many mitochondria and a well‐developed smooth endoplasmic reticulum. Interstitial cells were non‐contractile but exhibited regular spontaneous depolarisations in current clamp. These could be increased in frequency by noradrenaline and blocked by perfusion with calcium‐free solution. In voltage clamp they showed abundant calcium‐activated chloride current and spontaneous transient inward currents which could be blocked by chloride channel blockers. The majority of smooth muscle cells were vigorously contractile when stimulated but did not show spontaneous electrical activity in current clamp. In voltage clamp, smooth muscle cells showed very little calcium‐activated chloride current. We conclude that there are specialised pacemaking cells in the rabbit urethra that may be responsible for initiating the slow waves recorded from smooth muscle cells in the intact syncitium.

No abstract

SUMMARY1. Segments of bovine mesenteric lymphatic of varying diameter taken from different parts of the lymphatic tree were cut to 20 mm in length and set up so that measurements could be made of spontaneous isometric contractions.2. There was considerable variability in frequency of spontaneous contractions but this was independent of resting tension. There was no significant correlation between lymphatic diameter and inherent frequency of contraction.3. Isolated segments of bovine mesenteric lymphatic 70-80 mm in length were cannulated and set up in a three-compartment organ bath which allowed independent temperature control in each compartment. Pressure was recorded at inflow and outflow ends and experiments were video recorded.4. Contractile activity was normally initiated at the end of the lymphatic maintained at the higher temperature and the contractile wave was propagated along the length of the vessel.5. Propagation could occur either in the direction of valve orientation (orthograde) or retrogradely. The volume of fluid pumped was not significantly affected by the direction of propagation.6. Perfusion of the central compartment with Krebs solution at 0-2°C disrupted normal propagation and allowed the two parts of the lymphatic to contract at different frequencies although the two parts maintained an approximately 2:1 ratio.7. Perfusion of the central compartment with 10 mM-heptanol also disrupted normal propagation but the rates on either side of the partition bore no harmonic relationship to one another.8. These results suggest that relatively short segments of lymph duct have the ability to contract spontaneously and that their inherent frequencies are not determined by their position in the lymphatic tree. The results are consistent with the existence of electrical coupling along the lymphatic's length and they suggest that over distances of at least 80 mm independent pacemakers are capable of mutual entrainment at a frequency representing a compromise between the fastest and slowest components.

Measurements were made (using fast confocal microscopy) of intracellular Ca 2+ levels in fluo-4 loaded interstitial cells isolated from the rabbit urethra.

Specific classes of interstitial cells exist in visceral organs and have been implicated in several physiological functions including pacemaking and mediators in neurotransmission. In the bladder, Kit+ interstitial cells have been reported to exist and have been suggested to be neuromodulators. More recently a second interstitial cell, which is identified using antibodies against platelet-derived growth factor receptor-α (PDGFR-α) has been described in the gastrointestinal (GI) tract and has been implicated in enteric motor neurotransmission. In this study, we examined the distribution of PDGFR-α+ cells in the murine urinary bladder and the relation that these cells may have with nerve fibres and smooth muscle cells. Platelet-derived growth factor receptor-α+ cells had a spindle shape or stellate morphology and often possessed multiple processes that contacted one another forming a loose network. These cells were distributed throughout the bladder wall, being present in the lamina propria as well as throughout the muscularis of the detrusor. These cells surrounded and were located between smooth muscle bundles and often came into close morphological association with intramural nerve fibres. These data describe a new class of interstitial cells that express a specific receptor within the bladder wall and provide morphological evidence for a possible neuromodulatory role in bladder function.