The latch-bridge hypothesis of smooth muscle contraction

In contrast to striated muscle, both normalized force and shortening velocities are regulated functions of cross-bridge phosphorylation in smooth muscle. Physiologically this is manifested as relatively fast rates of contraction associated with transiently high levels of cross-bridge phosphorylation. In sustained contractions, Ca2+, cross-bridge phosphorylation, and ATP consumption rates fall, a phenomenon termed "latch". This review focuses on the Hai and Murphy (1988a) model that predicted the highly non-linear dependence of force on phosphorylation and a directly proportional dependence of shortening velocity on phosphorylation. This model hypothesized that (i) cross-bridge phosphorylation was obligatory for cross-bridge attachment, but also that (ii) dephosphorylation of an attached cross-bridge reduced its detachment rate. The resulting variety of cross-bridge cycles as predicted by the model could explain the observed dependencies of force and velocity on cross-bridge phosphorylation. New evidence supports modifications for more general applicability. First, myosin light chain phosphatase activity is regulated. Activation of myosin phosphatase is best demonstrated with inhibitory regulatory mechanisms acting via nitric oxide. The second modification of the model incorporates cooperativity in cross-bridge attachment to predict improved data on the dependence of force on phosphorylation. The molecular basis for cooperativity is unknown, but may involve thin filament proteins absent in striated muscle.

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.

HSP20 phosphorylation and interstitial metabolites in hypoxia-induced dilation of swine coronary arteries

OBJECTIVE

Hypoxia induces coronary artery dilation, but the responsible mechanism is largely unknown. Many stimuli induce arterial smooth muscle relaxation by reducing ser19-myosin regulatory light chain (MLC) phosphorylation. Other stimuli can induce smooth muscle relaxation without reductions in ser19-MLC phosphorylation. This form of relaxation has been termed force suppression and appears to be associated with heat shock protein 20 (HSP20) phosphorylation on ser16. We investigated whether hypoxia-induced sustained dilation in swine coronary arteries was promoted without ser19-MLC dephosphorylation and associated with ser16-HSP20 phosphorylation. Nitroglycerin vasodilation served as control.

METHODS

In a pressure myograph, the tunica media of intact pre-contracted (PGF(2alpha); 10(-5) m) porcine coronary artery segments were cannulated using a microdialysis catheter. Diameter responses and interstitial lactate/pyruvate ratios were studied during 90 min hypoxia, hypoxia + reoxygenation (60 min), nitroglycerin (100 microm, 90 min), and nitroglycerin + wash-out (60 min). The arterial segments were snap-frozen and analysed for ser16-HSP20 phosphorylation and ser19-MLC phosphorylation.

RESULTS

The normalized diameter responses to hypoxia (6.1 +/- 4.3%) and nitroglycerin (12.6 +/- 1.6%) were both significantly greater than normoxic control arteries (-10.5 +/- 1.8%, anova, P < 0.05). Ser16-HSP20 phosphorylation was increased with hypoxia and nitroglycerin treatment and ser16-HSP20 phosphorylation correlated with changes in diameters (n = 29, r2 = 0.64, P < 0.001). Ser19-MLC phosphorylation was not significantly altered by hypoxia. The lactate/pyruvate ratio was significantly increased in hypoxic arteries but did not correlate with diameters or ser16-HSP20 phosphorylation.

CONCLUSION

Ser16-HSP20 phosphorylation is a potential regulator of hypoxia-induced dilation in coronary arteries.

Serine 68 phospholemman phosphorylation during forskolin-induced swine carotid artery relaxation

BACKGROUND

Phospholemman (PLM) is an abundant phosphoprotein in the plasma membrane of cardiac, skeletal and smooth muscle. It is a member of the FXYD family of proteins that bind to and regulate the Na,K-ATPase. Protein kinase A (PKA) is known to phosphorylate PLM on serine 68 (S68), although the functional effect of S68 PLM phosphorylation is unclear. We therefore evaluated S68 PLM phosphorylation in swine carotid arteries.

METHODS

Two anti-PLM antibodies, one to S68 phosphorylated PLM and one to unphosphorylated PLM, were made to PLM peptides in rabbits and tested with purified PLM and PKA-treated PLM. Swine carotid arteries were mounted isometrically, contracted, relaxed with forskolin and then homogenized. Proteins were separated on SDS gels and the intensity of immunoreactivity to the two PLM antibodies determined on immunoblots.

RESULTS

The antipeptide antibody 'C2' primarily reacted with unphosphorylated PLM, and the antipeptide antibody 'CP68' detected S68 PLM phosphorylation. Histamine stimulation of intact swine carotid artery induced a contraction, increased the CP68 PLM antibody signal and reduced the C2 PLM antibody signal. High extracellular [K(+)] depolarization induced a contraction without altering the C2 or CP68 PLM signal. Forskolin-induced relaxation of histamine or extracellular [K(+)] contracted arteries correlated with an increased CP68 signal. Nitroglycerin-induced relaxation was not associated with changes in the C2 or CP68 PLM signal.

CONCLUSIONS

These data suggest that a contractile agonist increased S68 PLM phosphorylation. Agents that increase [cAMP], but not agents that increase [cGMP], increased S68 PLM phosphorylation. S68 PLM phosphorylation may be involved in cAMP-dependent regulation of smooth muscle force.

Children with obstructive sleep-disordered breathing generate high-frequency inspiratory sounds during sleep

STUDY OBJECTIVES

We observed that some children with adenotonsillar hypertrophy and obstructive sleep-disordered breathing (SDB) make high-frequency inspiratory sounds (HFIS) during sleep. Our objective was to determine whether HFIS occur in most children with obstructive SDB and adenotonsillar hypertrophy and whether adenotonsillectomy reduces HFIS.

DESIGN

Prospective consecutive-entry trial.

SETTING

Sleep laboratory.

PARTICIPANTS

Twenty-six children between 6 and 12 years of age with adenotonsillar hypertrophy suspected of having obstructive SDB.

MEASUREMENTS AND RESULTS

We performed polysomnography and measured sounds during sleep with a microphone suspended above the bed. Sounds were recorded on a computer at 44 kHz, analyzed with fast Fourier transformation for frequency content. HFIS were sounds occurring during an inspiration with frequencies greater than 2 kHz. HFIS were different from the low-frequency (< 2 kHz) sounds described in snoring adults. HFIS usually occurred in consecutive breaths, occasionally exceeding 100. We counted the number of HFIS that occurred per hour of sleep. Children who made more HFIS had more obstructive SDB than did those who did not make the HFIS, and there was a significant positive correlation between the number of HFIS and the obstructive apnea-hypopnea index. Children with more than 3 apneas and hypopneas per hour of sleep all made at least 10 HFIS per hour, and all children who had more than 10 HFIS per hour had obstructive apnea-hypopnea index values greater than 1. Children with adenotonsillar hypertrophy made more HFIS than did those children whose tonsils and adenoids had been removed.

CONCLUSIONS

HFIS may be a marker of disturbed breathing during sleep in children with adenotonsillar hypertrophy.

An upper airway resonator model of high-frequency inspiratory sounds in children with sleep-disordered breathing

The goal of this study was to determine how high-frequency inspiratory sounds (HFIS) are generated by sleeping children with obstructive sleep-disordered breathing (OSDB). We hypothesized that HFIS are generated when a high-velocity jet of air, generated by a narrowed upper airway, induces the upper airway to act as a resonating chamber. We tested two predictions of this hypothesis: 1) the upper airway is narrowed in children who make HFIS and 2) the length of the upper airway, calculated from HFIS harmonic intervals, is similar to that calculated from magnetic resonance imaging (MRI) scans. The study was conducted in the setting of a sleep laboratory. Participants included 29 children between 6 and 12 yr of age with adenotonsillar hypertrophy suspected of having OSDB. Minimum cross-sectional airway area and airway long dimensions (lips to larynx or soft palate) were measured in awake children with MRIs. Later that night, sound was recorded with a microphone suspended above their bed while the children underwent polysomnography. Sounds were later analyzed with fast Fourier transforms. We found that sleeping children who generated HFIS had significantly narrower upper airways compared with children who did not make HFIS [minimum airway area 20.5 +/- 4.4 vs. 70.9 +/- 22.5 mm(2) (mean +/- SE), respectively; P = 0.02]. There was a significant inverse correlation between the log(10) of the narrowest airway area and the number of HFIS recorded per hour (r(2) = 0.55, P < 0.00001). The harmonics characteristics of HFIS predicted that they were generated by sound resonating in chamber whose length was 12.0 +/- 0.9 cm, which is similar to the MRI measured distance from the lips to the larynx of 12.8 +/- 0.4 cm. In conclusion, these data suggest that children generate HFIS when 1) they have a narrowed upper airway and 2) their upper airway acts as a resonating chamber.

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.

Combination therapy of dyslipidemia in non-insulin-dependent diabetes mellitus and the metabolic syndrome

Non-insulin-dependent diabetes mellitus (NIDDM) and the metabolic syndrome separately and additively increase the risk for atherosclerotic cardiovascular disease. Considering the high cardiovascular risk associated with NIDDM and the metabolic syndrome, aggressive therapy of dyslipidemia with tailored combination therapy should be considered given informed consent and discussion of risks. In addition to statins, niacin, and fibrates, therapies shown to decrease the risk for atherosclerotic cardiovascular disease include omega-3 fatty acids, diet, exercise, and optimal blood pressure control with thiazides and blockers of the renin-angiotensin system. These therapies should also be considered to reduce the high cardiovascular risk associated with NIDDM and the metabolic syndrome.

Cooperative attachment of cross bridges predicts regulation of smooth muscle force by myosin phosphorylation

Serine 19 phosphorylation of the myosin regulatory light chain (MRLC) appears to be the primary determinant of smooth muscle force development. The relationship between MRLC phosphorylation and force is nonlinear, showing that phosphorylation is not a simple switch regulating the number of cycling cross bridges. We reexamined the MRLC phosphorylation-force relationship in slow, tonic swine carotid media; fast, phasic rabbit urinary bladder detrusor; and very fast, tonic rat anococcygeus. We found a sigmoidal dependence of force on MRLC phosphorylation in all three tissues with a threshold for force development of approximately 0.15 mol P(i)/mol MRLC. This behavior suggests that force is regulated in a highly cooperative manner. We then determined whether a model that employs both the latch-bridge hypothesis and cooperative activation could reproduce the relationship between Ser(19)-MRLC phosphorylation and force without the need for a second regulatory system. We based this model on skeletal muscle in which attached cross bridges cooperatively activate thin filaments to facilitate cross-bridge attachment. We found that such a model describes both the steady-state and time-course relationship between Ser(19)-MRLC phosphorylation and force. The model required both cooperative activation and latch-bridge formation to predict force. The best fit of the model occurred when binding of a cross bridge cooperatively activated seven myosin binding sites on the thin filament. This result suggests cooperative mechanisms analogous to skeletal muscle that will require testing.

Heat induced HSP20 phosphorylation without increased cyclic nucleotide levels in swine carotid media

BACKGROUND

Heat pretreatment of swine carotid artery has been shown to increase ser16-heat shock protein 20 (HSP20) phosphorylation and suppress force, i.e., reduce force with only minimal reduction in ser19-myosin regulatory light chain (MRLC) phosphorylation.

RESULTS

We further investigated this response in intact histamine stimulated swine carotid artery rings. There was a heat threshold such that increased ser16-HSP20 phosphorylation and force suppression were observed between 43 degrees C and 46 degrees C. The increased ser16-HSP20 phosphorylation persisted up to 16 hours after 44.5 degrees C heat treatment. Pretreatment of swine carotid media at 44.5 degrees C increased ser16-HSP20 phosphorylation without increases in [cAMP] or [cGMP], suggesting an alternate mechanism, perhaps phosphatase inhibition, for the increase in ser16-HSP20 phosphorylation. Heat pretreatment at 47.5 degrees C reduced force by decreasing MRLC phosphorylation rather than by large increases in ser16-HSP20 phosphorylation. HSP20 phosphorylation at the putative PKC site did not change with any treatment.

CONCLUSION

These results demonstrate that multiple mechanisms can induce force suppression that is correlated with ser16-HSP20 phosphorylation: 1) nitrovasodilators via cGMP, 2) forskolin via cAMP, and 2) thermal stress in a cyclic nucleotide independent manner.

Oral L-arginine can reverse digital necrosis in Raynaud's phenomenon

Raynaud's phenomenon is characterized by transient reduction in blood supply through the small arteries in the hands and feet. Severe Raynaud's phenomenon can cause digital necrosis. It has been hypothesized that nitric oxide may have a role in Raynaud's phenomenon. We report two cases in which oral L-arginine reversed digital necrosis in Raynaud's phenomenon and two additional cases in which the symptoms of severe Raynaud's phenomenon were improved with oral L-arginine. These reports suggest that a defect in nitric oxide synthesis or metabolism is present in Raynaud's phenomenon. They also suggest a potential role for oral L-arginine therapy in Raynaud's phenomenon, especially in Raynaud's phenomenon with digital necrosis.

Heat-induced force suppression and HSP20 phosphorylation in swine carotid media

In vascular smooth muscle, cyclic nucleotide-dependent phosphorylation of heat shock protein 20 (HSP20) on serine-16 (Ser16) has been suggested to cause force suppression, i.e., reduced force with only minimal myosin regulatory light chain (MRLC) dephosphorylation. We hypothesized that heat pretreatment also suppresses force by increasing HSP20 phosphorylation. After heat pretreatment of swine carotid artery at 44.5 degrees C for 4 h and reduction to 37 degrees C for 1 h, Ser16-HSP20 phosphorylation was increased and histamine-induced increases in contractile force were suppressed. Subsequent addition of nitroglycerin induced additive force suppression. Heat and nitroglycerin induced a similar relation between Ser16-HSP20 phosphorylation and force. Heat pretreatment induced a small, but significant, increase in total HSP20 immunostaining. These results demonstrate that vascular smooth muscle responds to thermal stress by increasing Ser16-HSP20 phosphorylation in addition to a possible small increase in total HSP20 concentration. The resulting heat-induced reduction in force should be considered "force suppression" because histamine-induced increases in MRLC phosphorylation were not significantly altered by heat pretreatment. These processes may bring about a resistance to contractile agonists, which could have clinical significance in conditions such as hyperthermia and/or sepsis with vasodilatory shock.

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.

cGMP-mediated phosphorylation of heat shock protein 20 may cause smooth muscle relaxation without myosin light chain dephosphorylation in swine carotid artery

Nitrovasodilators such as nitroglycerine, via production of nitric oxide and an increase in [cGMP], can induce arterial smooth muscle relaxation without proportional reduction in myosin light chain (MLC) phosphorylation or myoplasmic [Ca2+]. These findings suggest that regulatory systems, other than MLC phosphorylation and Ca2+, partially mediate nitroglycerine-induced relaxation. In swine carotid artery, we found that a membrane-permeant cGMP analogue induced relaxation without MLC dephosphorylation, suggesting that cGMP mediated the relaxation. Nitroglycerine-induced relaxation was associated with a reduction in O2 consumption, suggesting that the interaction between phosphorylated myosin and the thin filament was inhibited. Nitroglycerine-induced relaxation was associated with a 10-fold increase in the phosphorylation of a protein on Ser16. We identified this protein as heat shock protein 20 (HSP20), a member of a family of proteins known to bind to thin filaments. When homogenates of nitroglycerine-relaxed tissues were centrifuged at 6000 g, phosphorylated HSP20 preferentially sedimented in the pellet, suggesting that phosphorylation of HSP20 may increase its affinity for the thin filament. We noted that a domain of HSP20 is partially homologous to the 'minimum inhibitory sequence' of skeletal troponin I. The peptide HSP20110-121, which contains this domain, bound to actin-containing filaments only in the presence of tropomyosin, a characteristic of troponin I. High concentrations of HSP20110-121 abolished Ca2+-activated force in skinned swine carotid artery. HSP20110-121 also partially decreased actin-activated myosin S1 ATPase activity. These data suggest that cGMP-mediated phosphorylation of HSP20 on Ser16 may have a role in smooth muscle relaxation without MLC dephosphorylation. HSP20 contains an actin-binding sequence at amino acid residues 110-121 that inhibited force production in skinned carotid artery. We hypothesize that phosphorylation of HSP20 regulates force independent of MLC phosphorylation via binding of HSP20 to thin filaments and inhibition of cross-bridge cycling.

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.

The buffer barrier hypothesis, [Ca2+]i homogeneity, and sarcoplasmic reticulum function in swine carotid artery

1. The goal of this study was to evaluate the buffer barrier hypothesis in an intact arterial smooth muscle. Specifically, we investigated the interrelationships between intracellular [Ca2+] ([Ca2+]i) homogeneity and sarcoplasmic reticulum function in swine carotid artery. 2. We measured focal changes in [Ca2+]i by exploiting the different characteristics of several [Ca2+]i indicators: (1) aequorin, which can detect focal increases in [Ca2+]i such as those that occur in the subplasmalemmal region ([Ca2+]pm); (2) fura-2, which is primarily a measure of mean cytoplasmic [Ca2+] ([Ca2+]c); and (3) force, which reflects increases in [Ca2+] near the contractile apparatus. We then estimated the relative degree of [Ca2+]i homogeneity with the aequorin/fura-2 ratio. Finally, we inhibited sarcoplasmic reticulum Ca2+ pumping with cyclopiazonic acid (CPA), an inhibitor of the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA). 3. We found that, after Ca2+ depletion, the sarcoplasmic reticulum could be partially reloaded with Ca2+ by manipulations that increased the aequorin signal relatively more than the fura-2 signal. Complete reloading required large increases in the fura-2 signal. These data suggest that increases in [Ca2+]pm (as measured with aequorin) can partially reload the sarcoplasmic reticulum, but complete reloading required increases in [Ca2+]c (as measured with fura-2). Reloading could be partially inhibited by 10 microM CPA, indicating that SERCA function was important for reloading. 4. In unstimulated arteries, 10 microM CPA increased the fura-2 signal without altering the aequorin signal, thereby decreasing the aequorin/fura-2 ratio. Removal of extracellular Ca2+ without CPA also reduced the aequorin/fura-2 ratio. These data suggest that resting cells have a [Ca2+] gradient with [Ca2+]pm > [Ca2+]c; this gradient is maintained by SERCA function. 5. CPA slowed the decline in the fura-2 signal observed when histamine stimulation was removed. This result is consistent with the concept of vectorial Ca2+ efflux in which Ca2+ pumping by SERCA reduces [Ca2+]c after stimulation. 6. Ca2+ depletion by prior treatment with 100 microM histamine and CPA transiently attenuated subsequent histamine-induced aequorin and fura-2 transients. The effect on contraction was smaller: a delay in contraction of approximately 10 s. These data suggest that histamine-induced Ca2+ release has at least a small role in the initial phase of contraction; however, other contractile mechanisms appear to be able to compensate for loss of Ca2+ release with only modest changes in contraction kinetics. 7. These data suggest that there is a complex interrelationship between smooth muscle sarcoplasmic reticulum function and [Ca2+] in at least two cytoplasmic compartments. [Ca2+]pm and [Ca2+]c can differentially regulate sarcoplasmic reticulum Ca2+ filling; and sarcoplasmic reticulum function regulates [Ca2+]pm and [Ca2+]c.

Number needed to screen: development of a statistic for disease screening

OBJECTIVES

To develop the number needed to screen, a new statistic to overcome inappropriate national strategies for disease screening. Number needed to screen is defined as the number of people that need to be screened for a given duration to prevent one death or adverse event.

DESIGN

Number needed to screen was calculated from clinical trials that directly measured the effect of a screening strategy. From clinical trials that measured treatment benefit, the number needed to screen was estimated as the number needed to treat from the trial divided by the prevalence of heretofore unrecognised or untreated disease. Directly calculated values were then compared with estimate number needed to screen values.

SUBJECTS

Standard literature review.

RESULTS

For prevention of total mortality the most effective screening test was a lipid profile. The estimated number needed to screen for dyslipidaemia (low density lipoprotein cholesterol concentration >4.14 mmol/1) was 418 if detection was followed by pravastatin treatment for 5 years. This indicates that one death in 5 years could be prevented by screening 418 people. The estimated number needed to screen for hypertension was between 274 and 1307 for 5 years (for 10 mm Hg and 6 mm Hg diastolic blood pressure reduction respectively) if detection was followed by treatment based on a diuretic. Screening with haemoccult testing and mammography significantly decreased cancer specific, but not total, mortality. The number needed to screen for haemoccult screening to prevent a death from colon cancer was 1374 for 5 years, and the number needed to screen for mammography to prevent a death from breast cancer was 2451 for 5 years for women aged 50-59.

CONCLUSION

These data allow the clinician to prioritise screening strategies. Of the screening strategies evaluated, screening for, and treatment of, dyslipidaemia and hypertension seem to produce the largest clinical benefit.

Imaging bioluminescent indicators shows Ca2+ and ATP permeability thresholds in live cells attacked by complement

A series of permeability thresholds to Ca2+ metabolites and macromolecules, occurring at different times when cells are attacked by complement, has been established by imaging HeLa cells transiently expressing a recombinant cytosolic fusion protein of firefly luciferase and aequorin (luciferase-aequorin) to measure changes in ATP and cytosolic free Ca2+. Nuclear fluorescence of propidium was used as a measure of permeability to small molecules, and luciferase activity imaged to assess lysis. The rise in cytosolic free Ca2+ observed after C9 attack preceded by at least 60 s both the increase in propidium fluorescence, measured in single cells, and the decrease in ATP monitored by luciferase light emission. These effects were dependent on the concentration of C9. At concentrations of C9 up to 4 micrograms/ml no loss of luciferase-aequorin protein was detected at the end of the experiment. Thus the membrane integrity of the cells remained intact, even though the cells were permeable to propidium. These results confirmed our earlier observations that propidium permeability in cells attacked by complement was not a reliable measure of cell death. They also show that it is vital to take account of cellular heterogeneity if the mechanisms by which cells respond to membrane pore former attack are to be correctly interpreted.