Paul H. Ratz

Rapid effects of estrogen and progesterone on tone and spontaneous rhythmic contractions of the rabbit bladder.

Abstract Previous studies indicate that bladder instability in man may be associated with increased spontaneous rhythmic contractile activity. Ca2+ influx plays a central role in smooth muscle contractions, and recent evidence suggests that steroid hormones rapidly affect Ca2+ influx. Therefore we tested the hypothesis that estrogen and progesterone modulates spontaneous rhythmic detrusor contractions. Tissues were secured to isometric force (F) transducers in tissue baths and length-adjusted until K+-depolarization produced maximum contractions (Fo). Spontaneous rhythmic contractions (SRC) were sampled before and immediately after addition of estradiol or progesterone (10−5 M) to tissue baths. The average frequency and amplitude of SRC were, respectively, 0.156 Hz and 0.053 F/Fo (n = 24). Estradiol caused an immediate reduction in SRC, such that by 10 min, tone, frequency and amplitude were each reduced by, respectively, 36%, 46% and 47% (n = 7, P < 0.05). However, progesterone caused an immediate weak contraction, and at steady state (10 min), progesterone increased frequency of SRC by 152% but decreased SRC amplitude by 50% (n = 10, P < 0.05). Novel therapies using unique steroids that do not interact with genomic receptors may potentially reduce bladder smooth muscle activity, thereby reducing detrusor instability.

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

The subcellular mechanisms regulating stimulus‐contraction coupling in detrusor remain to be determined. We used Ca2+‐free solutions, Ca2+ channel blockers, cyclopiazonic acid (CPA), and RhoA kinase (ROK) inhibitors to test the hypothesis that Ca2+ influx and Ca2+ sensitization play primary roles. In rabbit detrusor, peak bethanechol (BE)‐induced force was inhibited 90% by incubation for 3 min in a Ca2+‐free solution. By comparison, a 20 min incubation of rabbit femoral artery in a Ca2+‐free solution reduced receptor‐induced force by only 5%. In detrusor, inhibition of sarcoplasmic reticular (SR) Ca2+ release by 2APB, or depletion of SR Ca2+ 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%. In the presence of the non‐selective cation channel (NSCC) inhibitor, LOE‐908, BE did not produce an increase in [Ca2+]i but did produce weak increases in myosin phosphorylation and force. Inhibitors of ROK‐induced Ca2+ sensitization, HA‐1077 and Y‐27632, inhibited BE‐induced force by ∼50%, and in combination with LOE‐908, nearly abolished force. These data suggest that two principal muscarinic receptor‐stimulated detrusor contractile mechanisms include NSCC activation, that elevates [Ca2+]i and ROK activation, that sensitizes cross bridges to Ca2+.

Temporal Aspects of Ca2+ and Myosin Phosphorylation during Myogenic and Norepinephrine-Induced Arteriolar Constriction

Previous studies demonstrated that maintenance of steady-state myogenic tone requires Ca2+-dependent myosin phosphorylation. The present studies furthered these observations by examining temporal relationships among Ca2+, myosin phosphorylation and vessel diameter during acute increases in intraluminal pressure and norepinephrine stimulation. Rat cremaster muscle arterioles were cannulated and loaded with the Ca2+-sensitive indicator fura-2. The extent of myosin phosphorylation was measured using two-dimensional gel electrophoresis. Acute increases in intraluminal pressure caused a biphasic increase in intracellular Ca2+ ([Ca2+]i), characterized by a transient peak followed by a decline to a steady-state level which remained significantly higher than control values. Peak [Ca2+]i was significantly related to vessel distension and increased with the change in wall tension. Increased intraluminal pressure resulted in a monophasic increase in myosin phosphorylation that was significantly correlated with instantaneous wall tension. In general, norepinephrine induced larger [Ca2+]i transients and a biphasic myosin phosphorylation pattern. The results demonstrate: (a) major roles for Ca2+ and myosin phosphorylation in arteriolar myogenic and norepinephrine-induced responses; (b) that changes in Ca2+ and phosphorylation during a myogenic response are related to changes in wall tension, and (c) differences in Ca2+ and phosphorylation patterns between the two modes of contraction reflect possible differences in underlying signaling mechanisms. The data further emphasize that spontaneous arteriolar tone represents a state of maintained smooth muscle activation that requires increases in [Ca2+]i and myosin light-chain phosphorylation.

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 (IC50 = 0.5–5 μM) but is a poor inhibitor of purified turkey smooth muscle myosin II (IC50 ∼80 μM). We found that blebbistatin potently (IC50 ∼3 μM) 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 (IC50 ∼5 μM). 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 β-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 (IC50 ∼20 μM). 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.

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.

Differential effects of sex hormones and phytoestrogens on peak and steady state contractions in isolated rabbit detrusor.

PURPOSE Recent evidence suggests that sex steroids may produce rapid inhibition of voltage operated Ca2+ channels (VOCCs). Detrusor smooth muscle is highly dependent upon Ca2+ influx for receptor-activated contractions. Thus, we examined the relative effectiveness of a select group of sex steroids and dietary phytoestrogens to relax detrusor contracted with the muscarinic receptor agonist, bethanechol (BE) and the purinergic P2X receptor agonist, alpha,beta-methylene ATP (alpha,beta-MeATP). MATERIALS AND METHODS Isolated strips of rabbit detrusor were secured to isometric force transducers in a tissue bath and length-adjusted until maximum contractions were achieved. Peak (P) contractile responses were recorded for alpha,beta-MeATP (P(ATP)) and BE (P(BE)) and steady-state (SS) responses were recorded for BE (SS(BE)) in the presence and absence of selected sex steroids and phytoestrogens (10 microM, unless indicated). RESULTS The L-type VOCC inhibitor, nifedipine (1 to 10 microM), completely inhibited P(ATP) but reduced SS(BE) by approximately 50%, whereas the VOCC and non-VOCC inhibitor, SKF 96365, inhibited SS(BE) by approximately 95%, suggesting that P(ATP) was entirely dependent on L-type VOCCs, but (BE)-induced contractions depended also on activation of non-VOCCs. 17Beta-estradiol (estradiol) and progesterone inhibited P(ATP) by approximately 60% and 20%, respectively, and 32 microM estradiol and ethinyl estradiol inhibited SS(BE) by approximately 80 and 95%, respectively. Inhibition by estradiol was potentiated, rather than blocked, by the nuclear estrogen receptor antagonist, tamoxifen. Moreover, tamoxifen alone nearly completely relaxed SS(BE). The inactive metabolite of estradiol, 17alpha-estradiol, inhibited both P(ATP) and P(BE) by approximately 40%. Testosterone had no effect on P(ATP) and P(BE). The phytoestrogen and tyrosine kinase inhibitor, genistein, inhibited SS(BE) by 44%, whereas daidzein, a phytoestrogen without tyrosine kinase inhibitory activity, produced only a 7% inhibition. None of the phytoestrogens examined inhibited P(BE), whereas all inhibited P(ATP) by approximately 20 to 35%. A comparison of inhibition of (BE) and alpha,beta-MeATP-induced contractions by selected estrogen isomers showed some distinct differences. For example, estrone did not inhibit P(BE) or SS(BE), but inhibited P(ATP) by approximately 20%, whereas DES inhibited SS(BE) by nearly 90%, but P(ATP) by a lesser degree (approximately 70%). CONCLUSIONS Our data support the hypothesis that 17beta-estradiol, ethinyl estradiol, DES, tamoxifen and genistein may relax detrusor contractions by inhibition of both VOCCs and non-VOCCs. Moreover, our data show that genistein, a dietary phytoestrogen with tyrosine kinase inhibitory activity, selectively reduced alpha,beta-MeATP-induced peak and BE-induced steady-state contractions, sparing the maximum response to BE. Lastly, the inactive isomer, 17alpha-estradiol, inhibited both BE- and alpha,beta-MeATP-induced contractions. These data suggest that certain dietary phytoestrogens (for example, genistein) or sex steroids, especially those with weak activity at the nuclear steroid site (for example, 17alpha-estradiol), or tamoxifen may prove therapeutically useful in treating overactive bladder caused by elevated muscarinic and purinergic receptor activation.

RhoA kinase and protein kinase c participate in regulation of rabbit stomach fundus smooth muscle contraction

The degree to which the RhoA kinase (ROK) blockers, Y‐27632 (1 μM) and HA‐1077 (10 μM), and the PKC blocker, GF‐109203X (1 μM), reduced force produced by carbachol, a muscarinic receptor agonist, and phenylephrine, an α‐adrenoceptor agonist, was examined in rabbit stomach fundus smooth muscle. When examining the effect on cumulative carbachol concentration‐response curves (CRCs), ROK and PKC blockers shifted the potency (EC50) to the right but did not reduce the maximum response. In a single‐dose carbachol protocol using moderate (∼EC50) and maximum carbachol concentrations, Y‐27632 and HA‐1077 reduced peak force, but GF‐109203X had no effect. By contrast, all three agents inhibited the carbachol contractions of rabbit bladder (detrusor) smooth muscle. Compared to carbachol, phenylephrine produced a weaker maximum response that was not inhibited by phentolamine, atropine nor capsaicin but was inhibited by Y‐27632, HA‐1077 and GF‐109203X. In detrusor, classical down‐regulation occurred, but in fundus, up‐regulation of responsiveness occurred. This up‐regulation in fundus may have been a post‐receptor event, because a KCl‐induced contraction produced after a carbachol CRC was stronger than one produced before the carbachol stimulus. In conclusion, these data suggest that ROK plays a critical role in the regulation of rabbit fundus smooth muscle contraction, which is distinct from chicken gizzard smooth muscle, where ROK is reported to exist but to not play a role in muscarinic receptor‐induced contraction. Additional unique findings are that PKC participates in phenylephrine‐ but not carbachol‐induced contraction in fundus, that carbachol does not activate identical subcellular signalling systems in fundus and detrusor, and that fundus, unlike detrusor, responds to carbachol stimulation with post‐receptor up‐regulation of contraction.

BETHANECHOL ACTIVATES A POST-RECEPTOR NEGATIVE FEEDBACK MECHANISM IN RABBIT URINARY BLADDER SMOOTH MUSCLE

PURPOSE Recent studies using vascular and gut smooth muscles indicate that contractile receptor agonists may activate post-receptor down-regulatory mechanisms causing a temporary reduction in the strength of subsequent contractions. Our data indicate a similar mechanism exists in detrusor smooth muscle of the urinary bladder. MATERIALS AND METHODS Each isolated strip of female rabbit detrusor was placed in a tissue bath, secured to an isometric force transducer, and length-adjusted until depolarization with 110 mM KCl produced a maximum contraction (S0). Subsequent contractions were normalized to S0 (S/S0) or to a first stimulus with 30 mM KCl or caffeine (S/S1). Tissues were pretreated with the muscarinic receptor agonist, bethanechol (BE), then stimulated with KCl, caffeine, or Bay k 8644 to identify potential post-receptor down-regulation. RESULTS Contractions induced by 30 mM KCl had three phases labeled fast peak (FP), slow peak (SP) and steady-state (SS). In tissues exposed for 30 min. to a maximum BE concentration then washed for 5 min., the KCl-induced FP and SP, but not SS, responses were reduced by approximately 40%. Smaller reductions in peak KCl-induced contractions occurred in tissues pretreated for a shorter duration or with a 100-fold lower BE concentration. This down-regulation induced by bethanechol pretreatment was reversible, lasting approximately 1-2 h. Not only were KCl-induced contractions reduced by BE pretreatment, but also those produced by the intracellular Ca(2+)-mobilizer, caffeine, and the L-type Ca2+ channel agonist, Bay k 8644. CONCLUSIONS Pretreatment of isolated strips of rabbit detrusor with a muscarinic receptor agonist produced short-term down-regulation of KCl-induced peak contractions that may have involved inhibition of both influx of extracellular Ca2+ and release of intracellular Ca2+. Reductions in the degree of this novel modulatory response during disease conditions and aging could enhance contractile activity, possibly causing detrusor instability.

Role of Protein Kinase Cζ 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 Ca2+ 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 PKCζ (PIPKCζ) inhibited KCl-induced tonic F. A myristoylated pseudosubstrate inhibitor of PKCα/β 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 μM GF-109203X inhibited only KCl-stimulated phosphorylation, not basal MYPT1-pT853. In fura-2-loaded tissues, GF-109203X and PIPKCζ elevated basal [Ca2+]i (calcium) and potentiated KCl-induced tonic increases in calcium while reducing KCl-induced tonic increases in F. Blockade by nifedipine of Ca2+ entry through voltage-operated Ca2+ channels reduced KCl-induced Ca2+ sensitization and KCl-stimulated but not basal MYPT1-pT853. These data together support a model in which ROCK and PKCζ 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 PKCζ to feed forward and cause additional MYPT1-pT853 above that induced by constitutive ROCK, permitting Ca2+ sensitization and strong F maintenance. Active PKCζ also feeds back to attenuate the degree of KCl-induced increases in calcium.

Adaptation of the length-active tension relationship in rabbit detrusor

Studies have shown that the length-tension (L-T) relationships in airway and vascular smooth muscles are dynamic and can adapt to length changes over a period of time. Our prior studies have shown that the passive L-T relationship in rabbit detrusor smooth muscle (DSM) is also dynamic and that DSM exhibits adjustable passive stiffness (APS) characterized by a passive L-T curve that can shift along the length axis as a function of strain history and activation history. The present study demonstrates that the active L-T curve for DSM is also dynamic and that the peak active tension produced at a particular muscle length is a function of both strain and activation history. More specifically, this study reveals that the active L-T relationship, or curve, does not have a unique peak tension value with a single ascending and descending limb, but instead reveals that multiple ascending and descending limbs can be exhibited in the same DSM strip. This study also demonstrates that for DSM strips not stretched far enough to reveal a descending limb, the peak active tension produced by a maximal KCl-induced contraction at a short, passively slack muscle length of 3 mm was reduced by 58.6 +/- 4.1% (n = 15) following stretches to and contractions at threefold the original muscle length, 9 mm. Moreover, five subsequent contractions at the short muscle length displayed increasingly greater tension; active tension produced by the sixth contraction was 91.5 +/- 9.1% of that produced by the prestretch contraction at that length. Together, these findings indicate for the first time that DSM exhibits length adaptation, similar to vascular and airway smooth muscles. In addition, our findings demonstrate that preconditioning, APS and adaptation of the active L-T curve can each impact the maximum total tension observed at a particular DSM length.