Novel drug targets for the pharmacotherapy of benign prostatic hyperplasia (BPH).

Benign prostatic hyperplasia (BPH) is the major cause of lower urinary tract symptoms in men aged 50 or older. Symptoms are not normally life threatening, but often drastically affect the quality of life. The number of men seeking treatment for BPH is expected to grow in the next few years as a result of the ageing male population. Estimates of annual pharmaceutical sales of BPH therapies range from $US 3 to 10 billion, yet this market is dominated by two drug classes. Current drugs are only effective in treating mild to moderate symptoms, yet despite this, no emerging contenders appear to be on the horizon. This is remarkable given the increasing number of patients with severe symptoms who are required to undergo invasive and unpleasant surgery. This review provides a brief background on prostate function and the pathophysiology of BPH, followed by a brief description of BPH epidemiology, the burden it places on society, and the current surgical and pharmaceutical therapies. The recent literature on emerging contenders to current therapies and novel drug targets is then reviewed, focusing on drug targets which are able to relax prostatic smooth muscle in a similar way to the α(1) -adrenoceptor antagonists, as this appears to be the most effective mechanism of action. Other mechanisms which may be of benefit are also discussed. It is concluded that recent basic research has revealed a number of novel drug targets such as muscarinic receptor or P2X-purinoceptor antagonists, which have the potential to produce more effective and safer drug treatments.

Heterogeneity amongst 5-HT3 receptor subunits: is this significant?

The serotonin 3 (5-HT3) receptor is a ligand gated ion channel unlike the other 5-HT receptors which are G protein coupled receptors. The functional 5-HT3 receptor forms a pentamer of five symmetrically arranged subunits surrounding a central pore. The 5-HT(3A) subunit was first identified at a molecular level and can form functional homomers or heteromers with the 5-HT(3B) subunit. Recently, three new 5-HT3 subunits have been discovered and these can also form functional heteromers with the 5-HT(3A) subunit. In addition, splice variants of the 5-HT3 subunits have also been reported. These findings have markedly increased the complexity of the 5-HT3 receptor and may form part of the explanation of unresolved differences between studies investigating 5-HT3 receptor function in cell lines compared with native tissues. In this review we discuss the properties of the different subunits and their distribution to determine if they contribute to functional changes in the 5-HT3 receptor. Several recent pharmacogenomic studies have revealed single nucleotide polymorphisms (SNPs) and other variations in the different 5-HT3 receptor subunits that are associated with various clinical conditions. We discuss the implications of these findings with respect to drug design and tailored pharmacogenomic therapies.

K+ channel modulation of slow wave activity in the guinea-pig prostate.

BACKGROUND AND PURPOSE: The aim of this study was to investigate the role of different K(+) channel populations and the inhibitory effect of various exogenously applied K(+) channel openers in the regulation of slow wave activity in the guinea-pig prostate. EXPERIMENTAL APPROACH: Recordings of membrane potential were made using intracellular microelectrodes. KEY RESULTS: Tetraethylammonium (TEA 300 micro M and 1 mM), iberiotoxin (150 nM) and 4-aminopyridine (4-AP 1 mM) increased the frequency of slow wave discharge. Apamin (1-200 nM) and glibenclamide (1 micro M) had no effect on slow wave activity. Lemakalim (1 micro M) and PCO-400 (1 micro M) abolished the slow waves, as did sodium nitroprusside (SNP 10 micro M) and calcitonin gene-related peptide (CGRP 100 nM). The inhibitory effect of these agents was independent of a significant change in membrane potential. In the presence of 4-AP (1 mM), TEA (1 mM) or glibenclamide (1 micro M) the inhibitory actions of SNP (10 micro M) were attenuated. The inhibitory actions of CGRP (100 nM) were also reversed by glibenclamide (1 micro M). In contrast, isoprenaline (1 micro M) did not alter the frequency of slow wave discharge. CONCLUSIONS AND IMPLICATIONS: These results demonstrate that BK(Ca) and 4-AP-sensitive K(+) channels regulate the frequency of prostatic slow wave discharge. SNP and CGRP abolish slow waves in a hyperpolarisation-independent manner, partially via opening of K(ATP) channels. BK(Ca) and 4-AP-sensitive K(+) channels also play an important role in the SNP-induced inhibition of slow wave activity. The lack of membrane hyperpolarisation associated with the SNP- and CGRP-induced inhibition implies that the channels involved in this action are not predominantly located on the smooth muscle cells.

K(+) channel blocker modulation of the refractory period in spontaneously active guinea-pig ureters.

The effects of various K(+) channel blockers on the spontaneous electrical activity of the smooth muscle cells of the ureter still attached to its primary pacemaker regions were investigated using standard intracellular microelectrode recording techniques. Spontaneous action potentials in the ureter were complex, consisting of an initial rapidly rising spike which was followed by a period of membrane oscillation, a quiescent plateau phase and terminated by an abrupt repolarisation and an after-hyperpolarisation with a peak "diastolic" potential of -66 mV. This after-hyperpolarization decayed slowly over 5-20 s until the underlying triggering potentials achieved threshold for another action potential discharge. Application of the Ca(2+)-entry blocker, nifedipine (1 mu;M), blocked action potential discharge within 2-5 min, after which the membrane settled at a potential of -55 mV. 4-Aminopyridine (4-AP)(1 mM for 2 min) and Ba(2+) (100 mu;M for 2 min) both depolarized significantly the diastolic potential. In 4-AP, this membrane depolarisation was associated with a decreased amplitude of the initial spike and an increase in the half-amplitude duration. In contrast, tetraethylammonium (TEA) (0.5 mM for 2 min) only increased the frequency and half-amplitude duration of these ureteric action potentials. Apamin (200 nM), Cs(+) (1 mM) and glibenclamide (1 microM) had no significant effects on any parameters of the ureteric action potential. It was concluded that the refractory period of the spontaneous action potentials in the whole-mount preparation of the upper urinary tract was determined by the opening of at least three K(+) channel populations: large conductance ('maxi K') Ca(2+)-activated K(+) channels; Ca(2+)-insensitive transiently opening K(+) (I(Kto)) channels and K(+)-selective inward rectifier channels.

Effects of nerve stimulation on spontaneously active preparations of the guinea pig ureter.

The effects of intrinsic nerve stimulation on the spontaneous electrical activity of the smooth muscle cells of the guinea pig ureter still attached to its renal pelvis were investigated using standard intracellular microelectrode techniques. Action potentials discharged spontaneously at a frequency of 3.3 +/- 0.2 min(-1) (n = 67) and consisted of an initial rapidly rising spike, followed by a variable period (0.2-5 s) of membrane potential oscillation and a quiescent plateau phase which was terminated by an abrupt repolarisation and after-hyperpolarisation to -66 mV. Transmural electrical stimulation (20-50 Hz for 2 s) transiently decreased the frequency of action potential discharge; the half-amplitude duration of the following action potentials, however, was transiently increased to 156 +/- 12% of control. Substance P (1 microM applied for 2 min) or neurokinin A (100 nM for 2 min) transiently increased the frequency of action potential discharge to 155 +/- 19% and 142 +/- 21%, respectively, of control. The excitatory actions of nerve stimulation or agonist application were reduced by the tachykinin antagonist, MEN 10,627 (1-3 microM), while the inhibitory actions of nerve stimulation were enhanced by MEN 10,627 (1 microM) or thiorphan (1 microM). Capsaicin (10 microM for 10-15 min) also evoked a transient increase in the frequency and half-amplitude duration of the ureteric action potentials, in a manner blocked by MEN 10,627 (3 microM), which was followed by a long period of membrane potential quiescence. Human calcitonin gene related peptide (hCGRP) (100 nM applied for 2-5 min) induced a time-dependent decrease in the frequency amplitude and duration of the spontaneous action potentials, in a manner blocked by glibenclamide (1 microM). It was concluded that the nerve-evoked excitatory and inhibitory changes in the parameters of the spontaneous ureteric action potentials arise from the release of the sensory neuropeptides, tachykinins and CGRP, respectively.

Identification of the cells underlying pacemaker activity in the guinea-pig upper urinary tract.

1. The varying profile of cell types along the muscle wall of the guinea-pig upper urinary tract was examined electrophysiologically, using intracellular microelectrodes, and morphologically, using both electron and confocal microscopy. 2. Simple 'pacemaker' oscillations (frequency of 8 min-1) of the membrane potential were recorded in both the pelvi-calyceal junction (83 % of cells) and the proximal renal pelvis (15 % of cells), but never in the distal renal pelvis or ureter. When filled with the cell marker, neurobiotin, 'pacemaker' cells were spindle shaped and approximately 160 microm in length. 3. In most cells of the ureter (100 %) and in both the proximal (75 %) and distal (89 %) renal pelvis, spontaneous action potentials (frequency of 3-5 min-1) consisted of an initial spike, followed by a number of potential oscillations superimposed on a plateau phase. When filled with neurobiotin, cells firing these 'driven' action potentials, were spindle shaped and > 250 microm in length. 4. Greater than 80 % of smooth muscle cells in the pelvi-calyceal junction were 'atypical', having < 40 % of their sectional areas occupied by loosely packed contractile filaments. Most of the smooth muscle cells in the ureter (99.7 %) and both the proximal (83 %) and distal (97.5 %) renal pelvis were of 'typical' appearance in that they contained cytoskeletal and contractile elements occupying > 60 % of cross-sectional area. 5. A third type of spontaneously discharging cell fired 'intermediate' action potentials (3-4 min-1), consisting of a single spike followed by a quiescent plateau and an abrupt repolarization. These cells were morphologically similar to interstitial cells of Cajal (ICC). However, these 'ICC-like' cells were not immuno-reactive for c-Kit, the proto-oncogene for tyrosine kinase. 6. In summary, 'atypical' smooth muscle cells were predominant in the pelvi-calyceal junction and fired 'pacemaker' potentials at a frequency significantly higher than 'driven' action potentials recorded in 'typical' smooth muscle cells throughout the renal pelvis and ureter. 'Intermediate' action potentials were recorded in 'ICC-like' cells in both the pelvi-calyceal junction and renal pelvis. We suggest that these 'ICC-like' cells act as a preferential pathway, conducting and amplifying pacemaker signals to initiate action potential discharge in the driven areas of the upper urinary tract.

Electrical basis of peristalsis in the mammalian upper urinary tract.

1. Peristalsis in the mammalian upper urinary tract (UUT) is mostly myogenic in origin, originating predominately in the proximal pelvicalyceal regions of the renal pelvis, an area that is enriched with specialized smooth muscle cells termed 'atypical' smooth muscle cells. Propagating peristaltic contractions are little affected by blockers of either autonomic nerve function or nerve impulse propagation; however, blockers of sensory nerve function or prostaglandin synthesis reduce both the frequency and the strength of the spontaneous contractions underlying peristalsis. 2. The electrical drive for these peristaltic contractions has long been considered to involve mechanisms analogous to the heart, such that 'atypical' smooth muscle cells generate spontaneous 'pacemaker' action potentials. These pacemaker potentials trigger the firing of action potentials and contraction in the muscular regions of the renal pelvis, which propagate distally to the ureter, propelling urine towards the bladder. 3. Recent intracellular microelectrode and single cell/channel patch-clamp studies have revealed that the ionic conductances underlying the action potentials recorded in the UUT are likely to involve the opening and slow closure of voltage-activated 'L-type' Ca2+ channels, offset by the time-dependent opening and closure of both voltage- and Ca(2+)-activated K+ channels. 4. In the present review we summarize the current knowledge of the ionic mechanisms underlying action potential discharge in the UUT, as well as present our view on how this electrical activity supports the initiation and conduction of UUT peristalsis.

Effects of nerve stimulation on the spontaneous action potentials recorded in the proximal renal pelvis of the guinea-pig.

The effects of nerve stimulation on the electrical and mechanical activity of the smooth muscle of the proximal renal pelvis of the guinea-pig were investigated using standard tension and microelectrode recording techniques. Spontaneous action potentials were deemed to have been recorded from three cell types: (1) "pacemaker" cells (9 of > 120) had membrane potentials (MPs) of -42.1 +/- 2.9 mV and fired action potentials of a simple waveform; (2) "driven" cells (> 100) had more stable MPs of -56.1 +/- 1.2 mV (n = 36) and more complex "ureter-like" action potentials; (3) the remaining cells had MPs of -45.5 +/- 1.7 mV (n = 15) and action potentials with a waveform "intermediate" to groups (1) and (2). Nifedipine (0.1-1 microM) and Cd2+ (0.1-1 mM) blocked all spontaneous action potential discharge and depolarized the membrane to near -40 mV. Intramural nerve stimulation (10-50 Hz for 1-10 s) increased both the amplitude and frequency of the spontaneous contractile activity, this increase peaked in about 30 s and decayed slowly over several minutes. Nerve stimulation depolarized pacemaker and driven cells 9.1 +/- 3.5 (n = 3) and 1.6 +/- 0.7 (n = 6) mV, respectively; the frequency of their action potential discharge increased from 7.6 +/- 2.7 and 9.9 +/- 1.1/min to 17.3 +/- 0.5 and 11.1 +/- 1.4/min, respectively. The duration of the action potentials in driven cells also increased significantly for several minutes. All these effects were blocked by tetrodotoxin (TTX) (1.6 microM). It was concluded that the positive chronotropic and inotropic effects of nerve stimulation on renal pelvis contractility can be correlated with the changes in the frequency and duration of the action potentials recorded in driven cells.