Leaflet Angle Research Articles

Invited Commentary

It is generally recognized that mitral regurgitation (MR) improves after aortic valve replacement for aortic stenosis. In this important study by Waisbren and colleagues [1Waisbren EC, Stevens L-M, Avery EG, Picard MH, Vlahakes GJ, Agnihotri AK. Changes in mitral regurgitation after replacement of the stenotic aortic valve. Ann Thorac Surg 2008;86:56–63.Google Scholar], the management of mild to moderate MR in patients with severe aortic stenosis is clarified. Intraoperative transesophageal echocardiography (TEE) is known to underestimate MR. By including preoperative and late postoperative transthoracic echocardiography (TTE) this report provides much needed insight into the perioperative relationships between TTE and TEE. Although the initial postoperative MR improvement was impressive, this improvement diminished during the mean 2-year follow-up. When compared with preoperative TTE, late TTE studies were largely unchanged. Importantly, none of the patients progressed to severe MR. Vena contracta width (VCW) was used as the sole measure of MR grade. The VCW is a direct anatomic measurement and is therefore less likely to be influenced by varying inotropic and loading conditions as compared to jet area, proximal iso-velocity surface area (PISA) or similar Doppler measurements. Given that this is a retrospective study, selection of VCW as the single measurement was completely appropriate. However, contemporary evaluation continues to improve and is worthy of mention. Three-dimensional echocardiography has resulted in important MR quantification advancements. Vena contracta area (VCA) and planimetry from three-dimensional echocardiography are also direct anatomic measurements. Like VCW, they are load-independent. However unlike VCW, which assumes a circular or elliptical regurgitant orifice, VCA and planimetry directly measure the regurgitant orifice surface area and thereby substantially improve accuracy. In practice, an integrated approach including VCW, various Doppler measurements, VCA, and planimetry is considered optimal. Additional direct anatomic measurements associated with myopathic or “functional” MR have also been demonstrated to be useful. These include coaptive characteristics, tenting area, and closing angles, all of which can be predictive of late postoperative mitral function. Details of the echocardiographic analysis are delineated in an excellent article from Quebec, Canada, by Magne and colleagues [2Magne J. Pibarot P. Dagenais F. Hachicha Z. Dumesnil J.G. Sénéchal M. Preoperative posterior leaflet angle accurately predicts outcome after restrictive mitral valve annuloplasty for ishemic mitral regurgitation.Circulation. 2007; 115: 782-791Crossref PubMed Scopus (221) Google Scholar]. We consider these direct anatomic measurements to be mandatory prior to mitral repair. In summary, the authors have addressed an important management dilemma. A subgroup analysis, which includes TTE data, gives important insight into intermediate-term mitral function. The limitations of intraoperative TEE are acknowledged and appropriately addressed. This study supports their conclusion that a conservative, selective approach to mitral intervention is appropriate in patients presenting for correction of aortic stenosis with concomitant mild to moderate mitral regurgitation. The authors are to be congratulated on their excellent work. Changes in Mitral Regurgitation After Replacement of the Stenotic Aortic ValveThe Annals of Thoracic SurgeryVol. 86Issue 1PreviewConcomitant mitral regurgitation (MR) is frequently seen in patients undergoing aortic valve replacement (AVR) for aortic stenosis. This study was undertaken to characterize the magnitude of MR in these patients and identify factors associated with significant postoperative change. Full-Text PDF

A stereo imaging system for measuring structural parameters of plant canopies

Plants constantly adapt their leaf orientation in response to fluctuations in the environment, to maintain radiation use efficiency in the face of varying intensity and incidence direction of sunlight. Various methods exist for measuring structural canopy parameters such as leaf angle distribution. However, direct methods tend to be labour-intensive, while indirect methods usually give statistical information on stand level rather than on individual leaves. We present an area-based, binocular stereo system composed of commercially available components that allows three-dimensional reconstruction of small- to medium-sized canopies on the level of single leaves under field conditions. Spatial orientation of single leaves is computed with automated processes using modern, well-established stereo matching and segmentation techniques, which were adapted for the properties of plant canopies, providing high spatial and temporal resolution (angle measurements with an accuracy of approx. +/-5 degrees and a maximum sampling rate of three frames per second). The applicability of our approach is demonstrated in three case studies: (1) the dihedral leaflet angle of an individual soybean was tracked to monitor nocturnal and daytime leaf movement showing different frequencies and amplitudes; (2) drought stress was diagnosed in soybean by quantifying changes in the zenith leaflet angle distribution; and (3) the diurnal course of the zenith leaf angle distribution of a closed soybean canopy was measured.

Preoperative Posterior Leaflet Angle Accurately Predicts Outcome After Restrictive Mitral Valve Annuloplasty for Ischemic Mitral Regurgitation

Ischemic mitral regurgitation (MR) often persists after restrictive mitral valve annuloplasty, in which case it is associated with worse clinical outcomes. The goal of the present study was to determine whether persistence of MR and/or clinical outcome could be predicted from preoperative analysis of mitral valve configuration. In 51 consecutive patients undergoing restrictive annuloplasty for ischemic MR, posterior leaflet (PL) angle, anterior leaflet angle, coaptation distance, and tenting area were quantified by echocardiography before surgery (6+/-3 days), and MR severity was assessed before and early after surgery (9+/-4 days). Postoperatively, persistence of mild to moderate MR (vena contracta > 3 mm) was observed in 11 (22%) of the patients. The best predictor of postoperative persistence of MR was a PL angle > or = 45 degrees (sensitivity 100%, specificity 97%, positive predictive value 92%, negative predictive value 100%). Patients with persistent MR had markedly lower 3-year event-free survival (26+/-20%) compared with those with nonpersistent MR (75+/-12%, P=0.01). Preoperative presence of a PL angle > or = 45 degrees also was associated with a markedly lower 3-year event-free survival (22+/-17% versus 76+/-12%; P<0.001). In patients undergoing restrictive annuloplasty for ischemic MR, persistence of MR and 3-year event-free survival can accurately be predicted by preoperative analysis of mitral valve configuration. Patients with a PL angle > or = 45 degrees (ie, with high PL restriction) should thus be considered poor candidates for this procedure, and concomitant or alternative procedures should be contemplated.

Determination of the optimal region for interaliasing distance measurement for flow regurgitant rate calculation: a fluid simulation study

Background Color Doppler imaging of the convergent region is promising for quantifying valvular regurgitation. Nevertheless, proximal isovelocity surface area method has limitations. We sought to determine the optimal localization to measure the most precise flow rate using a new approach: the interaliasing distance. Methods A finite volume-based program was used to perform simulations in unsteady flow conditions. Different instantaneous flow rates, leaflet angles, and orifice sizes were tested reproducing physiologic conditions. Relative difference between actual and interaliasing distance flow rate was calculated for each configuration. Results The relationship between the relative error and the aliasing velocity location was described by a third-order polynomial equation. The magnitude of relative error is a function of the flow rate, orifice size, and leaflet angle. Conclusion The optimal distance from the orifice to measure the interaliasing distance was when the closer aliasing was between 4 and 8 mm from the orifice.

Paraheliotropism in two Phaseolus species: combined effects of photon flux density and pulvinus temperature, and consequences for leaf gas exchange

Phaseolus species exhibit paraheliotropic (light-avoiding) leaf movements in response to environmental stimuli. We investigated the response of two ecologically contrasting species, Phaseolus vulgaris (mesic) and Phaseolus acutifolius (xeric), to various combinations of photosynthetic photon flux density (PFD) and pulvinus temperature to compare the control and consequences, in terms of photosynthetic gas exchange, of paraheliotropism under identical conditions. Leaflet angles were measured in a cuvette that exposed the leaf to controlled PFD and pulvinus temperature. A gas exchange system was used to determine carbon assimilation, stomatal conductance and water use efficiency (WUE) in response to environmental factors and leaf movements. The effect of tilting leaves was compared with an equivalent reduction of PFD for horizontal leaves. Both species showed significant (but different) responses to PFD and temperature, with increased light avoidance at high levels of both factors. P. acutifolius exhibited greater light avoidance under equivalent conditions. The gas exchange consequences of changing leaf angles were similar but not identical to those of an equivalent modification of radiation level. The photosynthetic cost of the leaf movements was negligible, but there was a substantial increase in WUE, especially for P. acutifolius. Thus, the benefits of paraheliotropism are achieved at virtually no cost to the plant.

Effects of soyabean leaflet inclination on some factors related to photosynthesis

Leaflet movement in soyabean (Glycine max (L.) Merr.) is considered to be an important physiological adaptation for intercepting sunlight for photosynthesis. Solar tracking may serve to increase plant production by maximizing light interception. The objective of this study was to determine the response of A (leaflet net assimilation rate), τ (stomatal conductance) and Ci (internal leaf carbon dioxide concentration) when penultimate fully expanded leaflet angles were changed. Soyabean (cv. Enrei, Maturity group VI) was planted at Niigata University, Japan, at two densities (25 and 16 plants/m2) in mid-May of 1992 and 1993. PFD (photon flux density), A, τ, and Ci were measured with a portable leaf photosynthesis instrument. The more towards vertical the leaflets were moved, the lower the PFD and A were. A measured at R5 showed no light saturation, but did show this at V13 and R3. While A measured at 09.00 h showed no saturation point, there was saturation at 12.00 h and 15.00 h at or less than 1500 μmol/m2/s. At R3 a high correlation (r = 0.89, P &lt; 0.01) was found between PFD and A for naturally oriented leaflets, but not for horizontal ones. These results suggest that leaflet movement is an adaptation to optimize A.

Aortic regurgitant flow by color Doppler measurement of the local velocity 7 mm above the leak orifice--Part 1: in vitro measurements.

The flow convergence method enables the determination of flow across restrictive orifices. It was validated for planar orifice plates and atrioventricular valves. However, its quantitative application to aortic regurgitation is complicated due to the complex valve anatomy which distorts the converging flow field. An open angle formed by the leaflets causes relatively lower velocities in the region near the orifice, resulting in underestimation of flow. Confinement of the flow convergence region by the ascending aorta relatively increases the velocities at greater distance to the orifice and can cause overestimation of flow. We hypothesized that there is a region at intermediate distance to the orifice, where both these effects on the flow field are minimal, so that the local velocity there is only a function of flow. In a flow model, aortic regurgitation was simulated. The flow convergence was imaged by color Doppler. Different scanning directions were used (analogue to apical and parasternal approach). Velocity profiles across the flow convergence were read along the line from the scanhead to the orifice. At a distance of 7 mm to the orifice, a uniform value was found for the ratio of local velocity v(7 mm)/flow (=0.28 cm(-2)). Variations in size (3.5 to 7 mm), site (central versus lateral) and leaflet angle (planar versus inverted funnel) of the orifice and in the scanning approach had only a minimal effect on this value. The aortic regurgitant flow convergence is characterized by a relatively uniform V (7 mm)/Q. Independent of variations in the anatomy and the scanning approach, this value directly reflects flow.

Tropic leaf movements, photosynthetic gas exchange, leaf δ13C and chlorophyll a fluorescence of three soybean species in response to water availability

Midday leaf angle, photosynthetic gas exchange, stable carbon isotope ratio (δ13C), and chlorophyll a fluorescence among three wild soybeans, Glycine soja, G. tomentella and G. tabacina, from habitats with different water availability were examined. Plants grown under low water availability had reduced leaf area, photosynthetic and electron transport rates, more positive δ13C values, and more vertical midday leaf angles. The three species differed in midday leaflet orientation, leaf size, photosynthesis and fluorescence responses to water availability. The species differences were consistent with the water availability of their habitat. G. soja, which grows in the wettest habitats, was shown to be the most photosynthetically susceptible to low water treatment and tended to have the most vertical midday leaflet angles. In contrast, G. tabacina, distributed in the driest habitats, had the least vertical midday leaflet angles and the lowest photosynthetic sensitivity to low water availability. G. tomentella, inhabiting an intermediate habitat, had intermediate midday leaf angles and photosynthetic responses. Our results support the hypothesis that paraheliotropic leaf movements respond in concert with photosynthetic characteristics in soybean leaves such that water use efficiency is enhanced and the risk of photoinhibition under water stress conditions is reduced.

Comparison of proximal isovelocity surface area method with pressure half-time and planimetry in evaluation of mitral stenosis

Objectives. This study sought to 1) compare the accuracy of the proximal isovelocity surface area (PISA) and Doppler pressure half-time methods and planimetry for echocardiographic estimation of mitral valve area; 2) evaluate the effect of atrial fibrillation on the accuracy of the PISA method; and 3) assess factors used to correct PISA area estimates for leaflet angulation.Background. Despite recognized limitations of traditional echocardiographic methods for estimating mitral valve area, there has been no systematic comparison with the PISA method in a single cohort.Methods. Area estimates were obtained in patients with mitral stenosis by the Gorlin hydraulic formula, PISA and pressure half-time method in 48 patients and by planimetry in 36. Two different factors were used to correct PISA estimates for leaflet angle (θ): 1) plane-angle factor (θ/180 [θ in degrees]); and 2) solid-angle factor [1 — cos(θ/2)].Results. After exclusion of patients with significant mitral regurgitation, the correlation between Gorlin and PISA areas (0.88) was significantly greater (p < 0.04) than that between Gorlin and pressure half-time (0.78) or Gorlin and planimetry (0.72). The correlation between Gorlin and PISA area estimates was lower in atrial fibrillation than sinus rhythm (0.69 vs. 0.93), but the standard error of the estimate was only slightly greater (0.24 vs. 0.19 cm2). The average ratio of the solid- to the plane-angle correction factors was approximately equal to previously reported values of the orifice contraction coefficient for tapering stenosis.Conclusions. 1) The accuracy of PISA area estimates in mitral stenosis is at least comparable to those of planimetry and pressure half-time. 2) Reasonable accuracy of the PISA method is possible in irregular rhythms. 3) A simple leaflet angle correction factor, θ/180 (θ in degrees), yields the physical orifice area because it overestimates the vena contracta area by a factor approximately equal to the contraction coefficient for a tapering stenosis.

Movement Influences Carambola Leaflet Chlorophyll Fluorescence and Temperature under Sunny Conditions

Leaflets of `Arkin', `B-10', `Kary', and `Sri Kembangan' carambola (Averrhoa carambola L.) trees were restrained in a horizontal position for 3.5 h during midday under full sun conditions to determine the influence of overriding natural leaflet movement on adaxial chlorophyll fluorescence and temperature. Induced chlorophyll fluorescence obtained after 30 minutes of dark adaptation following the period of full sun exposure was affected by leaflet movement. Restrained leaflets exhibited a variable fluorescence (Fv)/peak fluorescence (Fm) of 0.48, while that of unrestrained leaflets was 0.65. Adaxial leaflet temperature of restrained leaflets was 6C higher than that of leaflets that were allowed to move. The influence of leaflet movement on temperature or chlorophyll fluorescence was not different among the four cultivars. However, mean Fv/Fm of `Kary' and `Sri Kembangan' was lower than that of `B-10'. Our results indicate that the ability of carambola to change leaflet angle leads to lower temperature and higher photochemical efficiency than occurs when leaflets are not allowed to move naturally (vertically orient) under full sun conditions.

Phytochrome‐mediated closure of Albizzia lophantha leaflets as affected by sorbitol, mannitol and fusicoccin

The effect of sorbitol and mannitol and their interaction with the effect of fusiccocin has been investigated in phytochrome‐mediated nyctinastic closure of Albizzia lophantha leaflets. Treatments were commenced 2 h into the photoperiod. Pairs of leaflets were excised, floated on test solutions, and transferred to darkness immediately after red or far red irradiation. Leaflet angles were measured in darkness for 3 h. Fusicoccin (1 to 100 μM) inhibited nyctinastic closure and enhanced reopening in the presence of Pft (far‐red‐absorbing form of phytochrome). Fusicoccin effects on far red treated leaflets were not constant and whichever fusicoccin concentration was applied, the effects were small. These results confirm the role of the plasma membrane proton pump in extensor cells during opening. Sorbitol and mannitol (25–100 mM) increased nyctinastic closure for about 2 h and then enhanced reopening in the presence of Pfr. When sugar alcohols were applied together with fusicoccin, sorbitol and mannitol enhanced the fusicoccin inhibition of closure and a marked synergistic effect was observed. These results indicate that both fusiccocin and the reduction of external osmotic potential caused by sorbitol and mannitol supply may stimulate the activity of the extensor H+ pump promoting the reopening.

Frame-mounted porcine valve bioprostheses: Preparation during aortic-root dilation: Biomechanics and design considerations

The leaflet geometry and function of frame-mounted porcine bioprostheses prepared with dilation of the aortic root during frame mounting was investigated. The diameter of the porcine aortic root increased by 47% when dilated with a pressure of 120 mm Hg. In the absence of pressure dilation, the ratio of circumferential leaflet length to the radius of the aortic root (S/RA) was 2.7 +/- 0.1, and the angle of inclination of the leaflet to the base of the valve was 17 +/- 4.3 degrees. In this condition, the leaflet geometry was similar to that of some second-generation porcine bioprostheses, which demonstrated high open-leaflet bending strains at the commissures. Dilation of the porcine root with 120 mm Hg reduced the value of S/RA to 1.84 and produced a triangular open-leaflet configuration with minimum open-leaflet bending strains. Open-leaflet bending strains were also reduced in two prototype frame-mounted valves prepared with partial dilation of the aortic root, which had an S/RA ratio of less than 2.3 and a leaflet angle greater than 27 degrees. The study indicates that the next generation of porcine bioprostheses should be prepared with at least 17% dilation of the aortic root during frame mounting and with zero pressure difference across the leaflets during fixation. This can be achieved by applying an equal hydrostatic pressure to both sides of the valve leaflets during fixation and frame mounting.

Population variation of leaflet sleep movements in Oxalis grandis (Oxalidaceae)

To identify intraspecific variation in the expression of circadian leaflet movements, we observed changes in leaflet elevation angle in a controlled environment and in the field. Two morphologically and ecologically distinct populations of Oxalis grandis were compared: a typical mesic forest ecotype and an atypical, densely hirsute ecotype found on partially exposed shale outcroppings. Significant genotypic variation was detected within both populations. The controlled environment experiment revealed significant differences in the intrinsic rhythm between the two populations, primarily at the beginning and end of the photophase. We observed leaflet ascent and descent prior to lights‐on and lights‐off, respectively, processes we define as anticipatory. Hirsute ecotype plants showed a greater anticipation of lights‐on, whereas the mesic ecotype plants showed a greater anticipation of lights‐off. In the field, significant differences in leaflet elevation between the populations occurred in early morning and late afternoon, but not immediately preceding dusk. Contrary to the controlled environment experiment, mesic ecotype plants showed greater anticipation of dawn; plants from both populations showed similar anticipation of dusk. Plants of the hirsute ecotype exhibited higher sensitivity to sudden fluctuations in light level. We hypothesize that differences in light sensitivity partially explain the differences in the leaflet angle patterns between the field and controlled environments.

Follow-Up in mitral valve prolapse by phonocardiography, M-mode and two-dimensional echocardiography and Doppler echocardiography

To assess the serial phonocardiographic and echocardiographic change in patients with mitral valve prolapse (MVP), phonocardiograms and echocardiograms were reviewed retrospectively in 116 patients (48 men and 68 women, mean age 27 years) who had been determined to have MVP and were reexamined 4.3 years (range 1 to 14) later by phonocardiography and echocardiography between 1971 and 1988. Follow-up phonocardiograms showed periods when 5 of 18 patients with silent MVP developed mid- or late systolic clicks. Of 57 patients with mid- or late systolic clicks, 15 had silent MVP, 6 developed a late systolic murmur with or without systolic clicks and 1 developed a pansystolic murmur. Two of 9 patients with an isolated late systolic murmur developed a pansystolic murmur. M-mode echocardiograms showed that left atrial and left ventricular dimensions at end-diastole and end-systole increased in patients with systolic murmur (33 ± 10 vs 35 ± 11, 46 ± 6 vs 50 ± 7 and 29 ± 4 vs 31 ± S mm, respectively, all p < 0.001) and no statistically significant changes in any of these dimensions were found in patients without a systolic murmur. The degree of MVP evaluated by the anteroposterior mitral leaflet angle on the 2-dimensional echocardiogram was more severe in patients with a systolic murmur than in patients without systolic murmur (157 ± 12 vs 131 ± 16 °, p < 0.001). The degree of prolapse did not change during the follow-up periods. The number of patients with mitral regurgitation detected by pulsed Doppler echocardiography increased from 21 of 72 (29%) to 31 of 72 (43%). The present study suggests that the systolic murmur confirmed by phonocardiography should be taken into consideration in evaluating severity and in assessing prognosis of MVP.

The use of soybean leaflet angle data for irrigation scheduling

The feasibility of scheduling soybean [Glycine max (L.) Merr.] irrigation by using terminal leaflet angle data was evaluated by simultaneously measuring the angles to the horizontal of the uppermost terminal leaflet and soil moisture contents on plots with different soil moisture regimes during and following a drying cycle. Severe water stress was avoided. The percentage of leaflets with an angle greater than or equal to 45 degrees correlated significantly with both water content in the top 0.45 m of soil and the ET/Epan ratio.

Chronobiology of Aging in Albizzia julibrissin

We are using leaflet movements in Albizzia julibrissin as a model system for investigating the chronobiology of aging. To monitor leaflet movements during long dark periods with adequate temporal and spatial resolution, we designed an automated, computerized photoelectric monitoring system. Each of 12 leaflet pairs was positioned in an individual light-proof container, with one leaflet immobilized. The angle of the mobile leaflet was monitored by a photosensor array using a low intensity infrared beam. Leaflet position was determined by custom-developed software, using information on the shading patterns of the sensors. Data on leaflet angle as a function of time were collected and stored on a floppy disc and then printed in numerical and graphical form.Oscillations of young, middle-aged, and old leaflets persist during 7 d of darkness with a periodicity close to 24 h. Period length appears to be age-independent, but rhythmic wave form is age-dependent. The older the leaflet, the earlier and more completely it opens and the less completely it closes.

Leaflet angle and radiation avoidance by water stressed groundnut ( Arachis hypogaea L.) plants

The mechanism of alleviating water stress by leaflet angle variation was studied for cv. Tindivanam 2 (TMV 2) groundnut ( Arachis hypogaea L.) plants. A simple method of measuring leaflet angle was also devised. Radiation avoidance by leaf closure was exhibited to different degrees by water stressed and non-stressed plants. Leaflets in plants under continuous water stress had reduced areas and showed greater leaflet movement (leaf closure) compared to the amount of movement of leaflets on plants without water stress. Avoidance of radiation interception by leaflet movement occurs in groundnut plants under water stress.

Phytochrome and Circadian Clocks in Samanea

Previous investigations with the electron microprobe reveal that the movements of Samanea leaflets are correlated with massive redistribution of K within the pulvinus. Evidence is now presented that Cl moves with K, whether plants are in white light or darkness, whether or not the amplitude of free running oscillations has damped, and whether or not the rhythm has been rephased by phytochrome photoconversion. The mid-extensor to mid-flexor ratio of K + Cl is correlated with leaflet angle under all conditions. Total Cl in both inner cortex and motor region is approximately 0.6 as high as K. The stoichiometry between Cl and the migratory fraction of K is close to, but not precisely 1:1 in all regions of the pulvinus, suggesting that other ions or systems may also be involved in the balancing of electrical charges.

Dimensions and geometric relationships of the human aortic valve as a function of pressure.

In a continuing effort to develop improved prosthetic heart valves, a redefinition of the anatomy of the human aortic valve as a function of stress was undertaken. Dimensions and geometric relationships of the human aortic valve as a function of intraaortic pressure between 0 and 120 mm Hg were obtained from a series of silicone rubber valve casts. The axial length of the valve region was found to vary negligibly with pressure, but significant variations in geometry and angular dimensions were seen. The leaflet attachment annulus forms an ellipse at the plane of intersection with the cylindrical surface passing from the left ventricular tract through the aorta. Deductions from stress considerations for the measured geometry indicate that the loaded leaflet is a section of a cylindrical surface. The equation for this developed surface was obtained, and a prosthetic design was determined using average values at 100 mm Hg. The leaflet is developable onto a plane with a cut required along part of the junction line between the initially cylindrical part and the plane coapting surfaces. Optimum valve shape mandates a base angle between the cylindrical leaflet and the center axis of 70° (α =20-22°, where α is the leaflet angle).

Rhythmic Potassium Flux in Albizzia

Rhythmic leaflet movement in Albizzia is controlled by rhythmic K(+) flux in pulvinal motor cells. The angle assumed by darkened leaflets during the open phase of the rhythm can be altered by various compounds and changes in temperature; such treatments are ineffective during the closed phase. In all cases, effects on leaflet angle are correlated with and probably a consequence of K(+) flux in pulvinal motor cells. Incubation at low temperature (6C) or on sodium azide (1.0 mm) reduces K(+) in the ventral motor region and increases K(+) in the dorsal motor region, thereby decreasing leaflet angle. Incubation on cycloheximide (0.1 mm) or sodium acetate (0.05 m) inhibits protein synthesis; if the incubation period immediately precedes the opening phase, these compounds prevent both K(+) flux into the ventral motor cells and leaflet opening. Magnesium nitrate (0.05 m), supplied after leaflets have started to open, promotes K(+) secretion from the dorsal motor cells and increases the angle of opening.The data support the following hypothesis. Active K(+) transport into the ventral motor cells and out of the dorsal motor cells leads to opening; K(+) leakage in the opposite directions causes closure; and the interaction of these processes results in a rhythmic oscillation. Proteins in the ventral cell membranes that are required for active transport or membrane integrity turn over rhythmically and are resynthesized before opening. The availability of divalent cations determines the phase relationships between the K(+) rhythms in the dorsal and ventral motor cells.White light phases the rhythm. The "light on" signal turns on a K(+) secreting pump in the dorsal motor cells, while the "light off" signal initiates a period of protein synthesis which in turn leads to active K(+) transport into the ventral motor cells. Aminophylline (0.1-6.0 mm) inhibits white light-promoted opening and nyctinastic closure.