cGMP mediates corpus cavernosum smooth muscle relaxation with altered cross-bridge function

Insights on Platelet-Derived Growth Factor Receptor α-Positive Interstitial Cells in the Male Reproductive Tract

Male infertility is a significant factor in approximately half of all infertility cases and is marked by a decreased sperm count and motility. A decreased sperm count is caused by not only a decreased production of sperm but also decreased numbers successfully passing through the male reproductive tract. Smooth muscle movement may play an important role in sperm transport in the male reproductive tract; thus, understanding the mechanism of this movement is necessary to elucidate the cause of sperm transport disorder. Recent studies have highlighted the presence of platelet-derived growth factor receptor α (PDGFRα)-positive interstitial cells (PICs) in various smooth muscle organs. Although research is ongoing, PICs in the male reproductive tract may be involved in the regulation of smooth muscle movement, as they are in other smooth muscle organs. This review summarizes the findings to date on PICs in male reproductive organs. Further exploration of the structural, functional, and molecular characteristics of PICs could provide valuable insights into the pathogenesis of male infertility and potentially lead to new therapeutic approaches.

An immunochromatographic strip sensor for sildenafil and its analogues

In this study, a new hapten of sildenafil (Sild) was successfully synthesized and a sensitive monoclonal antibody (mAb) against Sild was produced based on this new hapten. The subclass of the mAb was IgG2a, and the half-maximal inhibitory concentration (IC50) of the mAb was 0.53 ng mL-1. Next, an immunochromatographic assay (ICA) was established for detecting Sild and its analogues in functional foods, where the visual detection limit (vLOD) and cut-off values were 0.5 and 20 μg kg-1, respectively. With the aid of a strip scan reader, the ICA can measure Sild with an LOD of 0.7 μg kg-1 and a line range of detection between 1.40 and 13.11 μg kg-1. The whole test process takes only 15 min. Therefore, the ICA provides a useful tool for the on-site detection and rapid initial screening of Sild in functional food samples.

Correlation between flow-mediated dilation and erectile dysfunction

We hypothesized that there is a correlation between the magnitude of endothelial-mediated dilation of brachial artery and erectile function in patients. Thus, flow-mediated dilation of the brachial artery (FMD)-used to assess the function of endothelium-was measured in 56 patients (aged approximately 35 years) having erectile dysfunction for 6-12 months. The patients were grouped based on International Index of Erectile Dysfunction: severe (5-10), moderate (11-16), mild to moderate (17-21), and mild (22-25). As compared to the mild group (8.8 +/- 1.7%), FMD was significantly reduced in the mild-to-moderate group (5.7 +/- 1.1%), moderate group (5.3 +/- 0.8%), and severe group (4.4 +/- 0.6%). Also, there was a positive correlation between the magnitude of endothelial and erectile dysfunction. Patients were treated with the 5-phosphodiesterase inhibitor sildenafil, known to elevate vascular cGMP level and thus the vascular efficacy of internal nitric oxide, for 3 to 6 months prior to the study. The mean doses of sildenafil used were as follows: severe group, 100 mg/event; moderate group, 86.1 +/- 21.4 mg/event; mild-to-moderate group, 71.8 +/- 23.2 mg/event; mild group, 25 mg/event. We found a positive correlation between the sildenafil dose requirement and the severity of erectile dysfunction. On the bases on these findings, together with the known mechanism of action of sildenafil, we propose that vascular endothelial dysfunction could contribute to erectile dysfunction and that erectile dysfunction may be an early marker of peripheral vascular disease.

Increased Expression of Heat Shock Protein 20 and Decreased Contractile Stress in Obstructed Rat Bladder

PURPOSE

Bladder outlet obstruction induces detrusor hypertrophy and it can eventually lead to decreased bladder smooth muscle contractility. Heat shock protein 20 is the proposed mediator of force suppression in vascular smooth muscle. We investigated whether heat shock protein 20 could also mediate the decreased contractility observed in partially obstructed rat bladders.

MATERIALS AND METHODS

Female Wistar rats (Harlan Laboratories, Indianapolis, Indiana) were randomized to partial urethral ligation or sham ligation. After 3 weeks the rats were sacrificed, and the bladders were harvested, frozen, homogenized and analyzed for heat shock protein 20 content by Western blot immunoreactivity. The content of myosin regulatory light chain, a constitutively expressed protein, was determined as a control. Bladder smooth muscle strips were dissected from some rats and mounted for force generation measurement.

RESULTS

At cystectomy obstructed bladders were significantly heavier and had more residual urine compared to sham operated bladders. Heat shock protein 20 immunoreactivity was significantly increased a mean +/- 1 SEM of 1.9 +/- 0.3-fold in obstructed vs sham operated bladders. Control protein myosin regulatory light chain immunoreactivity did not significantly differ in obstructed and sham operated bladders. Maximal stress, that is force per cross-sectional area, was significantly decreased in obstructed vs sham operated bladders. Human bladder was found to express immunoreactive heat shock protein 20.

CONCLUSIONS

We noted that partially obstructed rat bladders 1) express higher levels of heat shock protein 20 and 2) generate less stress than sham operated bladders. These data suggest the possibility that heat shock protein 20 over expression could at least partially mediate the decreased contractile activity observed with partial bladder outlet obstruction. The mechanism for increased heat shock protein 20 expression is unknown but it may involve increased mechanical stress or hypoxia from urethral obstruction. Human bladder expressed immunoreactive heat shock protein 20, suggesting that a similar mechanism could potentially occur in humans. If confirmed in humans, patients with clinical conditions that result in detrusor hypocontractility could potentially benefit from pharmacological interventions aimed at inhibiting heat shock protein 20.

Absence of force suppression in rabbit bladder correlates with low expression of heat shock protein 20

Background

Nitroglycerin can induce relaxation of swine carotid artery without sustained reductions in [Ca²⁺]_i or myosin regulatory light chain (MRLC) phosphorylation. This has been termed force suppression and been found to correlate with ser¹⁶-phosphorylation of heat shock protein 20 (HSP20). We tested for the existence of this mechanism in a smooth muscle that is not responsive to nitric oxide.

Methods

Isometrically mounted mucosa free rabbit bladder strips were contracted with carbachol and relaxed with 8-Br-cGMP, forskolin, or isoprenaline.

Results

Contraction was associated with a highly cooperative relation between MRLC phosphorylation and force such that very small increases in MRLC phosphorylation induced large increases in force. Relaxation induced by 8-Br-cGMP, forskolin, or isoprenaline did not shift the MRLC phosphorylation-force relation from that observed with carbachol alone, i.e. there was no force suppression. HSP20 content was negligible (approximately two hundred-fold less than swine carotid).

Conclusion

The lack of force suppression in the absence of HSP20 is consistent with the hypothesized role for HSP20 in the force suppression observed in tonic smooth muscles.

Phosphodiesterase-5 Inhibition: the Molecular Biology of Erectile Function and Dysfunction

This article discusses the role of phosphodiesterase-5 (PDE-5) inhibition in the molecular biology of erectile function and dysfunction. Commercially marketed PDE-5 inhibitors are highly specific for PDE-5, and in the face of continuing cyclic GMP (cGMP) synthesis,elevate cellular cGMP. This elevation results from direct competitive inhibition of PDE-5 and from blocking the negative feedback regulation of the enzyme. Elevation of cGMP activates cGMP-dependent protein kinase, which mediates the effects of the cGMP-signaling pathway to decrease smooth muscle tone and dilate penile vascular smooth muscle. By exploiting features of PDE-5 regulatory mechanisms that modulate PDE-5 function, the inhibitors enhance their own potencies.

Structural and functional features in human PDE5A1 regulatory domain that provide for allosteric cGMP binding, dimerization, and regulation

The cGMP-binding cGMP-specific phosphodiesterase (PDE5) contains a catalytic domain that hydrolyzes cGMP and a regulatory (R) domain that contains two GAFs (a and b; GAF is derived from the proteins mammalian cGMP-binding PDEs, Anabaena adenylyl cyclases, and Escherichia coli (FhlA)). The R domain binds cGMP allosterically, provides for dimerization, and is phosphorylated at a site regulated by allosteric cGMP binding. Quaternary structures and cGMP-binding properties of 10 human PDE5A1 constructs containing one or both GAFs were characterized. Results reveal that: 1) high affinity homo-dimerization occurs between GAF a modules (K(D) < 30 nM) and between GAF b modules (K(D) = 1-20 pM), and the sequence between the GAFs (Thr322-Asp403) contributes to dimer stability; 2) 176 amino acids (Val156-Gln331) in GAF a are adequate for cGMP binding; 3) GAF a has higher affinity for cGMP (K(D) < 40 nM) than does the isolated R domain (K(D) = 110 nM) or holoenzyme (K(D) = 200 nM), suggesting that the sequence containing GAF b and its flanking amino acids autoinhibits GAF a cGMP-binding affinity in intact R domain; 4) a mutant (Met1-Glu321) containing only GAF a has high affinity, biphasic cGMP-binding kinetics consistent with structural heterogeneity of GAF a, suggesting that the presence of GAF b is not required for biphasic cGMP-dissociation kinetics observed in holoenzyme or isolated R domain; 5) significant cGMP binding by GAF b was not detected; and 6) the sequence containing GAF b and its flanking amino acids is critical for cGMP stimulation of Ser102 phosphorylation by cyclic nucleotide-dependent protein kinases. Results yield new insights into PDE5 functions, further define boundaries that provide for allosteric cGMP binding, and identify regions that contribute to dimerization.

Regulation of intracellular Ca2+ release in corpus cavernosum smooth muscle: synergism between nitric oxide and cGMP

Tonic contraction of corpus cavernosum smooth muscle cells (SMCs) maintains the flaccid state of the penis, and relaxation is initiated by nitric oxide (NO), leading to erection. Our aim was to investigate the effect of NO on the smooth muscle cellular response to adrenergic stimulation in corpus cavernosum. Fura-2 fluorescence was used to record intracellular Ca(2+) concentration ([Ca(2+)](i)) from freshly isolated SMCs from rat and human. Phenylephrine (PE) transiently elevated [Ca(2+)](i) in the presence and absence of extracellular Ca(2+), indicating release from intracellular stores. Whereas the NO donor S-nitroso-N-acetylpenicillamine (SNAP) with sildenafil citrate (SIL) caused no change in basal [Ca(2+)](i), the PE-induced rise of [Ca(2+)](i) was reversibly inhibited by 27 +/- 7% (n = 21, P < 0.005) in rat and by 55 +/- 15% (n = 9, P < 0.01) in human SMCs. SNAP and SIL also reduced the contractile response to PE. To investigate the mechanism, we applied mediators alone or in combination. The soluble guanylyl cyclase inhibitor ODQ reduced the effect of SNAP and SIL. SIL, cGMP analogs, and NO donors without SIL did not reduce the PE-induced rise of [Ca(2+)](i). However, the combination of 8-bromo-cGMP with SNAP reduced the Ca(2+) peak by 42 +/- 9% (n = 22, P < 0.01). Our results demonstrate that NO and cGMP act synergistically to reduce Ca(2+) release from intracellular stores. Reduction of intracellular Ca(2+) release may contribute to relaxation of the corpus cavernosum, leading to erection.

Heat shock protein 20-mediated force suppression in forskolin-relaxed swine carotid artery

Increases in cyclic nucleotide levels induce smooth muscle relaxation by deactivation [reductions in myosin regulatory light chain (MRLC) phosphorylation (e.g., by reduced [Ca(2+)])] or force suppression (reduction in force without reduction in MRLC phosphorylation). Ser(16)-heat shock protein 20 (HSP20) phosphorylation is the proposed mediator of force suppression. We evaluated three potential hypotheses whereby Ser(16)-HSP20 phosphorylation could regulate smooth muscle force: 1) a threshold level of HSP20 phosphorylation could inactivate a thin filament as a whole, 2) phosphorylation of a single HSP20 could fully inactivate a small region of a thin filament, or 3) HSP20 phosphorylation could weakly inhibit myosin binding at either the thin- or thick-filament level. We tested these hypotheses by analyzing the dependence of force on Ser(16)-HSP20 phosphorylation in swine carotid media. First, we determined that swine HSP20 has a second phosphorylation site at Ser(157). Ser(157)-HSP20 phosphorylation values were high and did not change during contractile activation or forskolin-induced relaxation. Forskolin significantly increased Ser(16)-HSP20 phosphorylation. The relationship between Ser(16)-HSP20 phosphorylation and force remained linear and was shifted downward in partially activated muscles relaxed with forskolin. Neither forskolin nor nitroglycerin induced actin depolymerization as detected using the F/G-actin ratio method in smooth muscle homogenates. These results suggest that force suppression does not occur in accordance with the first hypothesis (inactivation of a thin filament as a whole). Our data are more consistent with the second and third hypotheses that force suppression is mediated by full or partial inhibition of local myosin binding at the thin- or thick-filament level.

Phosphodiesterase type 5 as a pharmacologic target in erectile dysfunction

The scientific rationale of pharmacologically inhibiting phosphodiesterase type 5 (PDE5) in the treatment of erectile dysfunction (ED) is reviewed. Published literature on the nitric oxide-cyclic guanosine monophosphate (cGMP) pathway for penile erection and on PDE5 inhibition using sildenafil as the model for pharmacologic PDE5 inhibition are assessed. The key second messenger in the mediation of penile erection is cGMP. PDE5 is the predominant PDE in the corpus cavernosum, and cGMP is its primary substrate. Therefore, in men with ED, elevation of cGMP in corpus cavernosal tissue via selective inhibition of cGMP-specific PDE5 is a means of improving erectile function at minimal risk of adverse events. This approach is validated by the clinical efficacy and safety of sildenafil, the pioneering drug for selective PDE5 inhibitor therapy for ED. Sildenafil exhibits inhibitory potency against PDE5 and a 10-fold lower dose-related inhibitory potency against rod outer segment PDE6, the predominant PDE in the phototransduction cascade in rods. Thus, its pharmacologic profile is predictable, with close correlation between pharmacodynamic and pharmacokinetic properties. Clinically, sildenafil improves erectile function in a large percentage of men with ED. The most common adverse events are due to PDE5 inhibition in vascular and visceral smooth muscle; similar adverse events are expected during therapeutic use of all PDE5 inhibitors. As free sildenafil plasma concentrations approach concentrations sufficient to inhibit retinal PDE6, usually at higher therapeutic doses, transient, reversible visual adverse events can occur, albeit infrequently. Selective inhibition of PDE5 is a rational therapeutic approach in ED, as proved by the clinical success of sildenafil.

Activation of myosin light chain phosphatase in intact arterial smooth muscle during nitric oxide-induced relaxation

We investigated whether myosin light chain phosphatase activity changes during nitric oxide-induced relaxation of contracted intact carotid media and how changes in phosphatase activity mediate this relaxation. We also investigated one mechanism for regulating this phosphatase. Myosin phosphatase activity, myosin light chain phosphorylation, guanosine 3',5'-cyclic monophosphate (cGMP) concentration, and phosphorylation of the inhibitory protein CPI-17 were all assayed in homogenates of one carotid media ring at each time point during nitric oxide-induced relaxation. The application of sodium nitroprusside to histamine-contracted media caused rapid declines in light chain phosphorylation and force. These were temporally correlated with a rapid elevation of cGMP and a large transient increase in myosin phosphatase activity. During the early response to nitroprusside, when force declined, increases in myosin phosphatase activity, concurrent with cGMP-mediated decreases in calcium and myosin light chain kinase activity, could accelerate light chain dephosphorylation. CPI-17 was dephosphorylated upon application of nitroprusside at the same time that myosin phosphatase activity increased, suggesting that the removal of inhibition by phospho-CPI-17 contributed to the increase in myosin phosphatase activity. After 20 min of nitroprusside, myosin phosphatase activity had declined to basal levels, however low force was sustained. Additional light chain phosphorylation-independent mechanisms may be involved in sustaining the relaxation.

Selected contribution: HSP20 phosphorylation in nitroglycerin- and forskolin-induced sustained reductions in swine carotid media tone

Cyclic nucleotide-induced relaxation of maximally activated arterial smooth muscle has two phases. 1) The initial relaxation transient is typically characterized by a rapid reduction in force associated with brief reductions in myoplasmic Ca(2+) concentration ([Ca(2+)](i)) and myosin regulatory light chain (MRLC) phosphorylation on serine (Ser)-19 (Ser(19)). 2) The sustained inhibitory response is typically associated with Ser(16) phosphorylation of heat shock protein 20 (HSP20) without sustained reductions in [Ca(2+)](i) or MRLC phosphorylation. We investigated whether the extent of Ser(16)-HSP20 phosphorylation quantitatively correlated with the sustained inhibitory response. With addition of nitroglycerin to histamine-stimulated swine carotid media, the initial relaxation transient was associated with a decrease in MRLC phosphorylation without an increase in Ser(16)-HSP20 phosphorylation. During the sustained phase of nitroglycerin-induced relaxation and during force redevelopment induced by washout of nitroglycerin in the continued presence of histamine, the level of Ser(16)-HSP20 phosphorylation, but not MRLC phosphorylation, correlated with inhibition of force. Forskolin, which increases cAMP concentration, also induced a sustained inhibitory response that was associated with increases in Ser(16)-HSP20 phosphorylation without reductions in MRLC phosphorylation levels. Forskolin increased Ser(16)-HSP20 phosphorylation to a greater extent and inhibited force more completely than that observed with nitroglycerin. Increases in Ser(16)-HSP20 phosphorylation correlated with the degree of force inhibition regardless of whether the relaxation was induced by nitroglycerin or forskolin. These data are consistent with the hypothesis that Ser(16)-HSP20 phosphorylation may be a cyclic nucleotide-dependent, yet MRLC phosphorylation-independent, inhibitor of smooth muscle contractile force.

Localization of heat shock protein 20 in swine carotid artery

BACKGROUND

Cyclic nucleotides can relax vascular smooth muscle by mechanisms distal to myosin regulatory light chain (MRLC) phosphorylation. This mechanism, termed relaxation without MRLC dephosphorylation, may be regulated by ser16 phosphorylation of heat shock protein 20 (HSP20).

RESULTS

Confocal imaging of HSP20 in smooth muscle tissues revealed that HSP20 was present throughout the cytoplasm, although some focal regions of the cytoplasm were found to contain more HSP20 than the remaining cytoplasm. The distribution of HSP20 within the cytoplasm was not altered by histamine, forskolin, or nitroglycerin.

CONCLUSION

Cytoplasmic localization of HSP20 is consistent with a potential function of HSP20 as a regulator of smooth muscle contractile force.

Inhibition of capacitative calcium entry is not obligatory for relaxation of the mouse anococcygeus by the NO/cyclic GMP signalling pathway

1. The object of this study was to determine whether inhibition of capacitative calcium entry is essential for relaxation of the mouse anococcygeus via the NO/cyclic GMP signalling pathway. 2. In intact muscles, thapsigargin (Tg; 100 nM)-induced tone was relaxed by NO, sodium nitroprusside (SNP), 8-Br-cyclic GMP, and nitrergic field stimulation. The relaxations were similar in magnitude to those observed against carbachol (50 microM) tone and, with the exception of those to 8-Br-cyclic GMP, were reduced by the soluble guanylyl cyclase inhibitor 1H-[1,2,4]oxodiazolo[4,3-a]quinoxalin-1-one (ODQ, 5 microM). 3. In single smooth muscle cells, loaded with Fura-2, both carbachol and Tg produced sustained elevations in cytoplasmic calcium levels ([Ca2+]i). SNP inhibited the rise in [Ca2+]i produced by carbachol, an effect attenuated by ODQ. In contrast, neither SNP nor 8-Br-cyclic GMP reduced the elevated [Ca2+]i associated with Tg. 4. In beta-escin skinned preparations, NO had no effect on tone induced by calcium (1 microM in the presence of 100 microM GTP). Carbachol and Tg produced further increases in calcium/GTP-induced tone and, in both cases, this additional tone was relaxed by NO and 8-Br-cyclic GMP. 5. The results support the hypothesis that the NO/cyclic GMP pathway inhibits capacitative calcium entry by refilling the internal stores, since reduction in [Ca2+]i was not observed in the presence of Tg. However, as muscle relaxation was still observed, impairment of capacitative calcium entry cannot be considered obligatory for relaxation. Results from skinned tissues suggest that inhibition of calcium sensitization processes, perhaps associated with store-depletion, may be an important mechanism of NO/cyclic GMP-induced relaxation.

Caldesmon and heat shock protein 20 phosphorylation in nitroglycerin- and magnesium-induced relaxation of swine carotid artery

Nitrovasodilators, high extracellular Mg(2+), and some other relaxing agents can cause smooth muscle relaxation without reductions in myosin regulatory light chain (MRLC) phosphorylation. Relaxations without MRLC dephosphorylation suggest that other regulatory systems, beyond MRLC phosphorylation, are present in smooth muscle. We tested whether changes in caldesmon phosphorylation, heat shock protein 20 (HSP20) phosphorylation, or intracellular pH (pH(i)) could be responsible for relaxation without MRLC dephosphorylation. In unstimulated tissues, caldesmon was phosphorylated 1.02+/-0.10 mol P(i)/mol caldesmon (mean+/-1 S.E.M.), HSP20 was phosphorylated 0.005+/-0.003 mol P(i)/mol HSP20, and estimated pH(i) was 7.21+/-0.07. Histamine stimulation induced a contraction, an intracellular acidosis, but did not significantly change caldesmon or HSP20 phosphorylation. Addition of nitroglycerin induced a relaxation, significantly increased HSP20 phosphorylation to 0.18+/-0.02 mol P(i)/mol HSP20, did not significantly change caldesmon phosphorylation, and pH(i) returned to near unstimulated values. Increase in extracellular Mg(2+) to 10 mM induced a relaxation, but did not significantly change HSP20 or caldesmon phosphorylation. These data suggest that changes in caldesmon phosphorylation, HSP20 phosphorylation, or pH(i) cannot be the sole explanation for relaxation without MRLC dephosphorylation. However, it is possible that HSP20 phosphorylation may be involved in nitroglycerin-induced relaxation without MRLC dephosphorylation.

Acute hyperglycemia depresses arteriolar NO formation in skeletal muscle

In the rat intestinal and cerebral microvasculatures, acute D-glucose hyperglycemia suppresses endothelium-dependent dilation to ACh without affecting endothelium-independent dilation to nitroprusside. This study determined whether acute hyperglycemia suppressed arteriolar wall nitric oxide concentration ([NO]) at rest or during ACh stimulation and inhibited nitroprusside-, ACh- or contraction-induced dilation of rat spinotrapezius arterioles. Vascular responses were measured before and after 1 h of topical 300 mg/100 ml D-glucose; arteriolar [NO] was measured with NO-sensitive microelectrodes. Arteriolar dilation to ACh was not significantly altered after superfusion of 300 mg/100 ml D-glucose. However, after hyperglycemia, arteriolar [NO] was not increased by ACh, compared with a 300 nM increase attained during normoglycemia. Arteriolar dilation to submaximal nitroprusside and muscle contractions was enhanced by hyperglycemia. These results indicated that in the rat spinotrapezius muscle, acute hyperglycemia suppressed arteriolar NO production while simultaneously augmenting vascular smooth muscle responsiveness to nitroprusside, presumably through cGMP-mediated mechanisms. In effect, this may have allowed ACh- and muscle contraction-induced vasodilation to be maintained during hyperglycemia despite an impaired NO system.

Molecular mechanisms of the effects of sildenafil (VIAGRA)

The molecular mechanisms of the effects of sildenafil, a specific inhibitor of cyclic guanosine monophosphate (cGMP) phosphodiesterases are briefly reviewed. The second messenger cGMP as well as its molecular targets (with the exception of the photoreceptor signal transduction machinery) have long played an underdog role compared with cyclic adenosine monophosphate and other signalling molecules such as inositoltrisphosphate. The same holds for guanylyl cyclase, which, albeit being the main effector molecule of the gaseous neurotransmitters carbon monoxide and nitric oxide (NO), has received much less attention relative to its activators and their synthases. Stimulation of the arginine --> NO --> cGMP pathway by bypassing NO-synthase is a well-established pharmacological principle in the treatment of cardiovascular disorders. In contrast, local application of NO-donors or oral feeding of excessive amounts of precursor amino acid L-arginine to treat erectile dysfunction were met with variable success or failure. The advent of a new principle, amplification of the NO-signaling cascade by means of target organ selective phosphodiesterase inhibition, has renewed interest in phosphodiesterases and cGMP.

Inhibitory mechanisms for cross-bridge cycling: the nitric oxide-cGMP signal transduction pathway in smooth muscle relaxation

Relaxation follows sequestration of Ca2+ mobilized by an excitatory stimulus in striated muscle. Removal of excitatory stimuli also relaxes smooth muscle in vitro after reductions in the myoplasmic [Ca2+] and dephosphorylation of the myosin regulatory light chains. However, there are several experimental procedures that produce relaxation in the presence of excitatory stimuli and elevated Ca(2+)-dependent cross-bridge phosphorylation. Of potential widespread physiological importance are treatments that increase myoplasmic [cGMP] owing to the ubiquity of nitric oxide (NO) as a signalling molecule for endothelial-mediated vasodilation and inhibitory nerves in most types of smooth muscle. Several mechanisms are implicated in the NO-cGMP mediated relaxation. Most studies support reductions in myoplasmic Ca2+. However, there is evidence that increases in cGMP also lower the Ca(2+)-sensitivity of cross-bridge phosphorylation. This would contribute to a decline in force through actions on the myosin light chain kinase/phosphatase system. In addition, changes in the dependence of force on phosphorylation are observed in tissues partially relaxed by treatments that elevate cGMP. This demonstrates that either the attachment and cycling of phosphorylated cross-bridges is impaired or blocked, or that the formation of dephosphorylated, force-generating cross-bridges ('latch-bridges') is reduced. Protein kinase G-catalysed phosphorylation of either a thin filament protein that blocks attachment of cross-bridges or a protein that inhibits myosin light chain phosphatase may explain the NO-induced relaxation with elevated cross-bridge phosphorylation.