A change of heart: cardiovascular development in the shrimp Metapenaeus ensis.

The larval development of penaeid shrimp is among the most complicated in crustaceans. In Metapenaeus ensis, there are six naupliar, three protozoeal and three mysid larval instars, followed by postlarval development. Irregular heartbeat begins late in naupliar instar 6. Co-ordinated beating at 400-600 beats min(-1) commences in the first protozoeal instar and continues throughout larval life. Initially, the contractile region is located more posteriorly in the cephalothorax and has a single pair of ostia, and the arterial distribution is limited to a single anterior vessel. In later mysid instars, a second cardiac pumping site develops posterior to, but connected with, the original site. This extension is more muscular, contains additional ostia and develops additional distribution vessels supplying the cephalothorax and abdominal areas. The original site is gradually merged into the new extension and only small refinements in the circulation occur in postlarval and juvenile life. Changes in physiological responses of the heart also occur throughout development. Responses to intra-pericardial microinjection of 5-hydroxytryptamine change drastically during development, as do cardiac responses to ambient hypoxia. Similarly, heartbeat of later juvenile instars is inhibited by injection of tetrodotoxin, while heartbeat of larval and early juvenile instars is not, suggesting that neurogenic regulation via the cardiac ganglion arises later in development. Our present studies attempt to integrate the anatomical and physiological changes in the development of the crustacean heart.

Respiratory and circulatory compensation to hypoxia in crustaceans.

Crustaceans are often tolerant of hypoxic exposure and many regulate O(2) consumption at low ambient O(2). In acute hypoxia, most increase branchial water flow, and many also increase branchial haemolymph flow, both by an increase in cardiac output and by shunting flow away from the viscera. The O(2)-binding affinity of crustacean O(2) carriers increases in hypoxic conditions, as a result of hyperventilation induced alkalosis. In chronic hypoxic exposure some crustaceans do not sustain high ventilatory pumping levels but increased effectiveness of O(2)-uptake across the gills is maintained as a result of the build up of metabolites such as lactate and urate which also function to increase the haemocyanin O(2)-binding affinity. Chronic exposure to hypoxia also may increase O(2)-binding capacity and promote the synthesis of new high O(2)-affinity carrier molecules. Exposure to untenable rates or levels of O(2) depletion causes many decapodan crustaceans to surface and ventilate the gills with air. Burrowing crayfish provide an example of animals, which excel in all these mechanisms. Control mechanisms involved in compensatory responses to hypoxia are discussed.

Control of cardiovascular function and its evolution in Crustacea.

Work in the last decade has shown that crustacean open circulatory systems are highly efficient and controlled in a complex manner. Control occurs at several levels. Myocardial contraction is initiated in the cardiac ganglion but constantly modulated by the central nervous system, both directly via the cardioregulatory nerves and indirectly via the neurohormonal system. Heart rate and stroke volume can be controlled independently and measurements of both are needed to assess cardiac output accurately. Haemolymph outflow from many arthropod hearts is via a complex multiarterial distribution system, and the regional distribution of cardiac output is tightly controlled via cardioarterial valves at the base of each artery. These valves contain innervated muscle, and differential contraction serves to regulate the efflux of oxygenated haemolymph into a particular system. The major influence on both the evolution and control of arthropod open blood vascular systems is efficiency of oxygen uptake and delivery. This influence is illustrated by reference to a variety of crustacean and other arthropod types.

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.

The FMRFamide-related peptides F1 and F2 alter hemolymph distribution and cardiac output in the crab Cancer magister.

The FMRFamide-related peptides F1 and F2, originally isolated from lobster pericardial organs, have been shown to exert cardioexcitatory effects on isolated or semi-isolated crustacean hearts. The present study sought to determine the in vivo effects of F1 and F2 on cardiac and circulatory performance of Cancer magister using a pulsed-Doppler technique. In general the effects of F1 and F2 were similar; however, F1 was more potent and its effects were of longer duration than those exerted by F2. Infusion of either F1 or F2 caused a decrease in heart rate and subsequent periods of acardia. These decreases in rate occurred concurrently with a short-term increase in stroke volume of the heart, followed by a longer-term decrease in stroke volume. Hemolymph flow rates through the anterior aorta, anterolateral arteries, sternal artery, and posterior aorta also showed the same trend, with an initial short-term increase in flow rate followed by a longer-term decrease with periods of ischemia. Hemolymph flow through the paired hepatic arteries simply decreased, but recovery to pretreatment levels was faster than in the other arterial systems. Threshold for these responses occurred at circulating concentrations between 10(-9) mol.l-1 and 10(-8) mol.l-1 for F1 and somewhat higher, between 10(-8) mol.l-1 and 10(-7) mol.l-1, for F2.

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).

Acid-Base and Ionic Regulation, During and Following Emersion, in the Freshwater Bivalve, Anodonta grandis simpsoniana (Bivalvia: Unionidae).

Specimens of the boreal clam, Anodonta grandis simpsoniana were emersed at 10{deg}C for 6 days and then reimmersed for 24 h. The clams lost water at a rate of 1.6% total water per day. After 144 h of emersion, water weight had declined by almost 15%, while extracellular fluid (ECF) osmolality had increased 30% to 52 mOsm kg-1. Control levels were reattained after 6 h reimmersion. ECF Po2 declined rapidly in the first 24 h of emersion, but remained near 20 Torr for the full 6-day exposure. After an initial rapid fall, pH declined at a slower rate, reaching 7.494 +/- 0.037 (mean +/- SEM) at 144 h. Pco2 was elevated from 0.6 +/- 0.6 to 12.4 +/- 1.1 Torr after 96 h, but no further increase was noted. ECF [Ca] increased threefold to 13.1 +/- 0.8 mmol l-1, while [HCO3app] rose from 5.4 +/- 0.3 to a maximum of 12.9 +/- 0.8 mmol 1-1 after 144 h. ECF [Na] and [Cl] were not affected by emersion. On reimmersion, recovery was rapid, with pH, Po2 and Pco2 returning to control within 2 h, while [Ca] and [HCO3app] remained elevated until 24 h after reimmersion. A 1:1 stoichiometry between [Ca] and [HCO3app] existed throughout the emersion and reimmersion periods. In the absence of protein buffers, the fall in ECF pH was arrested by the mobilization of calcium carbonate, presumably from the shell. By 96 h emersion Pco2 and Po2 had stabilized, suggesting that diffusion gradients sufficient to allow limited gas exchange had been established.

Facilitation of CO2 excretion by carbonic anhydrase located on the surface of the basal membrane of crab gill epithelium.

An isolated perfused crab gill preparation was used to test the hypotheses that crab gill carbonic anhydrase (CA) catalyzes the efflux of CO2 from the hemolymph, which lacks the enzyme, to the ambient medium and that the CA is localized on the luminal surface of the basal membrane. It was found that the efflux of CO2 from the internal perfusate was sensitive to the flow rate of the internal perfusate through the gill (and thus the residence time within the gill). The sensitivity of the CO2 efflux to residence time was nearly abolished upon treatment of the gill with an impermeable dextran-bound CA inhibitor. It is concluded that CA present on the luminal surface of the gill epithelium facilitates CO2 excretion by catalyzing the dehydration of the large hemolymph bicarbonate pool to the more diffusible molecular CO. The action of the enzyme is important in maintaining a CO2 gradient between hemolymph and water in a situation where hemolymph PCO2 is normally low, water PCO2 is variable, and the gills themselves are a source of metabolic CO2.

Acid-base regulation during exercise and recovery in the blue crab, Callinectes sapidus.

During swimming activity Callinectes sapidus incurred a severe hemolymph acidosis due to elevated levels of PCO2 and lactate. Although hemolymph lactate concentration rose steadily during 1 h of exercise, hemolymph pH was maximally depressed within the first 15 min. A discrepancy between the quantities of lactate and H+ released from the tissues into the hemolymph is explained by a large apparent efflux of H+ into the ambient seawater, presumably via a branchial ion exchange process. Ammonia excretion increased 6 fold during exercise, but it is not clear if this contributed to the excretion of H+. The total quantity of H+ excreted into the environment during exercise and recovery far exceeded that which could be attributed to hemolymph lactic acid, indicating that some of the excreted H+ must have originated from other sources, such lactic acid which dissociated intracellularly, with the lactate anions remaining in the cells. Because lactate anions increase hemocyanin O2 affinity when unopposed by the Bohr shift, the excretion of a large portion of the metabolic H+ load, leaving lactate behind in the hemolymph, has important consequences for the regulation of O2 transport during swimming.

Non-equilibrium acid-base status in C. productus: role of exoskeletal carbonate buffers.

Hemolymph acid-base variables (pH, PCO2 and CCO2), hemolymph Ca2+ and Na+ concentrations, and osmolality were measured in unrestrained crabs, Cancer productus, before, during and following 4 hr emersion and 43 hr hyperoxia (460-510 Torr), both at 10 degrees C. Emersion and hyperoxia provoked an acidosis associated with elevation of hemolymph CCO2 and PCO2, yet attempts to calculate PCO2 from measured pH and CCO2 always resulted in values greater than those measured directly. This discrepancy between measured and calculated PCO2, was associated with base excess, and was eliminated upon in vitro equilibration of the hemolymph and more slowly in vivo, suggesting that metabolic compensation for the acidosis occurred more rapidly than could acid-base equilibration. During emersion, increases of CCO2 and [Ca2+] provide evidence that the internal CaCO3 stores, possibly from the exoskeleton, were mobilized during acid-base compensation. Hyperoxia provoked no such increase in Ca2+, and branchial uptake of HCO3- may make a major contribution to the elevation of CCO2 during hyperoxia. It is suggested that shell buffering by aquatic crustaceans provides a means of compensation for acidosis under conditions during which branchial function is impaired.