Testing the constant-volume hypothesis by magnetic resonance imaging of Mytilus galloprovincialis heart

Hypertonic water reabsorption with a parallel-current system via the glandular and saccular renal tubules of Ruditapes philippinarum

We investigated the renal function of the brackish water clam, Ruditapes philippinarum, employing magnetic resonance imaging (MRI). The R. philippinarum kidney consists of two renal tubules, a glandular (GT) and a saccular (ST) tubule. After exposure to seawater containing manganese ion (Mn2+) at 20°C, the intensity of the T1-weighted MRI and longitudinal relaxation rates (1/T1=R1) of the kidney were increased. In the ST, haemolymph containing Mn2+ entered directly from the auricle, and the Mn2+ concentration ([Mn2+]) increased in the initial part of the ST. Thereafter, [Mn2+] was almost constant until the posterior end of the kidney. The GT received haemolymph from the pedal sinus via the visceral sinus. The GT runs parallel inside the ST, and [Mn2+] increased progressively until it merged with the ST. In a range of seawater with [Mn2+] from 1 to 30 µmol l-1, the [Mn2+] increased 12-fold in the posterior part of the ST, compared with the ambient [Mn2+]. Based on these results, the epithelium of the initial part of the ST reabsorbs water from luminal fluid, building up a higher osmotic pressure. Using this osmotic gradient, hypertonic water is reabsorbed via the epithelium of the GT to the ST, and then transferred to the haemolymph via the epithelium of the ST. Excess water is excreted as urine. This model was supported by the increases in [Mn2+] in the ST when the clams were exposed to seawater containing Mn2+ at salinity from 26.0 to 36.0‰, showing that the parallel-current system works in hypotonic seawater.

Foot extension and retraction in the clam Calyptogena okutanii without any Keber's valve: an inflatable fastener bag model

In order to investigate the foot manipulation of a clam without a Keber's valve, Calyptogena okutanii was examined by light microscopy, magnetic resonance imaging and computed tomography. The foot chamber was divided into two compartments by a dense muscle fastener zone (FZ) comprising a pedal artery and sinuses in the mid-sagittal plane in between muscles running in the anterior-posterior oblique direction. The distal part of the foot chamber (inflatable fastener bag, IFB) had a loose superficial muscle layer. The proximal part of the foot chamber (visceral reservoir, VR) was covered by a dense superficial muscle layer. The outlet of the VR was connected with the hinge ligament duct, consisting of the hinge ligament, a pair of shells and the pericardium. Based on these anatomical structures, foot extension starts from contraction of muscles in the FZ, so that flow in the FZ is stopped. Then, the superficial muscles of the foot contract, and the pressure of the IFB increases so that the foot can extend. Foot retraction starts from the relaxation of muscles in the FZ so that the hemolymph returns to the VR. The hinge ligament duct allows a constant return flow from the foot chamber to the gills and the heart. The heart rate and the flow in the FZ, which decreased and increased during the foot extension and retraction, respectively, supported this model. In conclusion, the FZ of Calyptogena okutanii could be an alternative to Keber's valve in Anodonta, playing a similar role.

Size-selective filtration of the atrial wall estimated from the accumulation of tracers in the kidney of the mussel Mytilus galloprovincialis

In order to determine the molecular weight cut-off (MWCO) for the atrial wall filtration into kidneys of the Mytilus galloprovincialis, we employed five magnetic resonance (MR) tracers: manganese chloride (Mn²⁺), gadolinium chloride (Gd³⁺), manganese-ethylenediaminetetraacetic acid (MnEDTA), gadolinium-diethylenetriamine pentaacetic acid (GdDTPA) and oligomer-based contrast agent (CH3-DTPA-Gd). After injection of the MR tracers (1 or 2 mmol l^-1×0.1 ml) into the visceral mass, T ₁-weighted MR imaging (T _1w-MRI) and the longitudinal relaxation rates (1/T ₁=R ₁) were measured at 20°C. The MR tracers were distributed uniformly in the visceral mass within 1 h after injection. The T _1w-MRI intensity and R ₁ of the kidney (R _1K) were increased by Mn²⁺ and MnEDTA, with urine concentrations estimated at 210 and 65 µmol l^-1, respectively. The rest of the tracers showed only minimal or no increase. When the mussels were additionally incubated in seawater with 10 µmol l^-1 MnCl₂, R _1K was increased in the GdDTPA group, but not in the GdCl₃ group. Therefore, Gd³⁺ might have inhibited renal accumulation of Mn²⁺ and Gd³⁺ Incubation in seawater with 10 µmol l^-1 MnEDTA showed no increase in the R _1K, but additional incubation with 10 µmol l^-1 MnCl₂ caused an increase in R _1K It is suggested that injected MnEDTA was filtrated as MnEDTA per se, and not likely separated into free Mn²⁺ Thus, we concluded that the MWCO of the atrial wall of the M. galloprovincialis is around 0.5 kDa, which is almost 1/100 of that for vertebrate animals, and suggests a reduction in efforts to reabsorb metabolites and osmolytes from the urine.

Studying the cardiovascular system of a marine crustacean with magnetic resonance imaging at 9.4 T

OBJECTIVES

An approach is presented for high-field MRI studies of the cardiovascular system (CVS) of a marine crustacean, the edible crab Cancer pagurus, submerged in highly conductive seawater.

MATERIALS AND METHODS

Structure and function of the CVS were investigated at 9.4 T. Cardiac motion was studied using self-gated CINE MRI. Imaging protocols and radio-frequency coil arrangements were tested for anatomical imaging. Haemolymph flow was quantified using phase-contrast angiography. Signal-to-noise-ratios and flow velocities in afferent and efferent branchial veins were compared with Student's t test (n = 5).

RESULTS

Seawater induced signal losses were dependent on imaging protocols and RF coil setup. Internal cardiac structures could be visualized with high spatial resolution within 8 min using a gradient-echo technique. Variations in haemolymph flow in different vessels could be determined over time. Maximum flow was similar within individual vessels and corresponded to literature values from Doppler measurements. Heart contractions were more pronounced in lateral and dorso-ventral directions than in the anterior-posterior direction.

DISCUSSION

Choosing adequate imaging protocols in combination with a specific RF coil arrangement allows to monitor various parts of the crustacean CVS with exceptionally high spatial resolution despite the adverse effects of seawater at 9.4 T.

Roles of Keber's valve and foot chamber for foot manipulation in the mussel Nodularia douglasiae

In order to analyse the roles of Keber's valve for foot manipulation in the mussel Nodularia douglasiae, the anatomy and haemolymph flow in the cardiovascular system were detected by magnetic resonance imaging. The superficial layer of the foot was covered by a dense muscle layer, which extended to the dorsal side and connected with the shell. This closed space, the foot chamber, had an inlet (anterior aorta) and an outlet (Keber's valve). At rest, in the beginning of the systolic phase, flows in the anterior aorta and the pedal artery increased, followed by the pedal and visceral sinuses. Then these flows ceased at the end of the systolic phase, followed by inflow to the ventricle in the diastolic phase; therefore, the compliance of the foot chamber is low enough to transfer pressure pulses to the visceral sinus. Extension of the foot started with relaxation of the foot muscle, so the compliance of the foot chamber increased. Then, Keber's valve closed so that the haemolymph filled the foot haemocoel. Retraction of the foot is initiated by the opening of Keber's valve. Judging from these results Keber's valve and the foot chamber are essential for circulation at rest, foot extension and retraction.This article has an associated First Person interview with the first author of the paper.

Accumulation and excretion of manganese ion in the kidney of M ytilus galloprovincialis

T ₁-weighted magnetic resonance imaging (T _1w-MRI) was employed to detect the accumulation of manganese ion (Mn²⁺) in urine in the kidney of the mussel Mytilus galloprovincialis, and the longitudinal relaxation rates (1/T ₁=R ₁) were measured. When the mussel was exposed to seawater containing 10 µmol l^-1 Mn²⁺, the T _1w-MRI intensity and R ₁ of the kidney, stomach and digestive glands were increased. Mn²⁺ might be taken into the hemolymph via the gastrointestinal tract, and then filtrated into the pericardium via the auricles. Although the image intensity in the pericardium was not affected by manganese, an image intensity enhancement was observed in the distal part of the renopericardial communication canals between the pericardium and the kidneys, indicating Mn²⁺ was concentrated in the excretion pathway. As the seawater Mn²⁺ concentration ([Mn²⁺]_SW) was increased from 3 to 50 µmol l^-1, R ₁ of the kidney (R _1K) was elevated. When the mussels were immersed in 3-10 µmol l^-1 [Mn²⁺]_SW for 24 h, the Mn²⁺ concentration in the kidney ([Mn²⁺]_K) showed a 15-fold increase compared with the ambient [Mn²⁺]_SW In the range of [Mn²⁺]_SW from 10 to 50 µmol l^-1, R _1K reached a plateau level that corresponded to 200 µmol l^-1 [Mn²⁺]_K As [Mn²⁺]_K fell transiently, voluntary excretion of urine from the kidney was assumed. The decreases in intensity were not synchronized between the right and left kidneys, and the closure of the shells might not be essential for urinary excretion. The voluntary excretion suggested an additional explanation for the large range in metal concentratons in the kidneys of the mussel.

Does the membrane pacemaker theory of metabolism explain the size dependence of metabolic rate in marine mussels?

According to the Membrane Pacemaker Theory of metabolism (MPT) allometric scaling of metabolic rate in animals is determined by the composition of cellular and mitochondrial membranes that changes with body size in a predictable manner. MPT has been elaborated from interspecific comparisons in mammals. It projects that the degree of unsaturation of membrane phospholipids decreases in larger organisms, thereby lowering ion permeability of the membranes and making cellular and thus whole animal metabolism more efficient. Here we tested the applicability of the MPT to a marine ectotherm, the mussel Mytilus edulis at the intraspecific level. We determined effects of body mass on whole organism, tissue and cellular oxygen consumption rates, on heart rate, metabolic enzyme activities and on the lipid composition of membranes. In line with allometric patterns the organismal functions and processes such as heart rate, whole animal respiration rate and phospholipid contents showed a mass-dependent decline. However, the allometry of tissue and cellular respiration and activity of metabolic enzymes was poor; fatty acid unsaturation of membrane phospholipids of gill tissue was independent of animal size. It is thus conceivable that most of the metabolic allometry observed at the organismal level is determined by systemic functions. These whole organism patterns may be supported by energy savings associated with growing cell size but not by structural changes in membranes. Overall, the set of processes contributing to metabolic allometry in ectotherms may differ from that operative in mammals and birds, with a reduced involvement of the mechanisms proposed by the MPT.

A portable infrared photoplethysmograph: heartbeat of Mytilus galloprovincialis analyzed by MRI and application to Bathymodiolus septemdierum

Infrared photoplethysmogram (IR-PPG) and magnetic resonance image (MRI) of the Mytilus galloprovincialis heart were obtained simultaneously. Heart rate was varied by changing temperature, aerial exposure and hypoxia. Higher heart rates (35-20 beat min⁻¹) were usually observed at 20°C under the aerobic condition, and typical IR-PPG represented a single peak (peak v). The upward and downward slopes of the peak v corresponded to the filling and contracting of the ventricle, respectively. A double-peak IR-PPG was observed in a wide range of heart rates (5 to 35 beats min⁻¹) under various conditions. The initial peak v corresponded to the filling of the ventricle, and the origin of the second peak (v’) varied with the heart rate. A flat IR-PPG with a noise-level represented cardiac arrest. Although large movement of the shells and the foot caused slow waves or a baseline drift of the IR-PPG, the heart rate can be calculated from the v-v interval. Based on these results, we assembled a portable IR-PPG recording system, and measured the heartbeats of Bathymodiolus septemdierum (Mytilidae) for 24 h on a research vessel just after sampling from the deep sea, showing that IR-PPG is a noninvasive, economical, robust method that can be used in field experiments.

Summary: Infrared photoplethysmogram of Mytilus heart was analyzed by magnetic resonance imaging. Portable photoplethysmographs provide a noninvasive, economical and robust method to monitor the heartbeat of mussels in field experiments.

Magnetic resonance imaging analysis of water flow in the mantle cavity of live Mytilus galloprovincialis

Water flow inside the shell of Mytilus galloprovincialis was measured by phase-contrast magnetic resonance imaging (MRI). In seawater without algal cells at 23 °C, water approached the mussel from the posterior-ventral side, and entered through the inhalant aperture at a velocity of 40-20 mm s(-1). The flow rate in the lower mantle cavity decreased to 10-20 mm s(-1), the water flowed in the anterior-dorsal direction and approached the demibranches at a velocity of 5-10 mm s(-1). After passing through the lamellae to the upper mantle cavity, the water stretched the interlamellar cavity, turned to the posterior-dorsal direction and accumulated in the epibranchial cavity. The water flows came together at the ventral side of the posterior adductor muscle. The velocity increased more to than 50 mm s(-1) in the exhalant siphon, and exhaled out in the posterior-dorsal direction. The anterior-posterior direction of the flow was imaged every 1.92 s by the inflow effect of T1-weighted MRI. The flow seemed to be constant, and no cyclic motion of the mantles or the gills was detected. Spontaneous closure of the shells caused a quick drop of the flow in the mantle cavity. In the opening process of the shells, water flow in the interlamellar cavities increased before the opening, followed by an increase of flows in the exhalant siphon and inhalant aperture with minimum delay, reaching a plateau within 1 min of the shells opening. This provides direct evidence that the lateral cilia drive water in the mussel M. galloprovincialis.