Downregulation of cGMP-dependent protein kinase-1 activity in the corpus cavernosum smooth muscle of diabetic rabbits

Increased guanosine 3',5'-cyclic monophosphate (cGMP), induced by nitric oxide release, is crucial for corpus cavernosum smooth muscle (CCSM) relaxation within the penis. This CCSM relaxation (necessary for penile erection) is impaired in men with erectile dysfunction (ED), especially those men with diabetes. One of the effector proteins for cGMP is cGMP-dependent protein kinase-1 (PKG-1). PKG-1 knockout mice exhibit detrusor overactivity (Am J Physiol Regul Integr Comp Physiol 279: R1112-R1120, 2000) and, more relevant to this study, ED (Proc Natl Acad Sci USA 97: 2349-2354, 2000), suggesting an in vivo role for PKG-1 in urogenital smooth muscle relaxation. In the current study, using normal rabbit CCSM, Western blot analysis revealed high expression of PKG-1 at levels almost equivalent to aorta (previously shown to have high PKG-1 expression) and that the two known alternatively spliced isoforms of PKG-1 (alpha and beta) are expressed in nearly equal amounts in the CCSM. However, in response to alloxan-induced diabetes, there was a decrease in expression of both PKG-1 isoforms at the mRNA and protein levels as determined by real-time RT-PCR and Western blotting, respectively, but with the PKG-1alpha isoform expression decreased to a greater extent. Moreover, diabetes was associated with significantly decreased PKG-1 activity of CCSM in vitro, correlating with decreased CCSM relaxation. Immunofluorescence microscopy revealed a diabetes-associated decrease in PKG-1 in the CCSM cells. In conclusion, our results demonstrate for the first time a significant downregulation of PKG-1 expression associated with decreased PKG-1 activity in the CCSM in response to diabetes. Furthermore, these results suggest a mechanistic basis for the decreased efficacy of phosphodiesterase V inhibitors in treating diabetic patients with ED.

Diabetes decreases rabbit bladder smooth muscle contraction while increasing levels of myosin light chain phosphorylation

The effect of diabetes mellitus on the regulation of urinary bladder smooth muscle contraction was studied. Diabetes was induced in the rabbit by alloxan injection followed by 16 wk of housing. The bladder was harvested and strips of wall devoid of both mucosa and serosa were examined. Intact strips of bladder smooth muscle from diabetic animals produced less stress in response to membrane depolarization than muscle from control animals; sensitivity to KCl was not changed. Carbachol responses were similar in muscle strips from the two animal groups. Basal myosin light chain (MLC) phosphorylation levels were significantly elevated in response to most stimuli in muscle strips from diabetic animals, although levels of stress were either unchanged or lower. alpha-Toxin-permeabilized strips that allow for control of the intracellular environment while maintaining excitation-contraction coupling showed increased levels of MLC phosphorylation but decreased sensitivity to activator Ca2+ in smooth muscle from diabetic animals. MLC phosphatase contents were similar in smooth muscle from the two animal groups; however, MLC phosphatase activity was greater in muscle from control compared with diabetic animals. These results suggest that diabetes mellitus uncouples basal MLC phosphorylation from force in the bladder smooth muscle cell.

Over Expression of Smooth Muscle Specific Caldesmon by Transfection and Intermittent Agonist Induced Contraction Alters Cellular Morphology and Restores Differentiated Smooth Muscle Phenotype

PURPOSE

The thin filament associated protein h-caldesmon (h-CaD) modulates actin myosin interaction and contraction. Bladder outlet obstruction and detrusor hypertrophy are associated with the over expression of the nonmuscle CaD isoform l-CaD. It implies a poorly differentiated state of bladder myocytes and cytoskeletal remodeling in detrusor hypertrophy. We determined if h-CaD expression can be increased in a unique bladder smooth muscle (BSM) cell line derived from obstructed rabbit bladder smooth muscle that over expresses l-CaD. We examined whether the genetic restoration of h-caldesmon is possible in bladder smooth muscle cells by transfection or by agonist mediated contraction and whether this manipulation would alter cellular morphology.

MATERIALS AND METHODS

BSM cells were transfected with chicken h-CaD cDNA inserted into a mammalian vector. In another experiment BSM cells underwent intermittent bethanechol induced stimulation. h-CaD mRNA and protein were quantified with reverse transcriptase-polymerase chain reaction and Western blot analyses. Cell morphology was assessed using phase, video and confocal microscopy after double immunostaining with antibodies against alpha-actin and caldesmon.

RESULTS

Reverse transcriptase-polymerase chain reaction using primers specific for the transfected vector and h-CaD cDNA confirmed stable transfection of cells and increased content of h-CaD mRNA. Following bethanechol induced intermittent contraction Western blotting revealed 80% relative over expression of h-CaD in treated transfected cell lines (p <0.05) and 74% (not significant) in treated nontransfected controls. Confocal immunofluorescence microscopy revealed CaD in the cytoplasmic filaments co-localized to alpha-actin in the main cell body and perinuclear region in transfected cells, in contrast to the diffuse, irregular distribution of these filaments in control cells.

CONCLUSIONS

A unique bladder myocyte cell line was successfully and stably transfected with h-CaD cDNA. We show that agonist induced intermittent contraction preferentially increases h-CaD expression, the predominant CaD in nonobstructed bladder smooth muscle, and the restoration of h-CaD alters cell morphology and the organization of cytoplasmic filaments in cells derived from obstructed rabbit detrusor musculature.

Why do gallium clusters have a higher melting point than the bulk?

Density functional molecular dynamical simulations have been performed on Ga17 and Ga13 clusters to understand the recently observed higher-than-bulk melting temperatures in small gallium clusters [Phys. Rev. Lett. 91, 215508 (2003)]]. The specific-heat curve, calculated with the multiple-histogram technique, shows the melting temperature to be well above the bulk melting point of 303 K, viz., around 650 and 1400 K for Ga17 and Ga13, respectively. The higher-than-bulk melting temperatures are attributed mainly to the covalent bonding in these clusters, in contrast with the covalent-metallic bonding in the bulk.

Smooth muscle hypertrophy following partial bladder outlet obstruction is associated with overexpression of non-muscle caldesmon

Partial bladder outlet obstruction (PBOO) induces remodeling of urinary bladder smooth muscle (detrusor). We demonstrate an increase in bladder wall mass, muscle bundle size, and a threefold increase in the cross-sectional area of detrusor myocytes following PBOO in male New Zealand White rabbits compared to that of controls. Some bladders with detrusor hypertrophy function close to normal (compensated), whereas others were dysfunctional (decompensated), showing high intravesical pressure, large residual urine volume, and voiding difficulty. We analyzed the expression of smooth muscle-specific caldesmon (h-CaD) and non-muscle (l-CaD) by Western blotting, RT-PCR, and real-time PCR. The expression of l-CaD is increased significantly at the mRNA and protein levels in the decompensated bladders compared to that of normal and compensated bladders. The CaD was also co-localized with myosin containing cytoplasmic fibrils in cells dissociated from obstructed bladders and cultured overnight. Our data show that the inability of decompensated bladders to empty, despite detrusor hypertrophy, is associated with an overexpression of l-CaD. The level of l-CaD overexpression might be a useful marker to estimate the degree of detrusor remodeling and contractile dysfunction in PBOO.

Decrease in Maximal Force Generation in the Neonatal Mouse Bladder Corresponds to Shift in Myosin Heavy Chain Isoform Composition

PURPOSE

A change in calcium handling has been proposed as the cause of decreased maximal force generation by neonatal bladders with growth. Recent studies suggest that increased myosin heavy chain isoform SM1 increases force generation. We studied force generation in neonatal mouse bladders to determine if decreases in SM1 corresponded with decreased force.

MATERIALS AND METHODS

C57Bl/6 mice were studied from birth to 12 weeks of life (adulthood). The bladder strip contractile response to KCl and bethanechol was followed by the inhibition of rho-kinase activity by Y-27632. The mRNA levels for SM1/SM2 were determined using reverse transcriptase-polymerase chain reaction and protein levels were determined using sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Muscle fraction per cross-sectional area was determined by trichrome staining.

RESULTS

Newborn bladders generated significantly more tension in response to KCl (43.3 vs 17.4 mN/mm2, p = 0.02) and bethanechol (40.6 vs 11.9 mN/mm2, p = 0.05) than adult bladders. Inhibition of rho-kinase resulted in similar decreases in tension in all bladders. SM1 mRNA decreased slightly from 60% at birth to 50% at 12 weeks. SM1 protein decreased from 72.5% at birth to 50% by 3 weeks and it remained stable at 12 weeks. Total myosin per gm protein remained stable. Muscle fraction decreased from 63.8% at birth to 58.6% at 12 weeks (p = 0.4).

CONCLUSIONS

We noted a decrease in SM1 that corresponded to a decrease in bladder force generation. The concept that SM1 contributes to the optimal assembly of myosin filaments suggests that changes in myosin isoforms may have a role in the decrease in voiding pressures seen in normal children.

Alteration in expression of myosin isoforms in detrusor smooth muscle following bladder outlet obstruction

Partial urinary bladder outlet obstruction (PBOO) in men, secondary to benign prostatic hyperplasia, induces detrusor smooth muscle (DSM) hypertrophy. However, despite DSM hypertrophy, some bladders become severely dysfunctional (decompensated). Using a rabbit model of PBOO, we found that although DSM from sham-operated bladders expressed nearly 100% of both the smooth muscle myosin heavy chain isoform SM-B and essential light chain isoform LC17a, DSM from severely dysfunctional bladders expressed as much as 75% SM-A and 40% LC17b (both associated with decreased maximum velocity of shortening). DSM from dysfunctional bladder also exhibited tonic-type contractions, characterized by slow force generation and high force maintenance. Immunofluorescence microscopy showed that decreased SM-B expression in dysfunctional bladders was not due to generation of a new cell population lacking SM-B. Metabolic cage monitoring revealed decreased void volume and increased voiding frequency correlated with overexpression of SM-A and LC17b. Myosin isoform expression and bladder function returned toward normal upon removal of the obstruction, indicating that the levels of expression of these isoforms are markers of the PBOO-induced dysfunctional bladders.

Obstruction-induced changes in urinary bladder smooth muscle contractility: a role for Rho kinase

Detrusor smooth muscle (DSM) undergoes hypertrophy after partial bladder outlet obstruction (PBOO) in male rabbits, as it does in men with PBOO induced by benign prostatic hyperplasia. Despite detrusor hypertrophy, some bladders are severely dysfunctional (decompensated). In this study, the rabbit model for PBOO was used to determine the biochemical regulation of the contractile apparatus and force maintenance by the detrusor from decompensated bladders (DB). Bladders from sham-operated rabbits served as a control. On stimulation with 125 mM KCl, the DSM from sham-operated (SB) rabbits showed phasic contractions, whereas the detrusor from DB was tonic, exhibiting slow development of force, a longer duration of force maintenance, and slow relaxation. The Rho kinase (ROK) inhibitor Y-27632 enhanced the relaxation of precontracted DSM strips from DB. The enhancement of relaxation of the KCl-induced contraction of DB by Y-27632 was associated with dephosphorylation of myosin light chain (MLC20). The DSM extract from DB showed low phosphatase activity compared with that from SB. The DB also showed more Ca2+-independent MLC20 phosphorylation, which was partially inhibited by Y-27632. RT-PCR and Western blotting revealed similar expression levels of MLC kinase and ROK-alpha in SB and DB, but ROK-beta was overexpressed in DB. These results suggest that the ROK-mediated pathway is partly responsible for the high degree of force maintenance and slow relaxation in the detrusor from DB.

Desensitization of the adrenocorticotrophin responses to arginine vasopressin and corticotrophin-releasing hormone in ovine anterior pituitary cells

Following repeated or prolonged exposure to either corticotrophin-releasing hormone (CRH) or arginine vasopressin (AVP), pituitary adrenocorticotrophin (ACTH) responsiveness is reduced. This study compared the characteristics of desensitization to CRH and AVP in perifused ovine anterior pituitary cells. Desensitization to AVP occurred at relatively low AVP concentrations and was both rapid and readily reversible. Treatment for 25 min with AVP at concentrations greater than 2 nM caused significant reductions in the response to a subsequent 5 min 100 nM AVP pulse (IC(50)=6.54 nM). Significant desensitization was observed following pretreatment with 5 nM AVP for as briefly as 5 min. Desensitization was greater following a 10 min pretreatment, but longer exposures caused no further increase. Resensitization was complete within 40 min following 15 min treatment with 10 nM AVP. Continuous perifusion with 0.01 nM CRH had no effect on AVP-induced desensitization. Treatment with 0.1 nM CRH for either 25 or 50 min caused no reduction in the response to a subsequent 5 min stimulation with 10 nM CRH. When the pretreatment concentration was increased to 1 nM significant desensitization was observed, with a greater reduction in response occurring after 50 min treatment. Recovery of responsiveness was progressive following 50 min treatment with 1 nM CRH and was complete after 100 min. Our data show that in the sheep AVP desensitization can occur at concentrations and durations of AVP exposure within the endogenous ranges. This suggests that desensitization may play a key role in regulating ACTH secretion in vivo. If, as has been suggested, CRH acts to set corticotroph gain while AVP is the main dynamic regulator, any change in responsiveness to CRH may significantly influence the overall control of ACTH secretion.

Increased contractility of diabetic rabbit corpora smooth muscle in response to endothelin is mediated via Rho-kinase beta

Corpus cavernosum smooth muscle (CCSM) from rabbits made diabetic for 6 months as a result of alloxan injection exhibited increased sensitivity (3vs 9 nM EC(50)) and generated 20-50% greater force to endothelin-1 (ET-1) compared to CCSM from normal rabbits. In contrast, the force produced by the CCSM in response to KCl and phenylephrine was not significantly altered in diabetic CCSM. The increased ET-1 sensitivity is associated with a two to three-fold upregulation of ET receptor A at both mRNA and protein levels in diabetic CCSM. ET-1-induced CCSM contraction is largely dependent upon Rho-kinase (ROK), since it is almost completely blocked by Y-27632 (a highly selective ROK inhibitor). Furthermore, expression of ROKbeta isoform is selectively upregulated in CCSM from diabetic rabbits. Thus, an increased CCSM tone, modulated by sensitization of the endothelin-mediated contractile pathway via ROK, may be a key component of the molecular mechanism of diabetes-induced erectile dysfunction.

An indigenously developed method for sweat collection and estimation of chloride for diagnosis of cystic fibrosis

An indigenously developed method for sweat collection and titration method for estimation of chloride was validated. The mean difference in estimated chloride value from the known strength of saline in 50 samples was -1.04 +/- 4.13 mEq/L (95% CI: -0.07 to 2.28). The mean difference in the estimated chloride values between two observers when the test was performed on known strengths of saline solution was -2.5 +/- 4.24 mEq/L (95% CI: -3.67 to 1.33). The inter observer variability between two observers when the test was performed on sweat samples obtained from 50 individuals was -1.12 +/- 4.34 mEq/L (95% CI: -2.23 to 0.8 ). Sweat weight of more than 100 mg could be collected in first attempt in 602 of 757 (80%) patient with an average sweat weight of 230 mg. This inexpensive method of sweat collection and chloride estimation has acceptable accuracy and repeatability and can be used in resource poor setting for making a diagnosis of cystic fibrosis.

Generation of a cell line with smooth muscle phenotype from hypertrophied urinary bladder

We have established a cell line from hypertrophied rabbit urinary bladder smooth muscle (SM) that stably expresses SM myosin (SMM). These cells, termed BSM, are spindle shaped and form swirls, similar to the "hills and valleys" described for cultured aortic SM cells. Western blotting revealed that BSM expresses the amino-terminal SMM heavy chain isoform SM-B, the carboxy-terminal SM1 and SM2 isoforms, and SM alpha-actin. In addition, they express cGMP-dependent protein kinase G, made by contractile SM cells in vitro but not by noncontractile cells synthesizing extracellular matrix. Immunofluorescence studies indicate a homogeneous population of cells expressing alpha-actin and SMM, including the SM-B isoform, and karyotyping demonstrates a stable 4N chromosomal pattern. These cells also express calcium-dependent myosin light chain kinase and phosphatase activity and contract in response to the muscarinic agonist bethanechol. To our knowledge, BSM is the first visceral SM cell line that expresses the SM-B isoform and might serve as a useful model to study the transcriptional regulation of tissue-specific SMM isoforms in differentiation and pathological SM.

Enhanced force generation by corpus cavernosum smooth muscle in rabbits with partial bladder outlet obstruction

PURPOSE

Growing clinical evidence suggests that benign prostatic hyperplasia induced partial bladder outlet obstruction is associated with an increased incidence of erectile dysfunction. We determined whether corpus cavernosum smooth muscle from rabbits with partial bladder outlet obstruction show any molecular or functional differences versus controls.

MATERIALS AND METHODS

Force generation and relaxation of corpus cavernosum smooth muscle 2 weeks after partial bladder outlet obstruction by 125 mM. KCl, phenylephrine and field stimulation were determined. Expression of total smooth muscle myosin and alternatively spliced smooth muscle myosin isoforms were determined by reverse transcriptase-polymerase chain reaction (RT-PCR), quantitative competitive RT-PCR, sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot analysis. Corpus cavernosum smooth muscle from sections of the penis were analyzed morphologically by immunofluorescence microscopy using antibodies to smooth muscle myosin and neurofilament protein.

RESULTS

Corpus cavernosum smooth muscle from rabbits with partial bladder outlet obstruction generated 40% to 50% more force than that of sham operated rabbits in response to KCl or phenylephrine and was more difficult to relax. Although quantitative competitive RT-PCR and sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that corpus cavernosum smooth muscle from rabbits with partial bladder outlet obstruction expressed only slightly more total smooth muscle myosin at the messenger RNA and protein levels, expression of the high adenosine triphosphatase isoform SM-B increased 2-fold. Morphological examination of corpus cavernosum smooth muscle sections revealed decreased innervation and increased smooth muscle bundle size.

CONCLUSIONS

We present the novel finding of molecular and functional changes in the corpus cavernosum smooth muscle associated with partial bladder outlet obstruction. Although the inclusion of sham operation ruled out direct injury during surgery, a change in corpus cavernosum smooth muscle innervation induced by nerve compression by the ligature is likely to induce trophic changes in the corpus cavernosum smooth muscle leading to over expression of the SM-B smooth muscle myosin isoform, increased contractility and impaired relaxation.

Acute and chronic regulation of pituitary receptors for vasopressin and corticotropin releasing hormone

At least two hypothalamic peptides, corticotropin releasing hormone (CRH) and vasopressin (VP), are important in regulating adrenocorticotropin (ACTH) release from the anterior pituitary. Both are secreted in a pulsatile manner and stimulate ACTH secretion by interacting with G protein-coupled receptors (GPCRs), namely the type 1 CRH receptor and V1b receptor, respectively. Repeated or prolonged stimulation with either peptide can cause reduced ACTH responsiveness or desensitisation, both in vivo and in vitro. Desensitisation of perifused sheep anterior pituitary cells to VP was found to be rapid and occurred following treatment with 5 nM VP for 5 min. This is within the range of concentrations and durations of VP pulses seen in sheep portal blood during acute stress. In contrast, significant desensitisation of the ACTH response to CRH required pre-treatment for longer than 25 min with a CRH concentration of 1 nM, suggesting that endogenous pulses may not elicit desensitisation. Although rapid GPCR desensitisation involves uncoupling of receptors from their G proteins, commonly mediated by receptor phosphorylation, and internalisation of receptors, desensitisation of neither the CRH nor VP receptor was mediated by PKA or PKC, respectively. Desensitisation of the response to VP was found to be dependent upon receptor internalisation, and resensitisation could be delayed by treatment with a protein phosphatase 2B inhibitor. The rapid kinetics of desensitisation of the ACTH response to VP suggest that this process is important in regulating the response to acute rather than chronic stress. If, as has been suggested, CRH acts in a permissive way to set corticotrope gain, desensitisation to CRH could also be important in long term regulation of ACTH secretion.

Osteopontin gene expression and immunolocalization in the rabbit urinary tract

PURPOSE

Osteopontin is a highly phosphorylated, calcium binding sialoprotein characterized by a conserved arginine-glycine-aspartate sequence. Vitronectin receptor (alphavbeta3 integrin) and hyaluronan receptor (CD44) are documented as receptors for osteopontin and their expression has been established in the bladder. Based on that finding and the fact that osteopontin protein is present in urine we hypothesized that osteopontin is expressed in the lower urinary tract.

MATERIALS AND METHODS

Osteopontin messenger (m)RNA and protein were analyzed in 5 adult urinary tracts and 5 neonatal bladders of New Zealand White rabbits using reverse transcriptase-polymerase chain reaction and immunohistochemical testing. Analysis of mRNA expression and localization of osteopontin receptors, alphavbeta3 integrin and CD44 were also performed in adult bladders and primary cultures of detrusor myocytes.

RESULTS

Adult renal pelvis, ureter, bladder and urethra, and neonatal bladders contained significant levels of osteopontin mRNA. Immunohistochemical staining revealed osteopontin expression in all layers of the transitional epithelium of the bladder, co-localizing with alphavbeta3 integrin mainly in the superficial layers and with CD44 mainly in the basal layers. Osteopontin was detected within the cytoplasm of smooth muscle cells, while alphavbeta3 integrin was located closer to the plasmalemma. Furthermore, primary cultured detrusor myocytes expressed osteopontin mRNA in stable fashion for up to 4 passages. Treating bladder myocyte cultures with insulin-like growth factor-1 and 17beta-estradiol resulted in up-regulation and down-regulation of osteopontin mRNA, respectively.

CONCLUSIONS

Adult and neonatal rabbit detrusors are a prominent source of osteopontin in vivo and in vitro. Epithelial osteopontin may be a source of osteopontin in urine. The co-localization of osteopontin in the bladder epithelium with alphavbeta3 integrin and CD44 suggests a role in maintaining the integrity of the transitional epithelium by providing the sealing and adhesiveness needed for the impermeable state of the bladder.

Heterogeneity of bladder myocytes in vitro: modulation of myosin isoform expression

We studied the expression of myosin heavy chain isoforms differing at the N-terminal (SM-A, SM-B) and the C-terminal (SM1, SM2) regions and non-muscle myosin heavy chain II-A and II-B (NMMHC II-A and B) in newborn and adult rabbit bladder smooth muscle cells (SMCs) and in cultures of enzymatically dissociated neonatal detrusor. RT-PCR analyses revealed that 94.5+/-3.27% of MHC transcripts of the adult bladder SMCs contained the 21-nucleotide insert (SM-B) compared with 83.8+/-3.2% in the newborn bladder, with the remainder of the mRNA being non-inserted (SM-A). In 3, 7, and 10 days of primary culture (proliferating, confluent, and post-confluent, respectively) and up to 4 subculture passages, bladder myocytes expressed predominantly SM-A. Immunofluorescence microscopy revealed heterogeneity in cultured myocytes, i.e. SM-B positive cells coexisting with negatively stained cells. In adult bladder, the C-terminal isoforms SM1 and SM2 represented, 43.1+/-4.3% and 56.89 + 4.3% of the mRNA, respectively, while newborn bladders expressed 72.5+/-7% SM1 and 27.5+/-7% SM2. Upon culturing, cells predominantly expressed SM1 at both the mRNA and protein levels. NMMHC II-A was expressed by both adult and newborn bladders and in culture, whereas NMMHC II-B was expressed at low levels only in newborn bladders, but upregulated in culture. These data indicate that bladder myocytes in vitro undergo modulation with relative overexpression of SM-A and SM1 and upregulation of NMMHC II-B. Information on the mechanisms responsible for this modulation in vitro might provide an understanding of the nature of altered myosin isoform expression associated with smooth muscle dysfunction in certain bladder diseases.

Estrogen modulates the expression of myosin heavy chain in detrusor smooth muscle

The effect of low serum estrogen levels on urinary bladder function remains poorly understood. Using a rabbit model, we analyzed the effects of estrogen on the expression of the isoforms of myosin, the molecular motor for muscle contraction, in detrusor smooth muscle. Expression of myosin heavy chain (MHC) isoforms, which differ in the COOH-terminal (SM1 and SM2) and the NH(2)-terminal (SM-A and SM-B) regions as a result of alternative splicing of the mRNA at either the 3'- or 5'-ends, was analyzed in age-matched female rabbits that were sham operated, ovariectomized (Ovx), and given estrogen after ovariectomy (4 rabbits/group). Ovx rabbits showed a significant decrease in the overall MHC content per gram of wet detrusor smooth muscle compared with controls (P < 0.04), which was reversed by estrogen replacement (P < 0.02). MHC content, as a proportion of total milligram of protein in the bladder tissue extracted, was also increased in estrogen-treated Ovx rabbits. Quantitative competitive RT-PCR revealed 1.72-, 2.63-, and 5.82 x 10(6) copies of MHC mRNA/100 ng total mRNA in Ovx, control, and estrogen-treated rabbits, respectively (P < 0.01). RT-PCR analysis using oligonucleotides specific for the region containing the SM1/SM2 MHC alternative splice sites indicated a lower SM2-to-SM1 ratio in estrogen-treated compared with control and Ovx rabbits (P < 0.05). Similarly, SDS-PAGE analysis of extracted myosin from estrogen-treated rabbits revealed a significantly lower SM2-to-SM1 isoform ratio compared with control and Ovx rabbits (P < 0.05). Expression of the SM-A and SM-B isoforms was not affected. These results indicate that myosin content is increased upon estrogen replacement in Ovx rabbits and that the abundance of SM1 relative to SM2 is greater in estrogen-treated rabbits compared with normal and Ovx rabbits. These data suggest that estrogen affects alternative splicing at the 3'-end of the MHC pre-mRNA to increase the proportion of SM1 vs. SM2.

Regional variation in myosin isoforms and phosphorylation at the resting tone in urinary bladder smooth muscle

Urinary bladder filling and emptying requires coordinated control of bladder body and urethral smooth muscles. Bladder dome, midbladder, base, and urethra showed significant differences in the percentage of 20-kDa myosin light chain (LC20) phosphorylation (35.45 +/- 4.6, 24.7 +/- 2.2, 13.6+/- 2.1, and 12.8 +/- 2.7%, respectively) in resting muscle. Agonist-mediated force was associated with a rise in LC20 phosphorylation, but the extent of phosphorylation at all levels of force was less for urethral than for bladder body smooth muscle. RT-PCR and quantitative competitive RT-PCR analyses of total RNA from bladder body and urethral smooth muscles revealed only a slight difference in myosin heavy chain mRNA copy number per total RNA, whereas mRNA copy numbers for NH2-terminal isoforms SM-B (inserted) and SM-A (noninserted) in these muscles showed a significant difference (2.28 x 10(8) vs. 1.68 x 10(8) for SM-B and 0.12 x 10(8) vs. 0.42 x 10(8) for SM-A, respectively), which was also evident at the protein level. The ratio of COOH-terminal isoforms SM2:SM1 in the urethra was moderately but significantly lower than that in other regions of the bladder body. A high degree of LC20 phosphorylation and SM-B in the bladder body may help to facilitate fast cross-bridge cycling and force generation required for rapid emptying, whereas a lower level of LC20 phosphorylation and the presence of a higher amount of SM-A in urethral smooth muscle may help to maintain the high basal tone of urethra, required for urinary continence.