Microplastic in Dredged Sediments: From Databases to Strategic Responses.

Microplastics (MPs) accumulate in sediments, yet guidelines for evaluating MP risks in dredged sediments are lacking. The objective of this study was to review existing literature on MPs in sediments to improve fundamental knowledge of MP exposures and develop a publicly available database of MPs in sediments. Twelve percent of the reviewed papers (nine studies) included sediment core samples with MP concentrations generally decreasing with depth, peaking in the top 15 cm. The remaining papers evaluated surficial grab samples (0 to 15 cm depth) from various water bodies with MPs detected in almost every sample. Median MP concentrations (items/kg dry sediment) increased in this order: lakes and reservoirs (184), estuarine (263), Great Lakes nearshore areas and tributaries (290), riverine (410), nearshore marine areas (487), dredge activities (817), and harbors (948). Dredging of recurrent shoaling sediments could be expected to contain MPs at various depths with concentrations influenced by the time elapsed since the last dredging event. These results offer key insights into the presence and variability of MPs in dredged sediments, informing environmental monitoring and risk assessment strategies.

Regulation of blood pressure in the land crab Cardisoma guanhumi.

We examined the cardiovascular responses to acute and chronic changes in blood volume (BV) in the land crab Cardisoma guanhumi. Acute reduction in BV caused an increase in activity in the dorsoventral muscles (DVMs) and to a lesser extent in the epimeral attractor muscles (EAMs). Contraction of the DVMs and EAMs will decrease the volume of the dorsal sinus and the thorax as a whole, respectively. BV reduction also caused bradycardia with frequent periods of cardiac arrest. There was a small drop in hemolymph pressure. BV expansion had the reciprocal effect on DVM and EAM activity but had no effect on heart rate (fH). After the cardioregulatory nerves were cut, acute hypovolemia had no effect on fH but still caused a moderate increase in DVM activity. After dehydration-induced BV reduction, DVM activity increased, whereas hemolymph pressure, fH, and EAM activity were maintained close to control levels.

Effects of walking on ventilatory and cardiac function in intact and cardiac-impaired lobsters.

Decapod crustaceans with normal heart function respond to the increased oxygen delivery requirements during walking with a step increase in heart and ventilation rate. In American lobsters, ventilation rate increased by the same amount during exercise at two walking speeds (2.4 and 8 m min(-1)); however, ventilation volume was significantly greater at the fastest walking speed (280 mL min(-1)) compared to animals at rest or walking at the slower speed (180 mL min(-1)). The heart responded in a similar manner to locomotion. Heart rate was elevated to the same level at the two different walking speeds, but cardiac stroke volume was greater, implying increased cardiac output, at the faster walking speed. The communication and compensation between the cardiac and ventilatory systems was revealed when the function of one was impaired. Ventilatory rate was significantly elevated when cardiac output was impaired by sectioning two of the alary ligaments and/or the regulatory nerves to the heart. When cardiac output was more severely impaired, ventilation rate was greater. Despite ventilatory compensation, anaerobic metabolism made a greater contribution to energy production with impaired heart function. Hemolymph lactate concentration was three to five times greater in impaired animals than controls. It is known that the ventilatory and cardiac systems of lobsters are coregulated. These data demonstrate that the performance of one system can respond to compensate for impaired function in the other.

Clinical signs and studies of the site of action of purified larkspur alkaloid, methyllycaconitine, administered parenterally to calves.

The purified diterpenoid alkaloid, methyllycaconitine was isolated from Delphinium brownii and injected intravenously into beef calves in order to observe the clinical signs. Following injection and depending upon dosage, calves showed a variety of signs; agitation, respiratory difficulty and loss of muscle control. Collapse occurred at higher doses of methyllycaconitine. Onset of clinical signs after injection occurred within two to three minutes, and recovery was rapid, clinical signs disappearing within five to ten minutes postinjection.The clinical signs observed are consistent with a skeletal neuromuscular site of action. Methyllycaconitine may have a curare-like action of postsynaptic blockage of nicotinic cholinergic receptors in cattle as has been demonstrated by in vitro experiments in other species. Physostigmine appears to be an effective antidote to methyllycaconitine.

Elasticity, unexpected contractility and the identification of actin and myosin in lobster arteries.

Lobster arteries, which exhibit non-uniform elasticity when stretched, have a trilaminar organization. The inner layer is an elastic connective tissue and the outer layer is a collagenous connective tissue; the middle layer of an artery is an aggregation of cells containing microfilaments. Arterial cells possess actin, myosin and tropomyosin. Except for the dorsal abdominal artery, striated muscle cells are not evident in the walls of any of the vessels. The neurotransmitter glutamic acid and the neurohormone proctolin elicit slow circumferential contractions in all of the arteries leaving the lobster heart. Only the dorsal abdominal artery contracts when stimulated electrically. Longitudinal strips of the arteries do not respond to either drugs or electrical stimulation. Arterial contraction will have profound effects on resistance to blood flow and may be an important component of the control mechanisms regulating blood distribution.

Sites and modes of action of proctolin and the FLP F2 on lobster cardiac muscle.

At the threshold concentration (1-10 pmol l(-1)), the neuropeptide hormones proctolin (PR) and the FLRFamide-like peptide (FLP) F(2) cause an increase in amplitude of electrically evoked contractions (each contraction is a brief tetanus) of lobster heart ostial muscle. At higher concentrations each peptide also induces an increase in tonus (contracture). The PR-induced contracture and augmentation of tetani are proportional to increases in [Ca2+]i. The rate of onset and recovery of peptide-induced effects on both tetani and contracture appeared to reduced by Ca2+ storage by the sarcoplasmic reticulum (SR). Enhanced tetani following a contracture may be due to enhanced voltage-gated Ca2+ current and sarcoplasmic reticular (SR) Ca2+ loading. The SR Ca2+ loading appears to be specific for PR and F2, since glutamic-acid-induced contractures are not followed by increased tetani. The prolonged elevation of [Ca2+]i during contracture causes a right-ward shift in the force-pCa curve indicating a decrease in myofibrillar sensitivity to Ca2+. Blocking voltage-gated Ca2+ channels with Cd2+, nifedipine or verapamil, while reducing tetani, does not prevent peptide-induced contracture and enhanced tetani. Opening SR Ca2+ channels and depleting SR Ca2+ with either caffeine or ryanodine blocked tetani but permitted accelerated peptide-induced contractures. We conclude that PR and F2 at low concentration enhance voltage-dependent Ca2+ induced Ca2+ release from the SR, while higher hormone levels directly gate Ca2+ entry across the sarcolemma.

Neuronal control of respiration in decapod crustacea.

Respiratory exchange in decapod crustacea requires the coordinated activity of the heart and the scaphognathites, appendages which ventilate the gills. There is common central nervous system neuronal modulation of both autogenically active systems as well as direct neuronal communication between both systems. The heart and scaphognathites also respond directly to oxygen tension. The neuronal control of the scaphognathites also respond directly to oxygen tension. The neuronal control of the scaphognathites is analyzed at several levels. Particular attention is directed toward the means by which the innately organized and stereotyped motor pattern for forward beating can be altered to produce reversed beating. The importance of sensory feedback in maintaining normal rates of scaphognathite beating is noted. And the phenomenon of bilateral coordination between the morphologically independent scaphognathites is described. Several different models of parts of the over-all scaphognathite neuronal circuitry are presented for heuristic purposes.

The Role of the Cardioregulatory Nerves in Mediating Heart Rate Responses to Locomotion, Reduced Stroke Volume, and Neurohormones in Homarus americanus.

Control of decapod crustacean heart activity is believed to be dependent on the regulation of the cardiac ganglion by external input from the central nervous system as well as by circulating neurohormones. This study investigated the roles of these inputs on the heart rates of lobsters exercising on a treadmill. Heart rate increased rapidly at the onset of walking in control animals. This rapid phase was lost after the regulatory nerves were cut, but small increases still occurred. When stroke volume was reduced by cutting alary ligaments, the animals compensated by increasing heart rate; this compensation was lost when the regulatory nerves were cut. In resting animals, injection of serotonin, octopamine, and dopamine induced increases in heart rate. After the regulatory nerves were cut, only dopamine and serotonin injections caused increases in heart rate, suggesting that these amines act on the cardiac ganglion as independent effectors.

Dopamine and nicotine, but not serotonin, modulate the crustacean ventilatory pattern generator.

Dopamine (DA) causes a dose-dependent increase in the frequency of motor neuron bursts [virtual ventilation (fR)] produced by deafferented crab ventilatory pattern generators (CPGv). Domperidone, a D2-specific DA antagonist, by itself reversibly depresses fR and also blocks the stimulatory effects of DA. Serotonin (5HT) has no direct effects on this CPGv. Nicotine also causes dramatic dose-dependent increases in the frequency of motor bursts from the CPGv. The action is triphasic, beginning with an initial reversal of burst pattern typical of reversed-mode ventilation, followed by a 2- to 3-min period of depression and then a long period of elevated burst rate. Acetylcholine chloride (ACh) alone is ineffective, but in the presence of eserine is moderately stimulatory. The inhibitory effects of nicotine are only partially blocked by curare. The excitatory action of nicotine is blocked by prior perfusion of domperidone, but not by SKF-83566.HCl, a D1-specific DA antagonist. SKF-83566 had no effects on the ongoing pattern of firing. These observations support the hypothesis that dopaminergic pathways are involved in the maintenance of the CPGv rhythm and that the acceleratory effects of nicotine may involve release of DA either directly or via stimulation of atypical ACh receptors at intraganglionic sites.

Comparison of Temperature Effects on Heart Performance of the Dungeness Crab, Cancer magister, in vitro and in vivo.

A large percentage of physiological studies are based on isolated components of complex systems, but the question can always be posed, are the responses the same in isolation as when these components are under the homeostatic controls that exist in vivo? For cardiac performance in Cancer magister, the responses to temperature variation over the range 4° to 20°C are different in semi-isolated hearts than in intact animals. Cardiac performance in semi-isolated hearts was measured with a pressure transducer, a flow transducer, and electromyogram (EMG) electrodes, and in intact animals with pulsed Doppler flow probes. Heart rate increase in semi-isolated hearts was about one-third of that in intact animals. The cardiac output of semi-isolated hearts decreased with increasing temperature, whereas that of intact animals increased. Stroke volume decreased linearly in semi-isolated hearts. In intact animals, stroke volume decreased from 4° to 12°C, but remained relatively stable from 12° to 20°C. The ventricular pressure and the EMG amplitude of semi-isolated hearts both decreased with increasing temperature. Double systolic contractions appeared both in semi-isolated hearts and in intact animals in the temperature range 13° to 20°C; this may represent a compensatory mechanism at extreme temperatures. The difference in cardiac performance between intact crabs and semi-isolated hearts reflects, almost certainly, extrinsic control in intact animals, including modulation by cardioregulatory nerves or neurohormonal modulation of the cardiac ganglion, myocardial contractility or changes in outflow resistance.

Patterns and bilateral coordination of scaphognathite rhythms in the lobster Homarus americanus.

The bilateral patterns of forward and reversed scaphognathite (SG) pumping are described for the American lobster. During forward pumping the two SGs usually function synchronously, but may also function independently. The nine muscles of one SG are arranged into four functional groups which are sequentially active during forward pumping. During reversed beats, motor neurones to one group of muscles are inactive while bursts to another are either delayed or missing. Reversal beats do not appear to alter the phasing of the central oscillators that generate the basic SG rhythm. Phase analysis of bilateral SG beating demonstrates two types of relationship: phase coupling or phase drifting with a tendency to couple. An animal may remain in one state for long periods of time or may alternate between states. Coupling can occur at more than one phase indicating phase multistability. The coupled state may remain constant at markedly different frequencies of beating, indicating phase rather than latency coupling between SGs. During the drifting state each SG tends to assume its "intrinsic" rate of oscillation. The drift state reflects the inherent asymmetry of the two SG systems. The influence of several parameters of sensory stimulation on phase and frequency of SG beating are analyzed.

Modulatory effects of proctolin on a crab ventilatory muscle.

Proctolin enhances nerve-evoked, phasic contractions of a selected respiratory muscle of the shore crab, Carcinus maenas, but has no effect on muscle tonus. Proctolin also increases the work and power output of this muscle. These effects are functionally appropriate in view of previous reports that proctolin stimulates the ventilatory rhythm. They also suggest that proctolin exerts coordinated modulatory control at the central and peripheral levels of the gill ventilatory system. In contrast, serotonin, dopamine and octopamine have no effect on this muscle.

The control of vascular resistance in the southern rock lobster, Jasus edwardsii (Decapoda: Palinuridae).

In Jasus edwardsii (Hutton) the vascular resistance of each of the seven major arterial systems leaving the heart was increased in response to several of the following neurotransmitters and neurohormones: acetylcholine, adrenalin, serotonin, dopamine, octopamine and peptides proctolin and FLRFamide-related peptide F(1). The resistance to flow through the infrabranchial sinus (IBS), part of the venous system, was also sensitive to these drugs. Unexpectedly, the responses of the IBS continued after removal of the gills. Differences in the profiles of responses of the arteries to individual hormones and in the magnitudes and the time courses of back pressure changes, eliminate a common downstream location such as the venous sinuses or gills, as the source of the arterial responses. Vasoactive drugs were effective when applied either via the lumen or, with longer delay, to the basal side of an artery via the IBS. It is concluded that the resistance of each of these sections of the vascular system is independently controllable by hormones.

Effects of scaphognathite nerve stimulation on the acutely deafferented crab ventilatory central pattern generator.

1. Sensory axons from crab (Carcinus maenas) scaphognathites enter the thoracic ganglion primarily via the LNb branch of the levator nerve. The LNa branch of the levator nerve and the depressor nerve each contain relatively few sensory axons. 2. Acutely deafferented ventilatory central pattern generators show a free running burst rate which is lower than that observed in intact crabs. Electrical stimulation of the levator nerve, or of its LNb branch, increases the burst rate in a frequency dependent manner. Stimulation at high enough intensity to recruit afferents will restart a paused motor rhythm. Stimulation of the levator nerve with short pulse trains phase resets and can entrain the rhythm. 3. In addition to increasing the burst rate, LNb stimulation also causes a progressive elimination of motor neurons from the bursts as the stimulating frequency increases, probably due to depolarization of the 3 oval organ 'giant' afferent axons in this branch. Intracellular depolarization of single oval organ afferents will also inhibit some motor neurons as well as slow or stop the rhythm. 4. Continuous stimulation of the depressor nerve does not affect the ganglionic burst rate and this nerve contains only a few small diameter afferent axons; however, brief trains of stimuli can reset the rhythm in a phase-dependent manner.

Cardiac performance in semi-isolated heart of the crab Carcinus maenas.

A semi-isolated, in situ heart preparation of the shore crab, Carcinus maenas, supported by its alary ligaments, pumps vigorously for hours at a mean heart rate of 49.7 beats/min and cardiac output of 30 ml.kg-1.min-1. These hearts show no adaptive responses to changes in pericardial sinus pressure, outflow resistance, or afterload. Direct perfusion-induced stretch of the heart wall causes increases in contractile force but minimal changes in heart rate. Stroke work and power are lower than comparable values for animals with myogenic hearts and closed circulatory systems. The values for heart rate and cardiac output are lower than in vivo values and may in part reflect the technique used as well as intrinsic performance of the heart without neural and neurohormonal inputs. Morphometrically the heart represents 0.2% of whole body weight, and the mean stroke volume of 0.35-0.45 ml/kg represents an ejection fraction of 27-34% of ventricular volume (1.4 ml/kg).

Physiology and excitation-contraction coupling in the intestinal muscle of the crayfish Procambarus clarkii.

The intestinal muscles of Procambarus clarkii are striated and yet they are specialized to produce slow peristaltic waves of contraction, not unlike those seen in vertebrate visceral smooth muscle. These muscles cannot be tetanized either by repetitive stimulation or by elevated potassium saline. The excitation-contraction (E-C) coupling mechanism was explored and compared with that known in crustacean skeletal muscle. Contraction is dependent on external Ca2+ which triggers the release of intracellular calcium from the sarcoplasmic reticulum (SR) via calcium-induced calcium release (CICR). Whereas contraction force is proportional to [Ca2+]o up to that in normal saline (13.4 mM), higher levels of Ca2+ reduce force. Ryanodine, which blocks calcium release from the SR, abolishes electrically stimulated contractions and CICR. Relaxation is achieved by removal of calcium from the cytosol in at least two ways, first by the re-loading of calcium into the SR by Ca2+-ATPases and second by the movement of calcium out of the cell by extruding it across the sarcolemma via Na+/Ca2+-exchangers. It is hypothesized that the inability of this muscle to show tetanus arises from inactivation of the voltage-gated calcium channels by high calcium. This is supported by the result that caffeine application causes an increase in tonus and size of phasic contractions by circumventing the sarcolemma and dumping SR calcium stores.

In situ and in vitro identification and characterization of cardiac ganglion neurons in the crab, Carcinus maenas.

The aim of this study was to investigate the intrinsic membrane properties and hormonal responses of individual central pattern generating neurons in the cardiac ganglion of the shore crab Carcinus maenas. Because the cardiac ganglion in this crustacean species is buried within the heart musculature and is therefore inaccessible for direct morphological and electrophysiological analysis, we developed two novel in vitro preparations. First, to make the ganglion accessible, we established a brief enzymatic treatment procedure that enabled us to isolate the entire cardiac ganglion, in the absence of muscle tissue. Second, a cell culture procedure was developed to isolate individual neurons in vitro. With the use of both isolated ganglionic and neuronal cell culture techniques, this study provides the first direct account of the neuroanatomy of the cardiac ganglion in shore crabs. We demonstrate that cultured neurons not only survived the isolation procedures, but that they also maintained their intrinsic membrane and transmitter response properties, similar to those seen in the intact ganglion. Specifically, we tested the peptides proctolin, crustacean cardioactive peptide, the FLRFamide-related peptide F2, and an amine (serotonin) on both isolated ganglion and in vitro culture neurons. We measured changes in neuronal burst rate, burst amplitude, pacemaker slope, and membrane potential oscillation amplitude in response to the above four hormones. Each hormone either increased neuronal activity in spontaneously bursting neurons, or induced a bursting pattern in quiescent cells. The in vitro cell culture system developed here now provides us with an excellent opportunity to elucidate cellular, synaptic and hormonal mechanisms by which cardiac activity is generated in shore crabs.

Neural Control of the Lateral Abdominal Arterial Valves in the Lobster Homarus americanus.

A dorsal abdominal artery in Homarus americanus runs the length of the abdomen, giving rise to one pair of large lateral arteries in each segment. These lateral arteries supply hemolymph to the abdominal muscles and the swimmerets. In addition, many small vessels leave the dorsal abdominal artery ventrolaterally to supply the gut and gonads. Bicuspid muscular valves are located at the junction of each segmental lateral artery with the dorsal abdominal artery, but not at the origin of the gut vessels. Nerves originating from the ventral abdominal ganglia travel along the lateral arteries to innervate the valves, providing both inhibitory and excitatory inputs. Inhibitory input produces hyperpolarizing inhibitory junctional potentials that relax the valve muscles, and in intact in situ perfused arteries causes increases in outflow from the affected lateral artery. Excitatory input produces depolarizing excitatory junctional potentials that close the valves and reduce perfusate outflow. The valve nerves also branch to innervate valves up to two segments anterior and one segment posterior. Application of exogenous {gamma}-aminobutyric acid hyperpolarizes valve muscle fibers. This and the hyperpolarizing effect of valve nerve stimulation are reversibly abolished by the application of picrotoxin (10-5 M). Acetylcholine (10-5 M), but not glutamate, causes depolarization and contraction of valves. The role of the valves in controlling the distribution of hemolymph flow is discussed.

Crustacean cardioexcitatory peptides may inhibit the heart in vivo.

Peptide neurohormones exist as functionally similar analogues in a wide variety of invertebrate and vertebrate phyla, and many have been implicated as cardiovascular regulators. In decapod crustaceans, these include the pentapeptide proctolin, crustacean cardioactive peptide (CCAP) and the FMRF amide-related peptides F1 and F2, all of which are found in the pericardial organs located immediately upstream of the heart. Cardioexcitatory activity has been demonstrated by these four peptides in both isolated and semi-isolated arthropod hearts; CCAP, however, has minimal effects on the heart of Cancer magister. In the present study, we determined the effects of proctolin, F1 and F2 on the heart of the crab C. magister in both in vitro (semi-isolated heart) and in vivo (whole animal) preparations. In semi-isolated hearts, infusion of each peptide caused cardioexcitation, increasing the rate and stroke volume of the heart. In whole crabs, the peptides were cardioinhibitory; the strongest effects were observed with F1 and F2, which dramatically decreased heart rate, cardiac stroke volume and cardiac output. These results cast doubt on current perceptions of the functional role of cardioactive peptides in the regulation of invertebrate cardiovascular performance in vivo.