Partial bladder outlet obstruction alters Ca2+ sensitivity of force, but not of MLC phosphorylation, in bladder smooth muscle

Detrusor contractility to parasympathetic mediators is differentially altered in the compensated and decompensated states of diabetic bladder dysfunction

Diabetic bladder dysfunction (DBD) affects up to 50% of all patients with diabetes, characterized by symptoms of both overactive and underactive bladder. Although most diabetic bladder dysfunction studies have been performed using models with type 1 diabetes, few have been performed in models of type 2 diabetes, which accounts for ~90% of all diabetic cases. In a type 2 rat model using a high-fat diet (HFD) and two low doses of streptozotocin (STZ), we examined voiding measurements and functional experiments in urothelium-denuded bladder strips to establish a timeline of disease progression. We hypothesized that overactive bladder symptoms (compensated state) would develop and progress into symptoms characterized by underactive bladder (decompensated state). Our results indicated that this model developed the compensated state at 1 wk after STZ and the decompensated state at 4 mo after STZ administration. Diabetic bladders were hypertrophied compared with control bladders. Increased volume per void and detrusor muscle contractility to exogenous addition of carbachol and ATP confirmed the development of the compensated state. This enhanced contractility to carbachol was not due to increased levels of M₃ receptor expression. Decompensation was characterized by increased volume per void, number of voids, and contractility to ATP but not carbachol. Thus, progression from the compensated to decompensated state may involve decreased contractility to muscarinic stimulation. These data suggest that the compensated state of DBD progresses temporally into the decompensated state in the male HFD/STZ model of diabetes; therefore, this male HFD/STZ model can be used to study the progression of DBD.

Alterations in the contractile phenotype of the bladder: lessons for understanding physiological and pathological remodelling of smooth muscle

The contractile properties of the urinary bladder are changed by the conditions of normal development and partial bladder outlet obstruction. This change in the contractile phenotype is accompanied by changes in the regulatory cascades and filaments that regulate contractility. This review focuses on such changes during the course of normal development and in response to obstruction. Our goal is to discuss the experimental evidence that has accumulated from work in animal models and correlate these findings with the human voiding phenotype.

Current perspectives on congenital obstructive nephropathy

Congenital obstructive nephropathy is the leading cause of chronic renal disease in children. As a result, it represents a tremendous societal burden in terms of morbidity and mortality, as well as in health care expenses of caring for children with chronic kidney disease and end-stage renal disease. The various diagnostic, prognostic, and therapeutic challenges associated with congenital obstructive nephropathy highlight the importance of developing effective experimental models for studying this disease process. In this review, we define the clinical entity that is congenital obstructive nephropathy, outline the current standards of diagnosis and care, and discuss the utilization of current experimental models designed to help clarify some of the clinical conundrums associated with this important disease.

Carbachol-induced rabbit bladder smooth muscle contraction: roles of protein kinase C and Rho kinase

Smooth muscle contraction is regulated by phosphorylation of the myosin light chain (MLC) catalyzed by MLC kinase and dephosphorylation catalyzed by MLC phosphatase. Agonist stimulation of smooth muscle results in the inhibition of MLC phosphatase activity and a net increase in MLC phosphorylation and therefore force. The two pathways believed to be primarily important for inhibition of MLC phosphatase activity are protein kinase C (PKC)-catalyzed CPI-17 phosphorylation and Rho kinase (ROCK)-catalyzed myosin phosphatase-targeting subunit (MYPT1) phosphorylation. The goal of this study was to determine the roles of PKC and ROCK and their downstream effectors in regulating MLC phosphorylation levels and force during the phasic and sustained phases of carbachol-stimulated contraction in intact bladder smooth muscle. These studies were performed in the presence and absence of the PKC inhibitor bisindolylmaleimide-1 (Bis) or the ROCK inhibitor H-1152. Phosphorylation levels of Thr(38)-CPI-17 and Thr(696)/Thr(850)-MYPT1 were measured at different times during carbachol stimulation using site-specific antibodies. Thr(38)-CPI-17 phosphorylation increased concurrently with carbachol-stimulated force generation. This increase was reduced by inhibition of PKC during the entire contraction but was only reduced by ROCK inhibition during the sustained phase of contraction. MYPT1 showed high basal phosphorylation levels at both sites; however, only Thr(850) phosphorylation increased with carbachol stimulation; the increase was abolished by the inhibition of either ROCK or PKC. Our results suggest that during agonist stimulation, PKC regulates MLC phosphatase activity through phosphorylation of CPI-17. In contrast, ROCK phosphorylates both Thr(850)-MYPT1 and CPI-17, possibly through cross talk with a PKC pathway, but is only significant during the sustained phase of contraction. Last, our results demonstrate that there is a constitutively activate pool of ROCK that phosphorylates MYPT1 in the basal state, which may account for the high resting levels of MLC phosphorylation measured in rabbit bladder smooth muscle.

Deletion of SM-B, the high ATPase isoform of myosin, upregulates the PKC-mediated signal transduction pathway in murine urinary bladder smooth muscle

Detrusor smooth muscle (DSM) hypertrophy induced by partial bladder outlet obstruction (PBOO) is associated with changes in the NH2-terminal myosin heavy chain isoform from predominantly SM-B to SM-A, alteration in the Ca2+ sensitization pathway, and the contractile characteristics from phasic to tonic in rabbits. We utilized the SM-B knockout (KO) mouse to determine whether a shift from SM-B to SM-A without PBOO is associated with changes in the signal transduction pathway mediated via PKC and CPI-17, which keeps the myosin phosphorylation (MLC20) level high by inhibiting the myosin phosphatase. DSM strips from SM-B KO mice generated more force in response to electrical field stimulation, KCl, carbachol, and phorbol 12,13-dibutyrate than that of age-matched wild-type mice. There was no difference in the ED50 for carbachol but the maximum response was greater for the SM-B KO mice. DSM from SM-B KO mice revealed increased mass and hypertrophy. The KO mice also showed an overexpression of PKC-alpha, increased levels of phospho-CPI-17, and an elevated level of IP3 and DAG upon stimulation with carbachol. Two-dimensional gel electrophoresis revealed an increased level of MLC20 phosphorylation in response to carbachol. Together, these changes may be responsible for the higher level of force generation and maintenance by the DSM from the SM-B KO bladders. In conclusion, our data show that ablation of SM-B is associated with alteration of PKC-mediated signal transduction and CPI-17-mediated Ca2+ sensitization pathway that regulate smooth muscle contraction. Interestingly, similar changes are also present in PBOO-induced DSM compensatory response in the rabbit model in which SM-B is downregulated.

Expression and function of COOH-terminal myosin heavy chain isoforms in mouse smooth muscle

Isoforms of the smooth muscle myosin motor, SM1 and SM2, differ in length at the carboxy terminal tail region. Their proportion changes with development, hormonal status and disease, but their function is unknown. We developed mice carrying the myosin heavy chain (MyHC) transgenes SM1, cMyc-tagged SM1, SM2, and V5-tagged SM2, and all transgenes corresponded to the SMa NH(2)-terminal isoform. Transgene expression was targeted to smooth muscle by the smooth muscle alpha-actin promoter. Immunoblot analysis showed substantial expression of the cMyc-tagged SM1 and V5-tagged SM2 MyHC protein in aorta and bladder and transgene mRNA was expressed in mice carrying unlabeled SM1 or SM2 transgenes. Despite significant protein expression of tagged MyHCs we found only small changes in the SM1:SM2 protein ratio. Significant changes in functional phenotype were observed in mice carrying unlabeled SM1 or SM2 transgenes. Force in aorta and bladder was increased (72 +/- 14%, 92 +/- 11%) in SM1 and decreased to 57 +/- 1% and 80 +/- 3% in SM2 transgenic mice. SM1 transgenic bladders had faster (1.8 +/- 0.3 s) and SM2 slower (7.1 +/- 0.5 s) rates of force redevelopment following a rapid step shortening. We hypothesize that small changes in the SM1:SM2 ratio could be amplified if they are associated with changes in thick filament assembly and underlie the altered contractility. These data provide evidence indicating an in vivo function for the COOH-terminal isoforms of smooth muscle myosin and suggest that the SM1:SM2 ratio is tightly regulated in smooth muscle tissues.

Partial bladder outlet obstruction selectively abolishes protein kinase C induced contraction of rabbit detrusor smooth muscle

PURPOSE

Despite the acute onset, partial bladder outlet obstruction in the rabbit induces detrusor remodeling similar to that in men with benign prostatic hyperplasia in terms of its impact on structural and functional alterations in smooth muscle. We determined if partial bladder outlet obstruction induced remodeling alters the protein kinase C signaling pathway that leads to contraction.

MATERIALS AND METHODS

Smooth muscle from control animals and those subjected to 2 weeks of partial bladder outlet obstruction were mounted for isometric force recording, measurement of myosin light chain phosphorylation and levels of adducin phosphorylation. Bladder muscle strips were stimulated by phorbol dibutyrate or carbachol in the presence and absence of bisindolylmaleimide-1.

RESULTS

Smooth muscle strips from animals subjected to partial bladder outlet obstruction showed little to no increase in stress in response to phorbol dibutyrate and no increase in myosin light chain phosphorylation levels. Muscle strips from control animals produced a robust contraction with concomitant increases in myosin light chain phosphorylation. Inhibition of protein kinase C by bisindolylmaleimide-1 significantly depressed carbachol induced contractions of muscle strips from control animals but it had no effect on carbachol induced contractions of muscle strips from outlet obstructed animals. Phorbol dibutyrate increased phospho-adducin levels in muscle strips from the 2 animal sources, suggesting that protein kinase C could be activated.

CONCLUSIONS

We propose that partial bladder outlet obstruction does not alter protein kinase C activation, but rather abolishes or uncouples the pathway(s) downstream of protein kinase C, leading to contraction. Loss of this pathway may contribute to the loss of normal voiding behavior and the resultant decompensated state.

PROCEEDINGS OF THE BALTIMORE SMOOTH MUSCLE MEETING: IDENTIFYING RESEARCH FRONTIERS AND PRIORITIES FOR THE LOWER URINARY TRACT

PURPOSE

The myocyte is a major parenchymal cell of the lower urinary tract (LUT) in men and women. Significant phenotypic diversity ensures that myocytes subserve their important role in the physiologically distinct tissues and organs of the LUT, including the ureters, bladder, urethra, prostate, penis, vagina and myometrium. Coordinated contraction and relaxation of myocytes is required for normal organ function, while alterations in myocyte structure/function are implicated in the etiology of various LUT diseases/disorders. LUT diseases/disorders will continue to increase in an ever aging American population. The purpose of the Baltimore Smooth Muscle Meeting was to begin to identify some research frontiers and priorities.

MATERIALS AND METHODS

A 1-day conference of some of the leading world experts in smooth muscle research was held at American Urological Association headquarters. These experts gave presentations in their areas of expertise and extensively discussed their work. This report details those interactions.

RESULTS

There is astonishing diversity in the contribution of the myocyte to LUT physiology and dysfunction. Novel tools, technologies and ideas have produced increased understanding and identified new frontiers.

CONCLUSIONS

An improved understanding of urogenital myocyte physiology, function and dysfunction is required better to elucidate disease mechanisms and develop novel therapeutics. The First Annual Baltimore Smooth Muscle Meeting provided the first step in this direction. More coordinated LUT myocyte funding initiatives, the further development of research resources, tools and technologies, and exploration of the urogenital system as a model system for studying systems biology and integrative physiology are among the highest research priorities.

Partial bladder outlet obstruction abolishes the receptor- and G protein-dependent increase in calcium sensitivity in rabbit bladder smooth muscle

Partial bladder outlet obstruction (PBOO) alters the function of the whole bladder and produces specific alterations in the contractility of the bladder smooth muscle cell. The goal of this study was to test the hypothesis that PBOO affects smooth muscle contraction at the level of the receptor- and G protein-dependent increase in myofilament Ca2+ sensitivity. To address this question, we used alpha-toxin-permeabilized strips of bladder smooth muscle from control animals and animals subjected to 2 wk of PBOO. Increasing free [Ca2+] increased force in permeabilized strips from control animals; the addition of 10 microM carbachol and 10 microM GTP increased both the Ca2+ sensitivity of the contractions and the maximal levels of force attained. In contrast, although increases in [Ca2+] increased force in permeabilized strips from PBOO animals, the addition of carbachol and GTP had no additional effects. Myosin light chain phosphorylation levels increased with [Ca2+], and although they tended to be higher in strips from PBOO animals, they did not reach statistical significance. Assessment of G protein activity from both animal models suggests this is not a site responsible for the loss of carbachol and GTP enhancement of myofilament Ca2+ sensitivity. The addition of phorbol dibutyrate increased the Ca2+ sensitivity of force development in strips from both animal models, suggesting that an alteration in PKC signaling is not involved. Our results are consistent with the hypothesis that PBOO decreases receptor-mediated myofilament calcium sensitization and that the site of action is downstream from either the G proteins or PKC.

Correlation between spontaneous electrical, calcium and mechanical activity in detrusor smooth muscle of the guinea-pig bladder

1. To investigate the cellular mechanisms underlying spontaneous excitation of smooth muscle of the guinea-pig urinary bladder, isometric tension was measured in muscle bundles while recording the membrane potential from a cell in the bundle with a microeletrode. Changes in the intracellular calcium concentration ([Ca(2+)](i); calcium transients) were recorded in strips loaded with the fluorescent dye, fura-PE3. 2. In 40% of preparations, individual action potentials and contractions, which were abolished by nifedipine (1 microm), were generated. In the remaining preparations, bursting action potentials and contractions were generated. Contractions were again abolished by nifedipine (1 microm), while higher concentrations of nifedipine (10-30 microm) were required to prevent the electrical activity. 3. Carbachol (0.1 microm) increased the frequency of action potentials and corresponding contractions. Apamin (0.1 microm) potentiated bursting activity and enhanced phasic contraction. Charybdotoxin (CTX, 50 nm) induced prolonged action potentials that generated enlarged contractions. In contrast, levcromakalim (0.1 microm) reduced the frequency of action potentials, action potential bursts and the size of the contractions. 4. Forskolin (0.1 microm), 8-bromoguanosin 3', 5' cyclic monophosphate (8Br-cGMP, 0.1 mm) and Y-26763 (10 microm) suppressed contractions without reducing the amplitude of either action potentials or Ca transients. 5. This paper confirms that action potentials and associated calcium transients are fundamental mechanisms in generating spontaneous contractions in smooth muscles of the guinea-pig bladder. However, in parallel with the excitation-contraction coupling, the sensitivity of the contractile proteins for Ca(2+) may play an important role in regulating spontaneous excitation and can be modulated by cyclic nucleotides and Rho kinase.