1 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. 2 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. 3 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. 4 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.

Isolated interstitial (“pacemaker”) cells from rabbit urethra were examined using the perforated-patch technique. Under voltage clamp at −60 mV, these cells fired large spontaneous transient inward currents (STICs), averaging −860 pA and >1 s in duration, which could account for urethral pacemaker activity. Spontaneous transient outward currents (STOCs) were also observed and fell into two categories, “fast” (1 s in duration). The latter were coupled to STICs, suggesting that they shared the same mechanism, while the former occurred independently at faster rates. All of these currents were abolished by cyclopiazonic acid, caffeine, or ryanodine, suggesting that they were activated by Ca²⁺release. Whend- myo-inositol 1,4,5-trisphosphate (IP₃)-sensitive stores were blocked with 2-aminoethoxydiphenyl borate, the STICs and slow STOCs were abolished, but the fast STOCs remained. In contrast, the fast STOCs were more nifedipine sensitive than the STICs or the slow STOCs. These results suggest that while fast STOCs are mediated by a mechanism similar to STOCs in smooth muscle, STICs and slow STOCs are driven by IP₃. These results support the hypothesis that pacemaker activity in the urethra is driven by the IP₃-sensitive store.

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 ofPDGFR‐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.

Freshly dispersed sheep mesenteric lymphatic smooth muscle cells were studied at 37°C using the perforated patch-clamp technique with Cs⁺- and K⁺-filled pipettes. Depolarizing steps evoked currents that consisted ofl-type Ca²⁺ [ I _Ca(L)] current and a slowly developing current. The slow current reversed at 1 ± 1.5 mV with symmetrical Cl⁻ concentrations compared with 23.2 ± 1.2 mV ( n = 5) and −34.3 ± 3.5 mV ( n = 4) when external Cl⁻ was substituted with either glutamate (86 mM) or I⁻ (125 mM). Nifedipine (1 μM) blocked and BAY K 8644 enhanced I _Ca(L), the slow-developing sustained current, and the tail current. The Cl⁻ channel blocker anthracene-9-carboxylic acid (9-AC) reduced only the slowly developing inward and tail currents. Application of caffeine (10 mM) to voltage-clamped cells evoked currents that reversed close to the Cl⁻ equilibrium potential and were sensitive to 9-AC. Small spontaneous transient depolarizations and larger action potentials were observed in current clamp, and these were blocked by 9-AC. Evoked action potentials were triphasic and had a prominent plateau phase that was selectively blocked by 9-AC. Similarly, fluid output was reduced by 9-AC in doubly cannulated segments of spontaneously pumping sheep lymphatics, suggesting that the Ca²⁺-activated Cl⁻ current plays an important role in the electrical activity underlying spontaneous activity in this tissue.

LINGO1 is a transmembrane protein that is up-regulated in the cerebellum of patients with Parkinson’s disease (PD) and Essential Tremor (ET). Patients with additional copies of the LINGO1 gene also present with tremor. Pharmacological or genetic ablation of large conductance Ca²⁺-activated K⁺ (BK) channels also result in tremor and motor disorders. We hypothesized that LINGO1 is a regulatory BK channel subunit. We show that 1) LINGO1 coimmunoprecipitated with BK channels in human brain, 2) coexpression of LINGO1 and BK channels resulted in rapidly inactivating BK currents, and 3) LINGO1 reduced the membrane surface expression of BK channels. These results suggest that LINGO1 is a regulator of BK channels, which causes a “functional knockdown” of these currents and may contribute to the tremor associated with increased LINGO1 levels. Significance Large conductance calcium-activated potassium (BK) channels are ubiquitously expressed and alter cellular excitability. These channels are formed by four pore-forming α subunits whose biophysical and pharmacological properties are modulated by regulatory β and γ subunits. LINGO1 is a protein, previously shown to be upregulated in both Parkinson’s disease and Essential Tremor. Consequently, we investigated its effects on BK channels and demonstrate that LINGO1 associates with these channels in human cerebellum. LINGO1 causes BK channels to inactivate and to open at more negative potentials. Furthermore, coexpression of BK with LINGO1 also led to a reduction in BK channels in the membrane. Our data support the idea that LINGO1 is a regulatory subunit of BK channels.