Regulation of ACh release from guinea pig bladder urothelial cells: potential role in bladder filling sensations

BACKGROUND AND PURPOSE

The aim of this study was to quantify and characterize the mechanism of non-neuronal ACh release from bladder urothelial cells and to determine if urothelial cells could be a site of action of anti-muscarinic drugs.

EXPERIMENTAL APPROACH

A novel technique was developed whereby ACh could be measured from freshly isolated guinea pig urothelial cells in suspension following mechanical stimulation. Various agents were used to manipulate possible ACh release pathways in turn and to study the effects of muscarinic receptor activation and inhibition on urothelial ATP release.

KEY RESULTS

Minimal mechanical stimulus achieved full ACh release, indicating a small dynamic range and possible all-or-none signal. ACh release involved a mechanism dependent on the anion channel CFTR and intracellular calcium concentration, but was independent of extracellular calcium, vesicular trafficking, connexins or pannexins, organic cation transporters and was not affected by botulinum-A toxin. Stimulating ACh receptors increased ATP production and antagonizing them reduced ATP release, suggesting a link between ACh and ATP release.

CONCLUSIONS AND IMPLICATIONS

These results suggest that release of non-neuronal ACh from the urothelium is large enough and well located to act as a modulator of ATP release. It is hypothesized that this pathway may contribute to the actions of anti-muscarinic drugs in reducing the symptoms of lower urinary tract syndromes. Additionally the involvement of CFTR in ACh release suggests an exciting new direction for the treatment of these conditions.

Pharmacological differences between the human and rat vanilloid receptor 1 (VR1)

Vanilloid receptors (VR1) were cloned from human and rat dorsal root ganglion libraries and expressed in Xenopus oocytes or Chinese Hamster Ovary (CHO) cells. Both rat and human VR1 formed ligand gated channels that were activated by capsaicin with similar EC(50) values. Capsaicin had a lower potency on both channels, when measured electrophysiologically in oocytes compared to CHO cells (oocytes: rat=1.90+/-0.20 microM; human=1.90+/-0.30 microM: CHO cells: rat=0.20+/-0.06 microM; human=0.19+/-0.08 microM). In CHO cell lines co-expressing either rat or human VR1 and the calcium sensitive, luminescent protein, aequorin, the EC(50) values for capsaicin-induced responses were similar in both cell lines (rat=0.35+/-0.06 microM, human=0.53+/-0.03 microM). The threshold for activation by acidic solutions was lower for human VR1 channels than that for rat VR1 (EC(50) pH 5.49+/-0.04 and pH 5.78+/-0.09, respectively). The threshold for heat activation was identical (42 degrees C) for rat and human VR1. PPAHV was an agonist at rat VR1 (EC(50) between 3 and 10 microM) but was virtually inactive at the human VR1 (EC(50)>10 microM). Capsazepine and ruthenium red were both more potent at blocking the capsaicin response of human VR1 than rat VR1. Capsazepine blocked the human but not the rat VR1 response to low pH. Capsazepine was also more effective at inhibiting the noxious heat response of human than of rat VR1.

The effects of pH on the interaction between capsaicin and the vanilloid receptor in rat dorsal root ganglia neurons

1. The vanilloid receptor of sensory neurons is a polymodal nociceptor sensitive to capsaicin, protons, heat and anandamide. Although it is known that interaction occurs between these different mediators the mechanism by which this occurs is poorly understood. In this study capsaicin elicited currents were recorded from vanilloid receptors found in adult rat isolated dorsal root ganglia (DRG) neurons under conditions of varying pH and the mechanism whereby protons can modulate this capsaicin response investigated. 2. Under whole-cell voltage clamp, modulating extracellular pH shifted the position of the capsaicin log(concentration)-response curve. Acidification from pH 9.0 to pH 5.5 lowered the EC50 values from 1150+/-250 nM to 5+/-2 nM with coincident change in the mean apparent slope factor from 2.3+/-0.3 to 0.9+/-0.2 and no change in maximal response. 3. The magnitude of the potentiation seen on reducing extracellular pH was not significantly affected by changes in extracellular calcium and magnesium concentration. 4. The response to capsaicin was not potentiated by a reduction in intracellular pH suggesting a site of action more accessible from the extracellular than the intracellular side of the membrane. 5. Potentiation by low pH was voltage independent indicating a site of action outside the membrane electric field. 6. At the single channel level, reducing extracellular pH increased channel open probability but had no significant effect on single channel conductance or open time. 7. These results are consistent with a model in which, on reducing extracellular pH, the vanilloid receptor in rat DRG neurons, changes from a state with low affinity for capsaicin to one with high affinity, coincident with a loss of cooperativity. This effect, presumed to be proton mediated, appears to involve one or more sites with pK(a) value 7.4-7.9, outside the membrane electrical field on an extracellularly exposed region of the receptor protein.

The amino terminus of receptor activity modifying proteins is a critical determinant of glycosylation state and ligand binding of calcitonin receptor-like receptor

The calcitonin receptor-like receptor (CRLR) can function as either a receptor for calcitonin gene-related peptide (CGRP) or for adrenomedullin (ADM), depending upon the coexpression of a novel family of single transmembrane proteins, which we have called receptor activity modifying proteins or RAMPs. RAMPs 1, 2, and 3 transport CRLR to the plasma membrane with similar efficiencies, however RAMP1 presents CRLR as a terminally glycosylated, mature glycoprotein and a CGRP receptor, whereas RAMPs 2 and 3 present CRLR as an immature, core glycosylated ADM receptor. Characterization of the RAMP2/CRLR and RAMP3/CRLR receptors in HEK293T cells by radioligand binding (125I-ADM as radioligand), functional assay (cAMP measurement), or biochemical analysis (SDS-polyacrylamide gel electrophoresis) revealed them to be indistinguishable, even though RAMPs 2 and 3 share only 30% identity. Chimeric proteins were created with the transmembrane and cytosolic portions of RAMP1 associated with the amino terminus of RAMP2 (RAMP2/1) and vice versa (RAMP1/2). Coexpression of RAMP2/1 with CRLR formed a core glycosylated ADM receptor, whereas the RAMP1/2 chimera generated both core glycosylated and mature forms of CRLR and enabled both ADM and CGRP receptor binding. Hence, the glycosylation state of CRLR appears to correlate with its pharmacology.

Capsaicin sensitivity is associated with the expression of the vanilloid (capsaicin) receptor (VR1) mRNA in adult rat sensory ganglia

A vanilloid receptor (VR1) has recently been cloned and shown to be a target for capsaicin, the excitotoxic component of capsicum peppers (Caterina, M.J., Schumacher, M.A., Tominaga, M., Rosen, T.A., Levine, J.D. and Julius, D., Nature, 389 (1997) 816-824). The effects of capsaicin appear to be selective for a subset of sensory neurones which includes polymodal nociceptors. The present study describes the distribution of VR1 mRNA, together with measurements of capsaicin sensitivity, in sensory nerve ganglia of different embryological origins and a single sympathetic ganglion, the superior cervical ganglion (SCG). In situ hybridisation revealed the expression of VR1 mRNA in small-to-medium-sized neurones of the dorsal root, trigeminal and vagal ganglia. No hybridisation signal was observed in the SCG neurones. This pattern of expression correlated with capsaicin sensitivity measured by whole-cell voltage clamp where, in similar sized cells, over 80% of neurones from dorsal root and vagal ganglia were capsaicin sensitive, but all SCG neurones were insensitive to capsaicin.

RAMPs regulate the transport and ligand specificity of the calcitonin-receptor-like receptor

Calcitonin-gene-related peptide (CGRP) and adrenomedullin are related peptides with distinct pharmacological profiles. Here we show that a receptor with seven transmembrane domains, the calcitonin-receptor-like receptor (CRLR), can function as either a CGRP receptor or an adrenomedullin receptor, depending on which members of a new family of single-transmembrane-domain proteins, which we have called receptor-activity-modifying proteins or RAMPs, are expressed. RAMPs are required to transport CRLR to the plasma membrane. RAMP1 presents the receptor at the cell surface as a mature glycoprotein and a CGRP receptor. RAMP2-transported receptors are core-glycosylated and are adrenomedullin receptors.

Voltage-dependent block by L- cis -diltiazem of the cyclic GMP-activated conductance of salamander rods

Block by L-cis-diltiazem of the cyclic GMP-activated conductance was studied in excised inside-out patches from the salamander rod outer segment. When L-cis-diltiazem was applied from the cytoplasmic face of the patch, current suppression increased monotonically with membrane depolarization, the ratio of blocked to unblocked current varying e-fold in 50 mV. This suggests that L-cis-diltiazem interacts with a binding site located about half-way across the membrane field, and is unable to fully traverse the cyclic GMP-activated channel. The kinetics of block were accelerated by increasing L-cis-diltiazem concentration and by depolarization. These results can be fitted by a single barrier model in which the barrier peak is located about a third of the way across the membrane field from the cytoplasmic face. Application of L-cis-diltiazem from the extracellular face of the patch also resulted in an enhancement of block with membrane depolarization. Indirect evidence supports the notion that this block resulted from partition of the unchanged form of the blocker across the membrane, and its subsequent interaction with the cytoplasmic face of the conductance.

The effect of pH on the block by L-cis-diltiazem and amiloride of the cyclic GMP-activated conductance of salamander rods

Block by L-cis-diltiazem and amiloride of the cyclic GMP-activated conductance was studied in inside-out patches excised from the salamander rod outer segment. When cytoplasmic pH was varied, the steady-state level of block by L-cis-diltiazem changed in the way that would be predicted if it were the protonated ammonium group that is responsible for effecting block. This is in contrast to the recent results of Haynes (J. gen. Physiol. 100, 783 (1992)) in catfish cones, where no such change was seen. Amiloride was found to block the conductance with a similar voltage dependence to that of L-cis-diltiazem. The dependence of amiloride block on cytoplasmic pH was found to be shifted relative to that of L-cis-diltiazem, consistent with the 1 pH-unit higher pKa value of amiloride and the idea that it is only the charged form of amiloride which can effect block. This suggests that the results seen with L-cis-diltiazem were indeed due to changes in the proportion of blocker in the protonated form, and not to effects of protons on the channel.