Microspectrofluorometric study of monoamines in the auricle of the heart of Protopterus aethiopicus

Summary The auricle of the heart of Protopterus aethiopicus contains large numbers of chromaffin cells, often lying immediately adjacent to the endothelium and displaying a bright blue-white fluorescence characteristic for catecholamines after formaldehyde treatment (Falck and Owman 1965). These results combined with X-ray microanalysis after initial fixation with glutaraldehyde and subsequent treatment with dichromate established that these chromaffin cells are the storage site of primary catecholamines (Scheuermann 1978, 1979, 1980; Scheuermann et al. 1980). The aim of the present pilot study was to demonstrate in these cells noradrenaline (NA) or dopamine (DA), or a mixture of both. The evaluation of the excitation spectra of the catecholamine fluorophore transformed by treatment with HCl vapour (excitation maxima at 320 and 370 nm) and the excitation-peak ratio analysis (peak ratio 370/320 nm =1.05–1.5; and 320/280 nm >1.5) identify DA as the primary catecholamine stored in these chromaffin cells. The low fading rate of the monoamine fluorescence after acidification confirms the presence of DA. These microspectrofluorometric findings demonstrate that chromaffin cells in the auricle of the Protopterus heart, which are a part of the medullary homologue of the adrenal gland of higher vertebrates, contain a primary catecholamine, namely DA.     

The morphology of the lung of the African lungfish,Protopterus aethiopicus

Summary The lung of the African lungfish (Protopterus aethiopicus) is paired, long and cylindrical. It is situated on the dorsal aspect of the coelomic cavity ventral to the ribs. Much of the gas exchange tissue is found in the proximal aspect of the lung with the caudal part largely taken up by a centrally situated air-duct with a few large peripherally located alveoli. Interalveolar septa, arranged at differing hierarchical levels from the air-duct, subdivide the lung into alveoli, the gas exchange compartments. The alveolar surface is covered by some cells characterized by microvilli on their free surface, while others are devoid of such structures. The general organization of the lung of Protopterus aethiopicus is similar to that of the other genera of Dipnoi, Neoceratodus and Lepidosiren, with the interalveolar septa increasing the surface area for gas exchange through pulmonary compartmentation. The abundant septal smooth muscle fibres and elastic tissue may contribute to the physiomechanical compliance of the lung. The undifferentiated alveolar pneumocytes and the double capillary system, observed in Protopterus, in general appear to characterize the very primitive lungs of the lower air-breathing vertebrates.     

A radiological study of the central circulation in the lungfish, Protopterus aethiopicus

The dipnoan heart is only in part structurally developed to support a separated circulation in pulmonary and systemic circuits. In the present investigation biplane angiocardiography has been used to describe the extent of such a double circulation and the factors which may modify it in the African lungfish, Protopterus aethiopicus. Contrast injections in the pulmonary vein revealed a clear tendency for aerated blood returing from the lungs to be selectively dispatched to the anterior branchial arteries giving rise to the major systemic circulation. Contrast injections in the vena cava delineated the sinus venosus as a large receiving chamber for systemic venous blood. Contraction of the sinus venosus discharged blood into the right, posterior part of the partially divided atrial space. Contrast injection in the pulmonary vein showed that vessel to pass obliquely from right to left such that blood was emptied distinctly into the left side of the atrium. During contraction the atrial space tended to retain a residual volume in its anterior undivided part which minized mixing. Ventricular filling occurred through separate right and left atrio-ventricular connections. Right-left separation in most of the ventricle was maintained by the partial ventricular septum, the trabeculated, spongelike myocardium and the mode of inflow from the atria. Mixing in the anterior undivided portion of the ventricle during the ejection phase was slight due to a streamlined ejection pattern. The outflow through the bulbus cordis occurred in discrete streams which in part were structurally separated by well developed spiral folds. In the anterior bulbus segment the spiral folds are fused and make completely separate dorsal and ventral outflow tracts. The ventral bulbus channel provides blood to the three anterior branchial arteries. The second and third branchial arteries are large and represent direct shunts to the dorsal aorta. The fourth and fifth branchial arteries are gill bearing and receive blood form the dorsal bulbus channel. The most posterior epibranchial vessels give rise to the pulmonary arteries.     

Intranuclear bundles of microfilaments and microtubules in chromaffin cells of the auricle of the heart of a lungfish, Protopterus aethiopicus

Intranuclear microtubular-microfilamentous rod-like inclusions were investigated in chromaffin cells of the auricle of the heart of lungfishes. In conventional electron microscopy, these inclusions reveal a wide variety in appearance, depending on their orientation to the plane of sectioning. Whereas originally they were merely interpreted as a bundle of microfilaments, application of a goniometer stage showed the rod- or spindle-shaped intranuclear inclusions to have a basic substructure of parallel arranged microtubules among microfilaments, which are clearly connected to chromatin granules, occasionally penetrating dense areas of chromatin. The chemical nature and biological significance of these structures, which so far remain enigmatic, are discussed.     

Ultrastructural study on nuclear-cytoplasmic relationships in oocytes of the African lungfish,Protopterus aethiopicus

Summary Growing oocytes of Protopterus, like those of some amphibians and teleosts, show an impressive development of the nucleolar apparatus. Numerous nucleolus-like bodies establish close spatial relationships with the nuclear envelope by extending pedicels and streams of finely dispersed material towards the inner membrane.At such contact points, gaps in the perinuclear cistern are more frequent than elsewhere along the nuclear boundary. Expansion of the outer nuclear membrane gives rise to blebs, with or without visible content, and these become pinched off to form small vesicles in the perinuclear cytoplasm.Small, electron dense aggregates, indistinguishable from nucleolar material occur on both sides of the nuclear envelope opposite to each other, some being connected by a slender portion of the same material within a nuclear pore. Such accumulations are interpreted as detached parts of nucleolar bodies in transit to cytoplasmic sites where they presumably participate in the biogenesis of ribosomes. At the height of nucleolar emission, nucleoplasm and perinuclear cytoplasm are so rich in small electron dense particles that they are almost indistinguishable from each other.At this stage of massive transport, the route provided by the nuclear pores seems to be insufficient and another, more spacious, gateway may be in operation. The latter involves direct passage of material across the nuclear membranes preferentially where these form blebs.This view is supported not only by the overt spatial relationships between nucleolar pedicels and blebs, but by the occurrence within perinuclear lacunae and blebs of particles that seem to be derived from nucleolar bodies. Furthermore, frequent interruptions in the nuclear membranes preferentially located where they expand into outpocketings suggest that at these sites temporary gateways may exist in the living cell that permit easy access of intranuclear components to the cytoplasm.Supported by grants AM-3984, NB-00840, and NB-05219 from the U.S.P.H.S.     

Respiratory allocation and standard rate of metabolism in the African lungfish, Protopterus aethiopicus

This paper quantifies the relationship between respiratory allocation (air vs. water) and the standard rate of metabolism (SMR) in the primitive air-breathing lungfish, Protopterus aethiopicus. Simultaneous measurements of oxygen consumed from both air and water were made to determine the SMR at ecologically relevant aquatic oxygen levels for juveniles 2 to 221 g. Total metabolic rate was positively correlated with body mass with a scaling exponent of 0.78. Aerial oxygen consumption averaged 98% (range=94% to 100%) of total respiratory allocation under low aquatic oxygen levels. Measurements of oxygen consumption made across a gradient of dissolved oxygen from normoxia to anoxia showed that P. aethiopicus maintains its SMR despite a change in respiratory allocation between water and air.     

Biology of the marbled lungfish, Protopterus aethiopicus Heckel, in Lake Baringo, Kenya

In this paper we provide life history characteristics for an introduced marbled lungfish (Protopterus aethiopicus) population in Lake Baringo, Kenya, including their length–weight relationship, body condition, fecundity, sex ratio, reproduction and food. These data are compared with those of other populations. Samples were obtained from the commercial fishery between January and October 2001. The lungfish exhibited positive allometric growth, with the length–weight relationship described by the equation: logW = ?6.41 + 3.52 logTL. The mean relative condition factor was close to unity, did not vary much between months and was not significantly different between sexes. The overall sex ratio was skewed in favour of females. The size of fish at first maturity was 70–76 cm total length (TL) for females and 82–88 cm TL for males. Fish in maturity stages III–IV occurred in all monthly samples suggesting year round spawning. The marbled lungfish in Lake Baringo is largely piscivorous and its impact on other fishes certainly warrants further research.     

Decline of the African lungfish (Protopterus aethiopicus) in Lake Victoria (East Africa)

Catch and effort data for the period 1973–1990 demonstrate a dramatic decline of lungfish in the Tanzanian waters of Lake Victoria. Bottom trawl catches in the Mwanza Gulf showed a decline in catch rates from 67.5 kg h⁻¹ in 1973 to 5.5 kg h⁻¹ in 1986. Trawling of commercial vessels in the Speke Gulf revealed a decline in lungfish catches from 1.3 kg h⁻¹ in 1986 to 0.07 kg h⁻¹ in 1990. The development of anoxia in the deeper waters of Lake Victoria, the algal blooms, and the decline of water transparency, all associated with eutrophication, are not likely to have contributed to the decreased catch rate. However, the lungfish decline may reflect the interaction of overexploitation by the fishery and a low level of Nile perch predation that restricts lungfish to wetland refugia. We suggest that this may have been reinforced over the past few decades by large-scale conversion of wetlands to agricultural land and harvesting of nest-guarding male lungfish leading to decreased recruitment of young.     

Gap junctions

Electrical coupling through gap junctions constitutes a mode of signal transmission between neurons (electrical synaptic transmission). Originally discovered in invertebrates and in lower vertebrates, electrical synapses have recently been reported in immature and adult mammalian nervous systems. This has renewed the interest in understanding the role of electrical synapses in neural circuit function and signal processing. The present review focuses on the role of gap junctions in shaping the dynamics of neural networks by forming electrical synapses between neurons. Electrical synapses have been shown to be important elements in coincidence detection mechanisms and they can produce complex input-output functions when arranged in combination with chemical synapses. We postulate that these synapses may also be important in redefining neuronal compartments, associating anatomically distinct cellular structures into functional units. The original view of electrical synapses as static connecting elements in neural circuits has been revised and a considerable amount of evidence suggests that electrical synapses substantially affect the dynamics of neural circuits.     

Evolution of tissue-specific keratins as deduced from novel cDNA sequences of the lungfish Protopterus aethiopicus

Lungfishes are possibly the closest extant relatives of the land vertebrates (tetrapods). We report here the cDNA and predicted amino acid sequences of 13 different keratins (ten type I and three type II) of the lungfish Protopterus aethiopicus. These keratins include the orthologs of human K8 and K18. The lungfish keratins were also identified in tissue extracts using two-dimensional polyacrylamide gel electrophoresis, keratin blot binding assays and immunoblotting. The identified keratin spots were analyzed by peptide mass fingerprinting which assigned seven sequences (inclusively Protopterus K8 and K18) to their respective protein spot. The peptide mass fingerprints also revealed the fact that the major epidermal type I and type II keratins of this lungfish have not yet been sequenced. Nevertheless, phylogenetic trees constructed from multiple sequence alignments of keratins from lungfish and distantly related vertebrates such as lamprey, shark, trout, frog, and human reveal new insights into the evolution of K8 and K18, and unravel a variety of independent keratin radiation events.     

Neuroepithelial endocrine cells in the lung of the lungfish Protopterus aethiopicus. An electron- and fluorescence-microscopical investigation

The occurrence and distribution of neuroepithelial endocrine (NEE) cells was demonstrated electron- and fluorescence-microscopically in the lungfish Protopterus aethiopicus. They were only found to occur solitarily in the basal part of the cilio-mucous epithelium which is restricted to the pneumatic duct and adjacent parts of the common anterior chamber. The NEE cells show a yellow, formaldehyde-induced fluorescence. Electron-microscopically, all the NEE cells are characterized by membrane-bound electron-dense secretory granules with varying diameters, ranging from 75 to 150 nm. These granules are distributed throughout the cytoplasm with a higher concentration in the basal region. The NEE cells were regularly found to contain paracrystalline inclusions with a tubule-like substructural arrangement. A small part of the NEE cells appeared to reach the luminal surface by means of a long slender process bearing specialized beaded microvilli on its apical pole. Intraepithelial nerve fibres, with the ultrastructural characteristics of afferent fibres, were found running parallel to the airway surface. Nerve profiles, largely resembling the latter, can be seen in the proximity of the basolateral plasma membrane of the NEE cells. In addition, nerve terminals containing an aggregation of small clear vesicles are in close contact with the NEE cells. In conclusion, it appears that, as has so far been assumed in higher vertebrates, the NEE cells in the lung of Protopterus may perceive changes in the airway gases whereupon they could respond by releasing a chemical modulator, influencing contacting afferent nerve terminals or nearby smooth muscle bundles. Furthermore, intraepithelial nerve fibres or NEE cells might be stretch-sensitive.     

Reproductive biology of the East African lungfish (Protopterus aethiopicus) in Mwanza Gulf, Lake Victoria

The population of lungfish (Protopterus aethiopicus (Heckel)) was sampled by bottom trawling for 12 months from June 1984 to May 1985. A total of 576 lungfish were investigated. The sex ratio in the population was 1 male : 1 -98 females. Average size at maturity was found to be 960cm total length (TL) but the smallest matured male measured 72-9 cm and the female 840 cm TL. It was not possible to establish spawning times and reproductive activity trends either from the variation in relative condition factor ‘kn’ or from gonadosomatic index (GSI). Whereas male lungfish seem to show no further increase in GSI after gonadal maturity stage III, females exhibit GSI values that vary directly with oocyte maturation and ovarian weights. Fat deposited along the gonads was found to be a better basis for assessing maturation and spawning in lungfish. The potential spawning period was found to be from September to May. Absence of lungfish with spent gonads over the 12-month period and the presence of different egg sizes in ovaries, suggest that the fish is a multiple spawner. Fecundity was found to range from 705 eggs (in 84-cm TL fish) to 14,922 eggs (in 130-cm TL Fish).     

Ornithine-urea cycle and urea synthesis in African lungfishes, Protopterus aethiopicus and Protopterus annectens, exposed to terrestrial conditions for six days

The objectives of this study were (1) to determine the type of carbamoyl phosphate synthetase (CPS) present, and the compartmentalization of arginase, in the livers of the African lungfishes, Protopterus aethiopicus and Protopterus annectens, and (2) to elucidate if these two lungfishes were capable of increasing the rates of urea synthesis and capacities of the ornithine-urea cycle (OUC) during 6 days of aerial exposure without undergoing aestivation. Like another African lungfish, Protopterus dolloi, reported elsewhere, the CPS activities from the livers of P. aethiopicus and P. annectens had properties similar to that of the marine ray (Taeniura lymma), but dissimilar to that of the mouse (Mus musculus). Hence, they possessed CPS III, and not CPS I as reported previously. CPS III was present exclusively in the liver mitochondria of both lungfishes, but the majority of the arginase activities were present in the cytosolic fractions of their livers. Glutamine synthetase (GS) activity was also detected in the hepatic mitochondria of both specimens. Therefore, our results suggest that the evolution of CPS III to CPS I might not have occurred before the evolution of extant lungfishes as suggested previously, prompting an examination of the current view on the evolution of CPS and OUC in vertebrates. Aerial exposure led to significant decreases in rates of ammonia excretion in P. aethiopicus and P. annectens, but there were no accumulations of ammonia in their tissues. However, urea contents in their tissues increased significantly after 6 days of aerial exposure. The estimated rates of urea synthesis in P. aethiopicus and P. annectens increased 1.2- and 1.47-fold, respectively, which were smaller than that in P. dolloi (8.6-fold) reported elsewhere. In addition, unlike P. dolloi, 6 days of aerial exposure had no significant effects on the hepatic CPS III activities of P. aethiopicus and P. annectens. In contrast, aerial exposure induced relatively greater degrees of reductions in ammonia production in P. aethiopicus (34%) and P. annectens (37%) than P. dolloi (28%) as previously reported. Thus, our results suggest that various species of African lungfishes respond to aerial exposure differently with respect to nitrogen metabolism and excretion, and it can be concluded that P. aethiopicus and P. annectens depended more on reductions in ammonia production than on increases in urea synthesis to ameliorate ammonia toxicity when exposed to terrestrial conditions.