The AMP-Dependent Protein Kinase (AMPK) Activator A-769662 Causes Arterial Relaxation by Reducing Cytosolic Free Calcium Independently of an Increase in AMPK Phosphorylation

Although recent studies reveal that activation of the metabolic and Ca²⁺ sensor AMPK strongly inhibits smooth muscle contraction, there is a paucity of information about the potential linkage between pharmacological AMPK activation and vascular smooth muscle (VSM) contraction regulation. Our aim was to test the general hypothesis that the allosteric AMPK activator A-769662 causes VSM relaxation via inhibition of contractile protein activation, and to specifically determine which activation mechanism(s) is(are) affected. The ability of A-769662 to cause endothelium-independent relaxation of contractions induced by several contractile stimuli was examined in large and small musculocutaneous and visceral rabbit arteries. For comparison, the structurally dissimilar AMPK activators MET, SIM, and BBR were assessed. A-769662 displayed artery- and agonist-dependent differential inhibitory activities that depended on artery size and location. A-769662 did not increase AMPK-pT172 levels, but did increase phosphorylation of the downstream AMPK substrate, acetyl-CoA carboxylase (ACC). A-769662 did not inhibit basal phosphorylation levels of several contractile protein regulatory proteins, and did not alter the activation state of rhoA. A-769662 did not inhibit Ca²⁺- and GTPγS-induced contractions in β-escin-permeabilized muscle, suggesting that A-769662 must act by inhibiting Ca²⁺ signaling. In intact artery, A-769662 immediately reduced basal intracellular free calcium ([Ca²⁺]_i), inhibited a stimulus-induced increase in [Ca²⁺]_i, and inhibited a cyclopiazonic acid (CPA)-induced contraction. MET increased AMPK-pT172, and caused neither inhibition of contraction nor inhibition of [Ca²⁺]_i. Together, these data support the hypothesis that the differential inhibition of stimulus-induced arterial contractions by A-769662 was due to selective inhibition of a Ca²⁺ mobilization pathway, possibly involving CPA-dependent Ca²⁺ entry via an AMPK-independent pathway. That MET activated AMPK without causing arterial relaxation suggests that AMPK activation does not necessarily cause VSM relaxation.

AICAR Administration Attenuates Hemorrhagic Hyperglycemia and Lowers Oxygen Debt in Anesthetized Male Rabbits

Background: Many strategies have been utilized to treat traumatic shock via improved oxygen delivery (DO₂), while fewer have been used to in an attempt to reduce oxygen demand (VO₂). The cellular energy sensor 5′ adenosine monophosphate-activated protein kinase (AMPK) has the potential to modulate both whole-body DO₂ and VO₂. Therefore, we determined the effect of the AMPK activator AICAR (5-aminoimidazole-4-carboxamide 1-β-D-ribonucleoside) given acutely or chronically on key metabolites, hemodynamics, and oxygen consumption/delivery before and during hemorrhage in anesthetized male rabbits.

Methods: Chronically treated animals received AICAR (40 mg/kg/day, IV) for 10 days prior to hemorrhage, while rabbits in the acute study were infused with AICAR (7.5 mg/kg bolus, 2 mg/kg/min infusion) or vehicle (0.3 ml/kg saline bolus, 0.03 ml/kg/min infusion) IV for 2 h prior to severe hemorrhage. Both acutely and chronically treated animals were sedated (ketamine/xylazine cocktail) the morning of the terminal experiment and surgically prepared for hemorrhage, including the implantation of arterial and venous catheters (for blood removal/sampling and drug/vehicle administration) and thoracotomy for implantation of transit-time flow transducers (for cardiac output determination).

Results: AICAR given acutely lowered arterial blood glucose and increased blood lactate levels before hemorrhage, and abolished the well-documented hemorrhage-induced hyperglycemia seen in vehicle treated animals. Animals given AICAR chronically had blunted hemorrhage-induced hyperglycemia without prior baseline changes. Chronically treated AICAR animals showed significantly lower lactate levels during hemorrhage. Rabbits receiving AICAR both acutely and chronically experienced similar falls in mean arterial pressure, cardiac output and hence DO₂ to their vehicle counterparts throughout the hemorrhage period. However, rabbits treated either acutely or chronically with AICAR accumulated lower oxygen deficits and debt during hemorrhage compared to vehicle-infused controls.

Conclusions: The oxygen debt data suggest that AMPK activation could decrease trauma associated morbidity and mortality, perhaps by mechanisms related to increased glucose utilization. Additional studies are needed to investigate the effects of AICAR and associated mechanisms of action when given during resuscitation from hemorrhage.

Metabolic Stress-Induced Activation of AMPK and Inhibition of Constitutive Phosphoproteins Controlling Smooth Muscle Contraction: Evidence for Smooth Muscle Fatigue?

Metabolic stress diminishes smooth muscle contractile strength by a poorly defined mechanism. To test the hypothesis that metabolic stress activates a compensatory cell signaling program to reversibly downregulate contraction, arterial rings and bladder muscle strips in vitro were deprived of O₂ and glucose for 30 and 60 min (“starvation”) to induce metabolic stress, and the phosphorylation status of proteins involved in regulation of contraction and metabolic stress were assessed in tissues under basal and stimulated conditions. A 15–30 min recovery period (O₂ and glucose repletion) tested whether changes induced by starvation were reversible. Starvation decreased basal phosphorylation of myosin regulatory light chain (MLC-pS19) and of the rho kinase (ROCK) downstream substrates cofilin (cofilin-pS3) and myosin phosphatase targeting subunit MYPT1 (MYPT1-pT696 and MYPT1-pT853), and abolished the ability of contractile stimuli to cause a strong, sustained contraction. Starvation increased basal phosphorylation of AMPK (AMPK-pT172) and 3 downstream AMPK substrates, acetyl-CoA carboxylase (ACC-pS79), rhoA (rhoA-pS188), and phospholamban (PLB-pS16). Increases in rhoA-pS188 and PLB-pS16 would be expected to inhibit contraction. Recovery restored basal AMPK-pT172 and MLC-pS19 to control levels, and restored contraction. In AMPKα₂ deficient mice (AMPKα2-/-), the basal level of AMPK-pT172 was reduced by 50%, and MLC-pS19 was elevated by 50%, but AMPKα2-/- did not prevent starvation-induced contraction inhibition nor enhance recovery from starvation. These results indicate that constitutive AMPK activity participates in constitutive regulation of contractile proteins, and suggest that AMPK activation is necessary, but may not be sufficient, to cause smooth muscle contraction inhibition during metabolic stress.

Slowly cycling Rho kinase-dependent actomyosin cross-bridge "slippage" explains intrinsic high compliance of detrusor smooth muscle

Biological soft tissues are viscoelastic because they display time-independent pseudoelasticity and time-dependent viscosity. However, there is evidence that the bladder may also display plasticity, defined as an increase in strain that is unrecoverable unless work is done by the muscle. In the present study, an electronic lever was used to induce controlled changes in stress and strain to determine whether rabbit detrusor smooth muscle (rDSM) is best described as viscoelastic or viscoelastic plastic. Using sequential ramp loading and unloading cycles, stress-strain and stiffness-stress analyses revealed that rDSM displayed reversible viscoelasticity, and that the viscous component was responsible for establishing a high stiffness at low stresses that increased only modestly with increasing stress compared with the large increase produced when the viscosity was absent and only pseudoelasticity governed tissue behavior. The study also revealed that rDSM underwent softening correlating with plastic deformation and creep that was reversed slowly when tissues were incubated in a Ca²⁺-containing solution. Together, the data support a model of DSM as a viscoelastic-plastic material, with the plasticity resulting from motor protein activation. This model explains the mechanism of intrinsic bladder compliance as "slipping" cross bridges, predicts that wall tension is dependent not only on vesicle pressure and radius but also on actomyosin cross-bridge activity, and identifies a novel molecular target for compliance regulation, both physiologically and therapeutically.

Vascular smooth muscle desensitization in rabbit epigastric and mesenteric arteries during hemorrhagic shock

The decompensatory phase of hemorrhage (shock) is caused by a poorly defined phenomenon termed vascular hyporeactivity (VHR). VHR may reflect an acute in vivo imbalance in levels of contractile and relaxant stimuli favoring net vascular smooth muscle (VSM) relaxation. Alternatively, VHR may be caused by intrinsic VSM desensitization of contraction resulting from prior exposure to high levels of stimuli that temporarily adjusts cell signaling systems. Net relaxation, but not desensitization, would be expected to resolve rapidly in an artery segment removed from the in vivo shock environment and examined in vitro in a fresh solution. Our aim was to 1) induce shock in rabbits and apply an in vitro mechanical analysis on muscular arteries isolated pre- and postshock to determine whether VHR involves intrinsic VSM desensitization, and 2) identify whether net VSM relaxation induced by nitric oxide and cyclic nucleotide-dependent protein kinase activation in vitro can be sustained for some time after relaxant stimulus washout. The potencies of phenylephrine- and histamine-induced contractions in in vitro epigastric artery removed from rabbits posthemorrhage were decreased by ∼0.3 log units compared with the control contralateral epigastric artery removed prehemorrhage. Moreover, a decrease in KCl-induced tonic, relative to phasic, tension of in vitro mesenteric artery correlated with the degree of shock severity as assessed by rates of lactate and K(+) accumulation. VSM desensitization was also caused by tyramine in vivo and PE in vitro, but not by relaxant agents in vitro. Together, these results support the hypothesis that VHR during hemorrhagic decompensation involves contractile stimulus-induced long-lasting, intrinsic VSM desensitization.

Elevated steady-state bladder preload activates myosin phosphorylation: detrusor smooth muscle is a preload tension sensor

In rabbit bladder wall (detrusor) muscle, the degree of tone induced during physiological filling (filling tone) is the sum of adjustable preload tension and autonomous contractile tension. The present study was designed to determine whether the level of filling tone is dependent on detrusor muscle length. Maximum active tension induced by KCl was parabolic in relation to length [tension increased from 70% to 100% of a reference length ( Lref) and decreased at longer muscle lengths]. Filling tone, however, increased in a linear fashion from 70% to 120% Lref. In the presence of ibuprofen to abolish autonomous contraction and retain adjustable preload tension, tension was reduced in strength but remained linearly dependent on length from 70% to 120% Lref. In the absence of autonomous contraction, stretching detrusor muscle from 80% to 120% Lref still caused an increase in tone during PGE2-induced rhythmic contraction, suggesting that muscle stretch caused increases in detrusor muscle contractile sensitivity rather than in prostaglandin release. In the absence of autonomous contraction, the degree of adjustable preload tension and myosin phosphorylation increased when detrusor was stretched from 80% to 120% Lref, but also displayed length-hysteresis, indicating that detrusor muscle senses preload rather than muscle length. Together, these data support the hypothesis that detrusor muscle acts as a preload tension sensor. Because detrusor muscle is in-series with neuronal mechanosensors responsible for urinary urgency, a more thorough understanding of detrusor muscle filling tone may reveal unique targets for therapeutic intervention of contractile disorders such as overactive bladder.

Prostaglandin E₂mediates spontaneous rhythmic contraction in rabbit detrusor muscle

INTRODUCTION

The purpose of this investigation was to determine if prostaglandin E₂(PGE₂) is produced by rabbit detrusor free of urothelium and demonstrate that PGE₂ is responsible for the generation of spontaneous rhythmic contraction (SRC).

METHODS

A bioassay was performed in which contraction frequency in strips of rabbit detrusor was compared before and after addition of superfusate from incubating sections of rabbit detrusor. Specificity was determined by testing the effects of SC-51089, a PGE₂(EP1) antagonist. Effects on development of tension were determined in artery segments after treatment with increasing doses of PGE₂, PGF₂α, and TXA₂, and a section of femoral artery was used as a negative control. Confirmation of PGE2 production was then determined using EIA kits.

RESULTS

Increased rhythmic frequency was identified after superfusate from a section of rabbit detrusor free of urothelium was added to strips of detrusor from the same animal. Additional experiments demonstrated that rhythmic frequency generated after treatment with PGE₂ was significantly reduced after treatment with SC-51089. In artery smooth muscle, prostaglandin dose response experiments demonstrated that only TXA₂ induced contraction at physiologic doses (<10⁻⁷M). As a negative control, subsequent treatment of a section of femoral artery with detrusor superfusate failed to increase tension, confirming a lack of TXA₂ production. EIA confirmed that PGE₂ production increased by 4.8-fold in strips of detrusor free of urothelium after 15 minutes of incubation and that this production was blocked by ibuprofen and a COX-1 inhibitor.

CONCLUSIONS

Rabbit detrusor produces PGE₂ which is the most likely mediator of SRC.

Active tension adaptation at a shortened arterial muscle length: inhibition by cytochalasin-D

Unlike the static length-tension curve of striated muscle, airway and urinary bladder smooth muscles display a dynamic length-tension curve. Much less is known about the plasticity of the length-tension curve of vascular smooth muscle. The present study demonstrates that there were significant increases of ∼15% in the phasic phase and ∼10% in the tonic phase of a third KCl-induced contraction of a rabbit femoral artery ring relative to the first contraction after a 20% decrease in length from an optimal muscle length (L(0)) to 0.8-fold L(0). Typically, three repeated contractions were necessary for full length adaptation to occur. The tonic phase of a third KCl-induced contraction was increased by ∼50% after the release of tissues from 1.25-fold to 0.75-fold L(o). The mechanism for this phenomenon did not appear to lie in thick filament regulation because there was no increase in myosin light chain (MLC) phosphorylation to support the increase in tension nor was length adaptation abolished when Ca(2+) entry was limited by nifedipine and when Rho kinase (ROCK) was blocked by H-1152. However, length adaptation of both the phasic and tonic phases was abolished when actin polymerization was inhibited through blockade of the plus end of actin by cytochalasin-D. Interestingly, inhibition of actin polymerization when G-actin monomers were sequestered by latrunculin-B increased the phasic phase and had no effect on the tonic phase of contraction during length adaptation. These data suggest that for a given level of cytosolic free Ca(2+), active tension in the femoral artery can be sensitized not only by regulation of MLC phosphatase via ROCK and protein kinase C, as has been reported by others, but also by a nonmyosin regulatory mechanism involving actin polymerization. Dysregulation of this form of active tension modulation may provide insight into alterations of large artery stiffness in hypertension.

Failure of Bay K 8644 to induce RhoA kinase-dependent calcium sensitization in rabbit blood vessels

BACKGROUND AND PURPOSE

RhoA kinase (ROCK) participates in K(+) depolarization (KCl)-induced Ca(2+) sensitization of contraction. Whether constitutive, depolarization- or Ca(2+)-activated ROCK plays the major role in this signalling system remains to be determined. Here, we determined whether Bay K 8644, a dihydropyridine that promotes Ca(2+) channel clusters to operate in a persistent Ca(2+) influx mode, could cause ROCK-dependent Ca(2+) sensitization.

EXPERIMENTAL APPROACH

Renal and femoral artery rings from New Zealand white rabbits were contracted with Bay K 8644. Tissues were frozen and processed to measure active RhoA and ROCK substrate (myosin phosphatase targeting subunit, MYPT1) and myosin light chain (MLC) phosphorylation, or loaded with fura-2 to measure intracellular free Ca(2+) ([Ca(2+)](i)). Effects of selective inhibitors of contraction were assessed in resting (basal) tissues and those contracted with Bay K 8644.

KEY RESULTS

Bay K 8644 produced strong increases in [Ca(2+)](i), MLC phosphorylation and tension, but not in MYPT1 phosphorylation. ROCK inhibition by H-1152 abolished basal MYPT1-pT853, diminished basal MLC phosphorylation and inhibited Bay K 8644-induced increases in MLC phosphorylation and tension. MLC kinase inhibition by wortmannin abolished Bay K 8644-induced contraction and increase in MLC phosphorylation but did not inhibit basal MYPT1-pT853. H-1152 and wortmannin had no effect on MYPT1-pT696, but 1 microM staurosporine inhibited basal MYPT1-pT853, MYPT1-pT696 and MLC phosphorylation.

CONCLUSIONS AND IMPLICATIONS

These data suggest that the constitutive activities of ROCK and a staurosporine-sensitive kinase regulate basal phosphorylation of MYPT1, which participates along with activation of MLC kinase in determining the strength of contraction induced by the Ca(2+) agonist, Bay K 8644.

Calcium-independent phospholipase A2 participates in KCl-induced calcium sensitization of vascular smooth muscle

In vascular smooth muscle, KCl not only elevates intracellular free Ca(2+) ([Ca(2+)](i)), myosin light chain kinase activity and tension (T), but also can inhibit myosin light chain phosphatase activity by activation of rhoA kinase (ROCK), resulting in Ca(2+) sensitization (increased T/[Ca(2+)](i) ratio). Precisely how KCl causes ROCK-dependent Ca(2+) sensitization remains to be determined. Using Fura-2-loaded isometric rings of rabbit artery, we found that the Ca(2+)-independent phospholipase A(2) (iPLA(2)) inhibitor, bromoenol lactone (BEL), reduced the KCl-induced tonic but not early phasic phase of T and potentiated [Ca(2+)](i), reducing Ca(2+) sensitization. The PKC inhibitor, GF-109203X (> or =3 microM) and the pseudo-substrate inhibitor of PKCzeta produced a response similar to BEL. BEL reduced basal and KCl-stimulated myosin phosphatase phosphorylation. Whereas BEL and H-1152 produced strong inhibition of KCl-induced tonic T (approximately 50%), H-1152 did not induce additional inhibition of tissues already inhibited by BEL, suggesting that iPLA(2) links KCl stimulation with ROCK activation. The cPLA(2) inhibitor, pyrrolidine-1, inhibited KCl-induced tonic increases in [Ca(2+)](i) but not T, whereas the inhibitor of 20-HETE production, HET0016, acted like the ROCK inhibitor H-1152 by causing Ca(2+) desensitization. These data support a model in which iPLA(2) activity regulates Ca(2+) sensitivity.

Potential for control of detrusor smooth muscle spontaneous rhythmic contraction by cyclooxygenase products released by interstitial cells of Cajal

Interstitial cells of Cajal (ICCs) have been identified as pacemaker cells in the upper urinary tract and urethra, but the role of ICCs in the bladder remains to be determined. We tested the hypotheses that ICCs express cyclooxygenase (COX), and that COX products (prostaglandins), are the cause of spontaneous rhythmic contraction (SRC) of isolated strips of rabbit bladder free of urothelium. SRC was abolished by 10 μM indomethacin and ibuprofen (non-selective COX inhibitors). SRC was concentration-dependently inhibited by selective COX-1 (SC-560 and FR-122047) and COX-2 inhibitors (NS-398 and LM-1685), and by SC-51089, a selective antagonist for the PGE-2 receptor (EP) and ICI-192,605 and SQ-29,548, selective antagonists for thromboxane receptors (TP). The partial agonist/antagonist of the PGF-2α receptor (FP), AL-8810, inhibited SRC by ∼50%. Maximum inhibition was ∼90% by SC-51089, ∼80–85% by the COX inhibitors and ∼70% by TP receptor antagonists. In the presence of ibuprofen to abolish SRC, PGE-2, sulprostone, misoprostol, PGF-2α and U-46619 (thromboxane mimetic) caused rhythmic contractions that mimicked SRC. Fluorescence immunohistochemistry coupled with confocal laser scanning microscopy revealed that c-Kit and vimentin co-localized to interstitial cells surrounding detrusor smooth muscle bundles, indicating the presence of extensive ICCs in rabbit bladder. Co-localization of COX-1 and vimentin, and COX-2 and vimentin by ICCs supports the hypothesis that ICCs were the predominant cell type in rabbit bladder expressing both COX isoforms. These data together suggest that ICCs appear to be an important source of prostaglandins that likely play a role in regulation of SRC. Additional studies on prostaglandin-dependent SRC may generate opportunities for the application of novel treatments for disorders leading to overactive bladder.

Role of protein kinase Czeta and calcium entry in KCl-induced vascular smooth muscle calcium sensitization and feedback control of cellular calcium levels

The degree of tonic force (F) maintenance induced in vascular smooth muscle upon K(+) depolarization with 110 mM KCl can be greatly reduced by inhibition of rhoA kinase (ROCK). We explored the possibility that a protein kinase C (PKC) isotype may also play a role in causing KCl-induced Ca(2+) sensitization. In isometric rings of rabbit artery, the PKC inhibitors, Go-6983 (3-[1-[3-(dimethylamino)propyl]-5-methoxy-1H-indol-3-yl]-4-(1H-indol-3-yl)-1H-pyrrole-2,5-dione), GF-109203X (2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl) maleimide), and a cell-permeable (myristoylated) pseudosubstrate inhibitor of PKCzeta (PI(PKCzeta)) inhibited KCl-induced tonic F. A myristoylated pseudosubstrate inhibitor of PKCalpha/beta that inhibited phorbol dibutyrate-induced F slightly potentiated KCl-induced tonic F and attenuated 30 mM KCl-induced F. Although the ROCK inhibitor, H-1152 [(S)-(+)-2-methyl-1-[(4-methyl-5-isoquinolinyl)-sulfonyl]-hexahydro-1H-1,4-diazepine dihydrochloride], reduced basal phosphorylation of myosin light-chain phosphatase-targeting subunit at Thr853 (MYPT1-pT853), 3 and 10 muM GF-109203X inhibited only KCl-stimulated phosphorylation, not basal MYPT1-pT853. In fura-2-loaded tissues, GF-109203X and PI(PKCzeta) elevated basal [Ca(2+)](i) (calcium) and potentiated KCl-induced tonic increases in calcium while reducing KCl-induced tonic increases in F. Blockade by nifedipine of Ca(2+) entry through voltage-operated Ca(2+) channels reduced KCl-induced Ca(2+) sensitization and KCl-stimulated but not basal MYPT1-pT853. These data together support a model in which ROCK and PKCzeta are constitutively active and function in "resting" muscle to regulate the basal levels of MYPT1-pT853 and calcium, respectively. In this model, KCl-induced increases in calcium activate PKCzeta to feed forward and cause additional MYPT1-pT853 above that induced by constitutive ROCK, permitting Ca(2+) sensitization and strong F maintenance. Active PKCzeta also feeds back to attenuate the degree of KCl-induced increases in calcium.

Stimulated calcium entry and constitutive RhoA kinase activity cause stretch-induced detrusor contraction

Urinary bladder wall muscle (i.e., detrusor smooth muscle; DSM) contracts in response to a quick-stretch, but this response is neither fully characterized, nor completely understood at the subcellular level. Strips of rabbit DSM were quick-stretched (5 ms) and held isometric for 10 s to measure the resulting peak quick-stretch contractile response (PQSR). The ability of selective Ca(2+) channel blockers and kinase inhibitors to alter the PQSR was measured, and the phosphorylation levels of myosin light chain (MLC) and myosin phosphatase targeting regulatory subunit (MYPT1) were recorded. DSM responded to a quick-stretch with a biphasic response consisting of an initial contraction peaking at 0.24+/-0.02-fold the maximum KCl-induced contraction (F(o)) by 1.48+/-0.17 s (PQSR) before falling to a weaker tonic (10 s) level (0.12+/-0.03-fold F(o)). The PQSR was dependent on the rate and degree of muscle stretch, displayed a refractory period, and was converted to a sustained response in the presence of muscarinic receptor stimulation. The PQSR was inhibited by nifedipine, 2-aminoethoxydiphenyl borate (2-APB), 100 microM gadolinium and Y-27632, but not by atropine, 10 microM gadolinium, LOE-908, cyclopiazonic acid, or GF-109203X. Y-27632 and nifedipine abolished the increase in MLC phosphorylation induced by a quick-stretch. Y-27632, but not nifedipine, inhibited basal MYPT1 phosphorylation, and a quick-stretch failed to increase phosphorylation of this rhoA kinase (ROCK) substrate above the basal level. These data support the hypothesis that constitutive ROCK activity is required for a quick-stretch to activate Ca(2+) entry and cause a myogenic contraction of DSM.

Sphingosine‐1‐phosphate and the immunosuppressant, FTY720‐phosphate, regulate detrusor muscle tone

Overactive bladder syndrome (OBS) results from disturbances of bladder function. Bladder smooth muscle (detrusor) exhibits spontaneous rhythmic activity (tone) independent of neurogenic control, which is enhanced in patients with OBS. We have now uncovered a prominent role for the bioactive sphingolipid metabolite, sphingosine-1-phosphate (S1P), in regulating rabbit detrusor smooth muscle tone and contraction. S1P-induced contraction of detrusor muscle was dependent on stretch and intracellular calcium. Although detrusor expresses the S1P receptors S1P1 and S1P2, only S1P2 appeared to be involved in S1P-induced contraction, since SEW2871 (S1P1 agonist) and dihydro-S1P (potent agonist for all S1P receptors except S1P2) were poor contractile agents. In agreement, the S1P2 antagonist JTE013 inhibited S1P-induced contraction. The fast, transient muscle contraction (phasic) mediated by S1P was dependent on phospholipase C (PLC) whereas the slower, sustained contraction (tonic) was not. Surprisingly, the immunosuppressant FTY720-phosphate, an agonist for all S1P receptors except S1P2, had distinct contractile properties and also induced slow, sustained contraction. Thus, FTY720-phosphate and/or S1P may regulate calcium channels in an S1P receptor-independent manner. Collectively, our results demonstrate that S1P may regulate detrusor smooth muscle tone and suggest that dysregulation of complex S1P signaling might contribute to OBS.

Potent inhibition of arterial smooth muscle tonic contractions by the selective myosin II inhibitor, blebbistatin

Blebbistatin is reported to be a selective and specific small molecule inhibitor of the myosin II isoforms expressed by striated muscles and nonmuscle (IC(50) = 0.5-5 microM) but is a poor inhibitor of purified turkey smooth muscle myosin II (IC(50) approximately 80 microM). We found that blebbistatin potently (IC(50) approximately 3 microM) inhibited the actomyosin ATPase activities of expressed "slow" [smooth muscle myosin IIA (SMA)] and "fast" [smooth muscle myosin IIB (SMB)] smooth muscle myosin II heavy-chain isoforms. Blebbistatin also inhibited the KCl-induced tonic contractions produced by rabbit femoral and renal arteries that express primarily SMA and the weaker tonic contraction produced by the saphenous artery that expresses primarily SMB, with an equivalent potency comparable with that identified for nonmuscle myosin IIA (IC(50) approximately 5 microM). In femoral and saphenous arteries, blebbistatin had no effect on unloaded shortening velocity or the tonic increase in myosin light-chain phosphorylation produced by KCl but potently inhibited beta-escin permeabilized artery contracted with calcium at pCa 5, suggesting that cell signaling events upstream from KCl-induced activation of cross-bridges were unaffected by blebbistatin. It is noteworthy that KCl-induced contractions of chicken gizzard were less potently inhibited (IC(50) approximately 20 microM). Adult femoral, renal, and saphenous arteries did not express significant levels of nonmuscle myosin. These data together indicate that blebbistatin is a potent inhibitor of smooth muscle myosin II, supporting the hypothesis that the force-bearing structure responsible for tonic force maintenance in adult mammalian vascular smooth muscle is the cross-bridge formed from the blebbistatin-dependent interaction between actin and smooth muscle myosin II.

Evidence for absence of latch-bridge formation in muscular saphenous arteries

Large-diameter elastic arteries can produce strong contractions indefinitely at a high-energy economy by the formation of latch bridges. Whether downstream blood vessels also use latch bridges remains unknown. The zero-pressure medial thickness and lumen diameter of rabbit saphenous artery (SA), a muscular branch of the elastic femoral artery (FA), were, respectively, approximately twofold and half-fold that of the FA. In isolated FA and SA rings, KCl rapidly (< 16 s) caused strong increases in isometric stress (1.2 x 10(5) N/m2) and intracellular Ca2+ concentration ([Ca2+]i; 250 nM). By 10 min, [Ca2+]i declined to approximately 175 nM in both tissues, but stress was sustained in FA (1.3 x 10(5) N/m2) and reduced by 40% in SA (0.8 x 10(5) N/m2). Reduced tonic stress correlated with reduced myosin light chain (MLC) phosphorylation in SA (28 vs. 42% in FA), and simulations with the use of the four-state kinetic latch-bridge model supported the hypothesis that latch-bridge formation in FA, but not SA, permitted maintenance of high stress values at steady state. SA expressed more MLC phosphatase than FA, and permeabilized SA relaxed more rapidly than FA, suggesting that MLC phosphatase activity was greater in SA than in FA. The ratio of fast-to-slow myosin isoforms was greater for SA than FA, and on quick release, SA redeveloped isometric force faster than FA. These data support the hypothesis that maintained isometric force was 40% less in SA than in FA because expressed motor proteins in SA do not support latch-bridge formation.

Convergence of Ca2+-desensitizing mechanisms activated by forskolin and phenylephrine pretreatment, but not 8-bromo-cGMP

Contractile stimuli can sensitize myosin to Ca2+ by activating RhoA kinase (ROK) and PKC that inhibit myosin light chain phosphatase (MLCP) activity. Relaxant stimuli, acting through PKA and PKG (cyclic nucleotide-dependent protein kinases), and pretreatment with contractile agents such as phenylephrine (PE), can desensitize myosin to Ca2+. It is unknown precisely how these stimuli cause Ca2+ desensitization. To test the hypothesis that PKA, PKG, and PE pretreatment signaling systems converge to cause relaxation by inhibition of ROK in intact, isolated tissues, we examined the effects of forskolin (FSK; PKA activation), 8-bromo-cGMP (8br-cGMP; PKG activation), and PE pretreatment on KCl-induced force maintenance in rabbit arteries, a response nearly completely dependent on ROK activation. PE pretreatment and agents activating PKA and PKG caused Ca2+ desensitization by inhibiting KCl-induced tonic force and MLC phosphorylation without inhibiting intracellular [Ca2+]. At pCa 5 in beta-escin-permeabilized muscle, FSK and 8b-cGMP accelerated the relaxation rate when tissues were returned to pCa 9, suggesting that both agents can elevate MLCP activity. However, a component of the Ca2+ desensitization attributed to PKG activation in intact tissues appeared to involve a MLC phosphorylation-independent component. Inhibition of KCl-induced tonic force by the ROK inhibitor, Y-27632, and by PE pretreatment, were synergistically potentiated by 8b-cGMP, but not FSK. FSK and PE pretreatment, but not 8b-cGMP, inhibited the KCl-induced increase in site-specific myosin phosphatase target protein-1 phosphorylation at Thr853. These data support the hypothesis that PKA and PE pretreatment converge on a common Ca2+-desensitization pathway, but that PKG can act by a mechanism different from that activated by PKA and PE pretreatment.

Regulation of smooth muscle calcium sensitivity: KCl as a calcium-sensitizing stimulus

KCl has long been used as a convenient stimulus to bypass G protein-coupled receptors (GPCR) and activate smooth muscle by a highly reproducible and relatively “simple” mechanism involving activation of voltage-operated Ca2+channels that leads to increases in cytosolic free Ca2+([Ca2+]i), Ca2+-calmodulin-dependent myosin light chain (MLC) kinase activation, MLC phosphorylation and contraction. This KCl-induced stimulus-response coupling mechanism is a standard tool-set used in comparative studies to explore more complex mechanisms generated by activation of GPCRs. One area where this approach has been especially productive is in studies designed to understand Ca2+sensitization, the relationship between [Ca2+]iand force produced by GPCR agonists. Studies done in the late 1980s demonstrated that a unique relationship between stimulus-induced [Ca2+]iand force does not exist: for a given increase in [Ca2+]i, GPCR activation can produce greater force than KCl, and relaxant agents can produce the opposite effect to cause Ca2+desensitization. Such changes in Ca2+sensitivity are now known to involve multiple cell signaling strategies, including translocation of proteins from cytosol to plasma membrane, and activation of enzymes, including RhoA kinase and protein kinase C. However, recent studies show that KCl can also cause Ca2+sensitization involving translocation and activation of RhoA kinase. Rather than complicating the Ca2+sensitivity story, this surprising finding is already providing novel insights into mechanisms regulating Ca2+sensitivity of smooth muscle contraction. KCl as a “simple” stimulus promises to remain a standard tool for smooth muscle cell physiologists, whose focus is to understand mechanisms regulating Ca2+sensitivity.

Length-dependent regulation of basal myosin phosphorylation and force in detrusor smooth muscle

Urinary bladder (detrusor) smooth muscle is active in the absence of an external stimulus. Tone occurs even "at rest" during the filling phase, and it is elevated in patients with overactive bladder. This study examined the role of muscle length on tone and the level of basal myosin light chain phosphorylation (MLC(20P)). MLC(20P) was 23.9 +/- 1% (n = 58) at short lengths (zero preload; L(z)). An increase in length from L(z) to the optimal length for contraction (L(o)) caused a reduction in MLC(20P) to 15.8 +/- 1% (n = 49). Whereas 10 microM staurosporine reduced MLC(20P) at L(z), 1 microM staurosporine, a Ca(2+)-free solution, and inhibitors of MLC kinase, protein kinase C (PKC) and RhoA kinase (ROK) did not. However, 1 microM staurosporine and inhibitors of ROK inhibited MLC(20P) and tone at L(o). These data support the hypothesis that a Ca(2+)-independent kinase, possibly ZIP-like kinase, regulates MLC(20P) at L(z), whereas in detrusor stretched to L(o), additional kinases, such as ROK, participate.

Dependency of detrusor contractions on calcium sensitization and calcium entry through LOE-908-sensitive channels

1. The subcellular mechanisms regulating stimulus-contraction coupling in detrusor remain to be determined. We used Ca(2+)-free solutions, Ca(2+) channel blockers, cyclopiazonic acid (CPA), and RhoA kinase (ROK) inhibitors to test the hypothesis that Ca(2+) influx and Ca(2+) sensitization play primary roles. 2. In rabbit detrusor, peak bethanechol (BE)-induced force was inhibited 90% by incubation for 3 min in a Ca(2+)-free solution. By comparison, a 20 min incubation of rabbit femoral artery in a Ca(2+)-free solution reduced receptor-induced force by only 5%. 3. In detrusor, inhibition of sarcoplasmic reticular (SR) Ca(2+) release by 2APB, or depletion of SR Ca(2+) by CPA, inhibited BE-induced force by only 27%. The CPA-insensitive force was abolished by LaCl3. By comparison, 2APB inhibited receptor-induced force in rabbit femoral artery by 71%. 4. In the presence of the non-selective cation channel (NSCC) inhibitor, LOE-908, BE did not produce an increase in [Ca(2+)]i but did produce weak increases in myosin phosphorylation and force. 5. Inhibitors of ROK-induced Ca(2+) sensitization, HA-1077 and Y-27632, inhibited BE-induced force by approximately 50%, and in combination with LOE-908, nearly abolished force. 6. These data suggest that two principal muscarinic receptor-stimulated detrusor contractile mechanisms include NSCC activation, that elevates [Ca(2+)]i and ROK activation, that sensitizes cross bridges to Ca(2+).