Effects of phosphorylation-related drugs on slow Ca2+ tail current in guinea-pig detrusor cells

Urinary bladder smooth muscle ion channels: expression, function, and regulation in health and disease

Urinary bladder smooth muscle (UBSM), also known as detrusor smooth muscle, forms the bladder wall and ultimately determines the two main attributes of the organ: urine storage and voiding. The two functions are facilitated by UBSM relaxation and contraction, respectively, which depend on UBSM excitability shaped by multiple ion channels. In this review, we summarize the current understanding of key ion channels establishing and regulating UBSM excitability and contractility. They include excitation-enhancing voltage-gated Ca2+ (Cav) and transient receptor potential channels, excitation-reducing K+ channels, and still poorly understood Cl- channels. Dynamic interplay among UBSM ion channels determines the overall level of Cav channel activity. The net Ca2+ influx via Cav channels increases global intracellular Ca2+ concentration, which subsequently triggers UBSM contractility. Here, for each ion channel type, we describe UBSM tissue/cell expression (mRNA and protein) profiles and their role in regulating excitability and contractility of UBSM in various animal species, including the mouse, rat, and guinea pig, and, most importantly, humans. The currently available data reveal certain interspecies differences, which complicate the translational value of published animal research results to humans. This review highlights recent developments, findings on genetic knockout models, pharmacological data, reports on UBSM ion channel dysfunction in animal bladder disease models, and the very limited human studies currently available. Among all gaps in present-day knowledge, the unknowns on expression and functional roles for ion channels determined directly in human UBSM tissues and cells under both normal and disease conditions remain key hurdles in the field.

Alpha1-, alpha2- and beta-adrenoceptors in the urinary bladder, urethra and prostate

1 We have systematically reviewed the presence, functional responses and regulation of alpha(1)-, alpha(2)- and beta-adrenoceptors in the bladder, urethra and prostate, with special emphasis on human tissues and receptor subtypes. 2 Alpha(1)-adrenoceptors are only poorly expressed and play a limited functional role in the detrusor. Alpha(1)-adrenoceptors, particularly their alpha(1A)-subtype, show a more pronounced expression and promote contraction of the bladder neck, urethra and prostate to enhance bladder outlet resistance, particularly in elderly men with enlarged prostates. Alpha(1)-adrenoceptor agonists are important in the treatment of symptoms of benign prostatic hyperplasia, but their beneficial effects may involve receptors within and outside the prostate. 3 Alpha(2)-adrenoceptors, mainly their alpha(2A)-subtype, are expressed in bladder, urethra and prostate. They mediate pre-junctional inhibition of neurotransmitter release and also a weak contractile effect in the urethra of some species, but not humans. Their overall post-junctional function in the lower urinary tract remains largely unclear. 4 Beta-adrenoceptors mediate relaxation of smooth muscle in the bladder, urethra and prostate. The available tools have limited the unequivocal identification of receptor subtypes at the protein and functional levels, but it appears that the beta(3)- and beta(2)-subtypes are important in the human bladder and urethra, respectively. Beta(3)-adrenoceptor agonists are promising drug candidates for the treatment of the overactive bladder. 5 We propose that the overall function of adrenoceptors in the lower urinary tract is to promote urinary continence. Further elucidation of the functional roles of their subtypes will help a better understanding of voiding dysfunction and its treatment.

Tyrosine kinase inhibitors and ATP modulate the conversion of smooth muscle L-type Ca2+ channels toward a second open state

Properties of smooth and cardiac L-type Ca2+ channels differ prominently in several physiological aspects, including sympathetic modulation. To assess the possible underlying mechanisms, we applied the whole cell patch-clamp technique to guinea pig detrusor smooth muscle cells, in which only L-type Ca2+ channel currents are observed in practice. During depolarization to large positive potentials, the conformation of the majority of L-type Ca2+ channels is converted from the normal (O1) to a second open state (O2), which undergoes little inactivation during depolarization. Extracellular application of genistein, a known tyrosine kinase inhibitor, significantly attenuated the voltage-dependent conversion of Ca2+ channels to O2, accompanied by reduction of availability, whereas genistin, an inactive analog, had little effect. In the absence of ATP in the patch pipette, intracellular application of either genistein or tyrphostin-47 suppressed the conversion to O2. Computer calculation revealed that the acceleration of the O1 to an inactivated state qualitatively reconstructs the unique effects of PTK inhibitors antagonized by ATP. We concluded that under normal conditions smooth muscle L-type Ca2+ channels are already modulated by tyrosine-kinase and ATP-related mechanism(s) and thereby easily achieve the second conversion, which yields voltage-dependent modulation of L-type Ca2+ current analogous to that in cardiac myocytes during beta-adrenoceptor stimulation.

Differential regulation of Ca2+-activated K+ channels by beta-adrenoceptors in guinea pig urinary bladder smooth muscle

Stimulation of beta-adrenoceptors contributes to the relaxation of urinary bladder smooth muscle (UBSM) through activation of large-conductance Ca(2+)-activated K(+) (BK) channels. We examined the mechanisms by which beta-adrenoceptor stimulation leads to an elevation of the activity of BK channels in UBSM. Depolarization from -70 to +10 mV evokes an inward L-type dihydropyridine-sensitive voltage-dependent Ca(2+) channel (VDCC) current, followed by outward steady-state and transient BK current. In the presence of ryanodine, which blocks the transient BK currents, isoproterenol, a nonselective beta-adrenoceptor agonist, increased the VDCC current by approximately 25% and the steady-state BK current by approximately 30%. In the presence of the BK channel inhibitor iberiotoxin, isoproterenol did not cause activation of the remaining steady-state K(+) current component. Decreasing Ca(2+) influx through VDCC by nifedipine or depolarization to +80 mV suppressed the isoproterenol-induced activation of the steady-state BK current. Unlike forskolin, isoproterenol did not change significantly the open probability of single BK channels in the absence of Ca(2+) sparks and with VDCC inhibited by nifedipine. Isoproterenol elevated Ca(2+) spark (local intracellular Ca(2+) release through ryanodine receptors of the sarcoplasmic reticulum) frequency and associated transient BK currents by approximately 1.4-fold. The data support the concept that in UBSM beta-adrenoceptor stimulation activates BK channels by elevating Ca(2+) influx through VDCC and by increasing Ca(2+) sparks, but not through a Ca(2+)-independent mechanism. This study reveals key regulatory molecular and cellular mechanisms of beta-adrenergic regulation of BK channels in UBSM that could provide new targets for drugs in the treatment of bladder dysfunction.

Inward current oscillation underlying tonic contraction caused via ETA receptors in pig detrusor smooth muscle

Endothelin-1 (ET-1) is a powerful vasoconstricting peptide. Recent studies showed synthesis of ET-1 and the presence of ET receptors in urinary bladder smooth muscle cells. In the present study, we investigated the possible role of ET-1 in detrusor contraction and its underlying mechanisms in terms of electrical activity. ET-1 caused dose-dependent tonic contraction of bladder smooth muscle strips. Whole cell patch-clamp experiments revealed that ET-1 induced a single transient inward current in the majority of detrusor cells and that additional inward current oscillations were induced in one-third of the cells. The inward current oscillation and tonic contraction shared several characteristic features: 1) both activities lasted for a considerable time after ET-1 washout and 2) only prior application of ETA receptor antagonists, not ETB receptor antagonists, significantly suppressed ET-1-induced contractions and the oscillating inward currents. It was concluded that the inward current oscillation underlies ET-1-induced tonic contraction. Experiments with ion substitution and channel blockers suggested that periodic activation of Ca2+-activated Cl- channels caused the oscillating inward currents.

Negative modulation of L-type Ca2+ channels via beta-adrenoceptor stimulation in guinea-pig detrusor smooth muscle cells

beta-Adrenergic stimulation enhances the activity of L-type Ca(2+) channels through mechanisms mediated by adenosine 3'5'-cyclic monophosphate (cAMP) and protein kinase A in cardiac myocytes. However, in smooth muscle cells, the effect of beta-adrenoceptor stimulation on the L-type Ca(2+) channel activity has been controversial, and the exact mechanism is still unclear. The present study was aimed at elucidating the effect of beta-adrenergic stimulation upon the activity of L-type Ca(2+) channels in guinea-pig detrusor smooth muscle cells. Isoproterenol (0.1-1 microM) inhibited Ba(2+) currents through L-type Ca(2+) channels (I(Ba)). Isoproterenol (0.1 microM) shifted the steady-state inactivation curve to negative voltages by 11 mV without affecting activation curves. The stimulation of cAMP-mediated signal transduction pathway by forskolin, 8-bromoadenosine 3'5'-cyclic monophosphate (8-Br-cAMP), or the intracellular application of cAMP also mimicked the effects of isoproterenol on I(Ba), which was blocked by the inhibition of protein kinase A. These results indicate that, in detrusor smooth muscles, the stimulation of beta-adrenoceptors exerts negative modulation of L-type Ca(2+) channels via cAMP/protein kinase A-dependent mechanism.

Slow deactivation and U-shaped inactivation properties in cloned Cav1.2b channels in Chinese hamster ovary cells

Whole-cell patch-clamp techniques were applied to Chinese hamster ovary cells stably expressing cloned smooth muscle Ca(2+) channel alpha(1)-subunits. In the presence of Ba(2+) as a charge carrier, U-shaped inactivation was observed in the presence and absence of Ca(2+) agonists. Also, tail currents deactivated slowly when conditioning steps of positive potential were applied. The deactivation time constant was decreased by hyperpolarizing the repolarization step. Application of ATP-gamma-S or H-7 had little effect on the conditions necessary to induce slow tail, suggesting involvement of physical processes in the channel protein. In the presence of Bay K 8644, additional application of nifedipine decreased the amplitudes of the test and tail currents induced by a test step preceded by a conditioning step to +80 mV, but did not affect the decay time constant of the tail current. From these results and assumptions we have drawn up a kinetic scheme with one closed state, two open states (O(1), O(2)) and two inactivated states linked to the closed state and open state O(1), respectively, i.e., open state O(2) protected from inactivation. Computer calculation reconstructed slow deactivation and U-shaped inactivation properties. A similar kinetic scheme with Ca(2+)-agonist-binding states accounted for the results in the presence of Ca(2+) agonists.

Ca(2+) channel properties in smooth muscle cells of the urinary bladder from pig and human

Ca(2+) channel properties of pig and human bladder smooth muscle were investigated utilizing standard whole-cell patch clamp techniques. Both the amplitude obtained and the current density of Ca(2+) channel current evoked by step depolarization were larger in human than in pig myocytes. The inward currents were sensitive to an L-type Ca(2+) channel antagonist, nifedipine, the effects of which were not significantly different between species. In both species, prior application of ATP (0.1 mM) had no effect on activation of this voltage-sensitive channel current, while a muscarinic receptor agonist, carbachol (0.1 mM), significantly attenuated the amplitude of this current. Furthermore, inclusion of GDP-beta-S or Heparin in the pipette abolished or had no effect on the suppression of Ca(2+) current by carbachol, respectively. These results forward the pig as a good model for the human in detrusor Ca(2+) channel properties, especially with regard to neural modulation, although voltage-sensitive Ca(2+) channels seem to make greater contribution in human bladder physiology.

The alpha 1-subunit of smooth muscle Ca(2+) channel preserves multiple open states induced by depolarization

The cloned alpha 1-subunits of the smooth muscle Ca(2+) channel (alpha (1C-b)) from rabbit lung were expressed in Chinese hamster ovary cells. The effect of large depolarizations was examined using cell-attached patch clamp techniques. After large, long-duration depolarizations (to +80 mV, 4 s), the cloned smooth muscle Ca(2+) channels were still open, and also showed slow channel closure upon repolarization. The sum of unitary channel currents revealed that the tail current seen after large conditioning depolarizations had a slower deactivation time constant compared to that seen when the cell membrane was depolarized briefly with a test step (to +40 mV), suggesting that large depolarizations transform the conformation of the Ca(2+) channels to a second open state. The decay time course of the tail current induced by large conditioning depolarizations was prolonged by reducing the negativity of the repolarization step, and vice versa. Using the slow deactivating characteristic, the current-voltage relationship was directly measured by applying a ramp pulse after a large depolarization. Its slope conductance was approximately 26 pS. Since the patch pipettes contained Ca(2+) agonists, the transition of the Ca(2+) channel conformation to the second, long open state during a large depolarization was distinct from that caused by Ca(2+) agonists, suggesting that the cloned alpha 1-subunits of smooth muscle Ca(2+) channels preserve the characteristic features seen in native smooth muscle Ca(2+) channels. In addition, when skeletal muscle beta-subunits were coexpressed with the alpha 1-subunits, the long channel openings after large, long-duration depolarizations were frequently suppressed. This phenomenon could be explained if the skeletal muscle beta-subunits increased the inactivation rate during the preconditioning depolarization.