Temperature acclimation alters cardiac performance in the lobster Homarus americanus

The American lobster is a poikilotherm that inhabits a marine environment where temperature varies over a 25°C range and depends on the winds, the tides and the seasons. To determine how cardiac performance depends on the water temperature to which the lobsters are acclimated we measured lobster heart rates in vivo. The upper limit for cardiac function in lobsters acclimated to 20°C is approximately 29°C, 5°C warmer than that measured in lobsters acclimated to 4°C. Warm acclimation also slows the lobster heart rate within the temperature range from 4 to 12°C. Both effects are apparent after relatively short periods of warm acclimation (3–14 days). However, warm acclimation impairs cardiac function at cold temperatures: following several hours exposure to frigid (<5°C) temperatures heart rates become slow and arrhythmic in warm acclimated, but not cold acclimated, lobsters. Thus, acclimation temperature determines the thermal limits for cardiac function at both extremes of the 25°C temperature range lobsters inhabit in the wild. These observations suggest that regulation of cardiac thermal tolerance by the prevailing environmental temperature protects against the possibility of cardiac failure due to thermal stress.     

Neuromuscular synapses on the dactyl opener muscle of the lobster Homarus americanus

The crustacean dactyl opener neuromuscular system has been studied extensively as a model system that exhibits several forms of synaptic plasticity. We report the ultrastructural features of the synapses on dactyl opener of the lobster (Homarus americanus) as determined by examination of serial thin sections. Several innervation sites supplied by an inhibitory motoneuron have been observed without nearby excitatory innervation, indicating that excitatory and inhibitory inputs to the muscle are not always closely matched. The ultrastructural features of the lobster synapses are generally similar to those described previously for the homologous crayfish muscle, with one major distinction: few dense bars are seen at the presynaptic membranes of these lobster synapses. The majority of the lobster neuromuscular synapses lack dense bars altogether, and the mean number of dense bars per synapse is relatively low. In view of the finding that the physiology of the lobster dactyl opener synapses is similar to that reported for crayfish, these ultrastructural observations suggest that the structural complexity of the synapses may not be a critical factor determining synaptic plasticity.This work was supported by funds from the University of Virginia Research and Development Committee.     

Electrophysiological and structural studies on the heart muscle of the lobster Homarus americanus

The electrophysiological properties and structure of heart muscle fibres of the lobster myocardium have been investigated. Discontinuities were observed in the expected decrement of electrotonic potentials in a study of the passive membrane properties of the muscle. Injection of Procion yellow revealed transverse barriers to dye movement, identified in electron micrographs as intercellular junctions. Unlike the intercalated discs of the vertebrate myocardium, which these regions superficially resemble, nexus regions of ;close' or ;gap' junctions are absent. The intercellular cleft is generally 400 A in width, locally reduced to 75-100 A The functional implications of these findings are discussed. Cardiac myoneural junctions, which include regularly repeating structures extending into the synaptic cleft from the postsynaptic membrane, are described.     

Nerve terminals and synapses in the cardiac ganglion of the adult lobster Homarus americanus

The cardiac ganglion in the lobster Homarus americanus was examined with a transmission electron microscope. Nerve terminals often existed in large aggregations surrounded by glial and connective tissue elements. Axo-axonic and axo-dendritic synapses were present. Six ultrastructurally different types of nerve terminal, each containing an abundance of vesicles, were distinguished: three formed discrete chemical synapses as indicated by typical release site morphology; three did not. The latter appear to be neurosecretory axon terminals of extrinsic neurons. More than one morphologically distinct type of synaptic vesicle occurred commonly in a given terminal, suggesting the presence of coexisting neurotransmitters and/or neuroregulatory factors. Symmetrical chemical synapses and electrotonic junctions between axons were present.     

Excitation-contraction coupling in cardiac muscle revisited.

According to the current views the direct and indispensable source of Ca2+ activating contraction is sarcoplasmic reticulum (SR). Ca2+ is released from the SR when its release channels (ryanodine receptors) are activated by Ca2+ influx through the L-type Ca2+ channels (dihydropyridine receptors). In contrast, ryanodine receptors of skeletal muscles are activated by conformational changes in dihydropyridine receptors induced by sarcolemmal voltage. Ca2+ influx is not necessary for their activation. In this review the papers not quite conforming with the current views are referred to and discussed. Their results suggest that SR is not an indispensable source of contractile Ca2+ at least in some mammalian species, and that cardiac ryanodine receptors may be activated by conformational changes in dihydropyridine receptors without Ca2+ influx (like in skeletal muscle). This may be a mechanism parallel to or accessory to the Ca2+ induced release of Ca2+ (CIRC).     

Dopamine and 5-hydroxytryptamine actions on the cardiac ganglion of the lobster Homarus americanus

The cardioexcitor monoamines dopamine (DA) and 5-hydroxytryptamine (5HT) accelerate bursting by isolated cardiac ganglia of the lobster Homarusamericanus most effectively when they act on a region of the ganglionic trunk anterior to the small cells which have been considered the pacemakers of the system. 5HT may exert its acceleratory action by depolarizing cell processes. Neither the somata nor the spike-initiating zones of the small cells have to be directly exposed to 5HT or DA in order for acceleration to occur. When 5HT is applied selectively to the small cells bursts are prolonged, probably as a result of increases in the duration of the endogenous burst-organizing potentials (driver potentials) generated by these neurons. This action on the small cells can lead to prolonged and intensified bursts of the full ganglion during the onset of 5HT action when the whole ganglion is exposed to the monoamine. Neither DA nor 5HT has a direct effect on the characteristics of large cell (motorneuron) driver potentials. Accepted: 3 September 1997     

Thermosensitivity of the lobster, Homarus americanus, as determined by cardiac assay

It is generally accepted that crustaceans detect, and respond to, changes in water temperature, yet few studies have directly addressed their thermosensitivity. In this investigation a cardiac assay was used as an indicator that lobsters (Homarus americanus) sensed a change in temperature. The typical cardiac response of lobsters to a 1-min application of a thermal stimulus, either warmer (n = 19) or colder (n = 17) than the holding temperature of 15 degrees C, consisted of a short bradycardia (39.5 +/- 8.0 s) followed by a prolonged tachycardia (188.2 +/- 10.7 s). Lobsters exposed to a range of rates of temperature change (0.7, 1.4, 2.6, 5.0 degrees C/min) responded in a dose-dependent manner, with fewer lobsters responding at slower rates of temperature change. The location of temperature receptors could not be determined, but lesioning of the cardioregulatory nerves eliminated the cardiac response. Although the absolute detection threshold is not known, it is conservatively estimated that lobsters can detect temperature changes of greater than 1 degree C, and probably as small as 0.15 degrees C. A comparison of winter and summer lobsters, both held at 15 degrees C for more than 4 weeks, revealed that although their responses to temperature changes were similar, winter lobsters (n = 18) had a significantly lower baseline heart rate (34.8 +/- 4.4 bpm) and a shorter duration cardiac response (174 s) than summer lobsters (n = 18; 49.9 +/- 5.0 bpm, and 320 s respectively). This suggests that some temperature-independent seasonal modulation of cardiac activity may be occurring.     

Properties of the 23,000-Da phosphoproteins in cardiac sarcolemma and sarcoplasmic reticulum

The calmodulin- and cAMP-dependent protein kinase-mediated phosphorylations of isolated sarcolemma and sarcoplasmic reticulum vesicles have been compared. Similarities in the calmodulin-mediated phosphorylation of the sarcolemma and sarcoplasmic reticulum 23,000-Da phosphoproteins included their Mg2+, Na+, Ca2+, and calmodulin sensitivities, as well as the size of their dissociated subunits. In contrast, a number of differences between these phosphoproteins were indicated in their sensitivity to detergents (Triton X-100 and sodium dodecyl sulfate) and calmodulin antagonists (R24571 and trifluoperazine). Furthermore, in contrast to the sarcoplasmic reticulum phosphoprotein, the sarcolemma phosphoprotein could not be affinity labeled with 125I-calmodulin. While these results indicate the probable chemical similarity of the sarcolemma and sarcoplasmic reticulum 23,000-Da phosphoproteins, they also indicate there are differences in the lipid/phosphoprotein interactions in these two membranes.     

Comparison of cyclic AMP-dependent phosphorylation of sarcolemma and sarcoplasmic reticulum from rat cardiac ventricle muscle

1. The phosphorylation by cAMP and protein kinase I of rat cardiac sarcolemma (SL) and sarcoplasmic reticulum (SR) isolated from the same homogenate, was compared. 2. In both fractions, the phosphate incorporation is strongly dependent on the ATP and the membrane protein concentration. 3. SDS-gel electrophoresis reveals that in the SL preparation a protein of Mr = 24,500 and a glycoprotein of Mr = 17,500 are mainly phosphorylated, while in the SR fraction the main phosphate incorporation is found in a protein having a Mr = 37,000. 4. Isoprenaline stimulates the phosphorylation of SL but not of SR. Propranolol abolished that stimulatory action of isoprenaline completely, suggesting that the beta-adrenoceptor is involved.     

A new mycosis of larval lobster (Homarus americanus).

Spodoptera exempta larvae were reared on semisynthetic maize diet. Pathogenicity studies were undertaken on first- to fifth-instar larvae fed a high dosage of Nosema necatrix spores. Larvae from the earlier instars were most susceptible to the microsporidan and also developed bacteriosis. A cytoplasmic polyhedrosis virus (CPV) was evident in some infected larvae but not in controls. The development of N. necatrix is redescribed using the light microscope. A disporoblastic life cycle was evident at 25°C and both a disporoblastic and an octosporoblastic life cycle at 20°C. The implications of the occurrence of bacteriosis and CPV and the possible biological significance of the two sporogonic sequences are discussed. The taxonomic position of N.necatrix is reviewed and, after comparison with existing species of the genera Nosema and Parathelohania, it is placed in the new genus Vairimorpha. The implications of polymorphism are discussed in relation to the classification of the Microsporida.     

The mandibular organ of the lobster,Homarus americanus

The lobster mandibular organ is well vascularized and its polygonal cells are arranged loosely around blood vessels and blood sinuses. Numerous mitochondria and microbodies (peroxisomes) give the acidophilic cytoplasm a finely granular appearance, but there is no evidence of secretory granules. The abundant endoplasmic reticulum is almost entirely agranular and occurs in two morphologically distinct forms: tubular and cisternal. The tubular reticulum is randomly distributed and may represent the site of synthesis and transport of the mandibular organ product. The cisternal reticulum is frequently associated with microbodies. Both forms of endoplasmic reticulum proliferate during mid to late premolt. Mandibular organ ultrastructure closely resembles that of cells known to synthesize steroids or lipids, which suggests that this organ may have a similar function. There is no functional evidence of involvement in molt control in Homarus, but ultrastructural and other evidence suggests an analogy with insect corpus allatum.     

Oxygen consumption of the lobster,Homarus americanus milne-edwards

Summary 1. Oxygen consumption by a group of 25 lobsters was essentially constant over a range of ambient oxygen concentrations from 1.0 to 8.5 mg/l. Consumption by groups of 35 and 50 lobsters at 15° C decreased as the concentration decreased.2. Oxygen consumption by individuals at 10° and 15° C increased as the oxygen concentration increased.3. Oxygen consumption increased as activity increased with crowding.4. Oxygen consumption almost doubled after feeding.5. Oxygen consumption per unit weight decreased with increasing size.6. The average rate of oxygen consumption by individuals doubled over the temperature range 12° to 25° C.7. Oxygen consumption in air at 6° to 25° C was much less than in water.

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Sauerstoffverbrauch des HummersHomarus americanus Milne-Edwards

Kurzfassung Bei einer Gruppe von 25 Hummern wurde der Sauerstoffverbrauch in Dauerfluß-Respirometern gemessen. Bei 10° C erwies er sich als im wesentlichen konstant über einen Bereich der Sauerstoffkonzentration im umgebenden Wasser von 1,0 bis 8,5 mg/l. Bei Gruppen von 35 und 50 Hummern, welche bei 15° C getestet wurden, nahm der Sauerstoffverbrauch jedoch mit fallender Sauerstoffkonzentration etwas ab. Anstieg der Individuenzahl pro Raumeinheit (crowding) führte zu steigender Bewegungsaktivität und zu erhöhtem Sauerstoffverbrauch. Nahrungsaufnahme verursachte fast eine Verdoppelung des Sauerstoffverbrauchs. Kleine Individuen verbrauchen pro Gewichtseinheit mehr Sauerstoff als große. In manometrischen Respirometern stieg der Sauerstoffverbrauch bei hohen Sauerstoffkonzentrationen im umgebenden Wasser mit der Temperatur. Bei 6° bis 25° C war der Sauerstoffverbrauch in der Luft wesentlich geringer als im Wasser.

     

Electron microscopy of stomatogastric ganglion in the lobster Homarus americanus

The stomatogastric ganglion and two of the associated afferent and efferent nerve trunks (stomatogastric and dorsal ventricular nerves) from Homarus americanus have been examined with light and electron microscopy after glutaraldehyde-osmium tetroxide fixation. The dorsally located neuron somata, rich in ribosomes and glycogen, are encased in multi-layered glial and fibrous sheaths. The synaptic neuropil regions occur scattered throughout the central and ventral part of the ganglion, interspersed amonglarger nerve fibres of extrinsic and intrinsic origin from which the neuropil is derived. Neural processes containing masses of small clear vesicles plus larger dense-core vesicles make apparent synaptic contacts at points of increased membrane density with smaller, non-vesicle-containing or sometimes other vesicle-containing nerve fibres.     

Neurohormonal alteration of integrative properties of the cardiac ganglion of the lobster Homarus americanus.

The spontaneous burst discharges of isolated lobster (Homarus americanus) cardiac ganglia were recorded with a spaced array of electrodes. Small regions (less than 1 mm) of the ganglion were exposed to the cardioexcitor neurohormone in extracts of pericardial organs (XPO) or to 10(-5) M 5-hydroxytryptamine (5HT). All axons were excited (increased mean firing frequency, f) by both substances, but only by applications in the region between the soma (but excluding it) and proximal site of impulse initiation. Units not so exposed changed their f relatively little despite f increases of as much as threefold in exposed units and changes in burst rate and overall length. Regularity and grouping of all impulse activity into bursts was never disturbed. 5HT increases burst rate at any point of application. The increases are larger if small cells are affected than if only large cells are exposed. Burst length decreases except when the pacemaker is affected. In contrast, XPO affects neither burst rate or length unless small cells are affected. Length is increased if non-pacemaker small cells are affected; both rate and length increase if the pacemaker is affected. The pacemaker usually exhibits an f of intermediate value. Rate changes are not simply related to its f. A small cell can "burst" in the absence of impulses from any other cells. XPO may enhance endogenous "driver potentials," while 5HT may excite by depolarizing at limited sites.