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Larval zebrafish (Danio rerio) of two different age classes ('swim-up' larvae, 9 days old; 'free-swimming' larvae, 21 days old) were exposed to either an endurance/continuous training or interval training. Control animals were kept in stagnant water. A comparison of cardiac activity of trained (either endurance or interval) and untrained animals at the end of the training regime revealed no differences in heart rate, end-diastolic and end-systolic ventricular volume, and cardiac output. Training also had no influence on the concentration of erythrocytes in the blood. Thus, at the level of total oxygen transport in the blood, training did not provoke any improvement during the first 32 days of development. Significant changes, however, were observed at the tissue level. In free-swimming larvae [i.e., between 21 and 32 days postfertilization (dpf)] endurance training increased the capillarization of both axial muscle caudal to the anus and the tail fin. In addition, mitochondrial density of red and intermediate muscle fibers increased significantly. In contrast to capillarization, even swim-up larvae, trained between 9 and 15 dpf, were affected. The observed increase in mitochondrial content indicates a high demand for oxygen and energy-rich metabolites for oxidative phosphorylation. In older larvae, this is met by the increase in capillarization that improves the blood supply and with it the required oxygen and metabolite supply of muscle tissue. Both of these adaptational changes result in a reduction of diffusion distances (between capillary and muscle fiber as well as mitochondria) and may contribute to a higher resistance toward oxygen deficiency. Furthermore, this study indicates that plasticity of muscle tissue is already established in early stages of development at both the tissue and cellular levels. interval training; endurance training; white muscle fiber; red muscle fiber; intermediate muscle fiber

Cardiac activity and anaerobic metabolism were analyzed in zebrafish larvae raised under normoxia (P[O.sub.2] = 20 kPa) and under chronic hypoxia (P[O.sub.2] = 10 kPa) at three different temperatures (25, 28, and 31[degrees]C). Heart rate increased with development and with temperature. Under normoxia, cardiac output increased significantly at high temperature (31[degrees]C), but not at 28 or at 25[degrees]C. Under chronic hypoxia, however, heart rate as well as cardiac output increased at all temperatures in larvae at about hatching time or shortly thereafter. Cardiac activity of larvae raised for 2 wk after fertilization with a reduced hemoglobin oxygen-carrying capacity in their blood (hypoxemia; due to the presence of CO or of phenylhydrazine in the incubation water) was not different from control animals. Whole body lactate content of these animals did not increase. Thus there was no indication of a stimulated anaerobic energy metabolism. The increase in cardiac activity observed during hypoxia suggests that at about hatching time receptors are present that sense hypoxic conditions, and this information can be used to induce a stimulation of convective oxygen transport to compensate for a reduction in bulk oxygen diffusion in the face of a reduced oxygen gradient between environmental water and tissues. Under normoxia, however, the P[O.sub.2] gradient between environmental water and tissues and diffusional oxygen transport assure sufficient oxygen supply even if hemoglobin oxygen transport in the blood is severely impaired. Thus, under normoxic conditions and with a normal metabolic rate of the tissues, convective oxygen transport is not required until ~2 wk after fertilization. anaerobic metabolism; cardiac output; convective oxygen transport

European eels were studied to determine the effect of extracellular pH on proton-secreting pathways and proton production. The results had established the importance of extracellular glucose as a substrate for proton production in swim bladder gas gland cells. The role of acid-secreting pathways utilizing sodium-dependent mechanisms was found to be more important at low pH compared to its role in the acid release gas gland cells.

Prem, C., W. Salvenmoser, J. Wurtz, and B. Pelster. Swim bladder gas gland cells produce surfactant: in vivo and in culture. Am J Physiol Regulatory Integrative Comp Physiol 279: R2336-R2343, 2000.--Electron microscopical examination of gas gland cells of the physostome European eel (Anguilla anguilla) and of the physoclist perch (Perca fluviatilis) revealed the presence of significant numbers of lamellar bodies, which are known to be involved in surfactant secretion. In the perch, in which the gas gland is a compact structure and gas gland cells are connected to the swim bladder lumen via small canals, lamellar bodies were also found in flattened cells forming the swim bladder epithelium. Flat epithelial cells are absent in the eel swim bladder, in which the whole epithelium consists of cuboidal gas gland cells. In both species, Western blot analysis using specific antibodies to human surfactant protein A (SP-A) showed a cross-reaction with swim bladder tissue homogenate proteins of ~65 kDa and in the eel occasionally of ~120 kDa, probably representing SP-A-like proteins in a dimeric and a tetrameric state. An additional band was observed at ~45 kDa. Western blots using antibodies to rat SP-D again resulted in a single band at ~45 kDa in both species, suggesting that there might be a cross-reaction of the antibody to human SP-A with an SP-D-like protein of the swim bladder tissue. To localize the surfactant protein, eel gas gland cells were cultured on permeable supports. Under these conditions, the gas gland cells regain their characteristic polarity. Electron microscopy confirmed the presence of lamellar bodies in cultured cells, and occasionally, exocytotic events were observed. Immunohistochemical staining using an antibody to human SP-A demonstrated the presence of surfactant protein only in luminal membranes and in adjacent lateral membranes. Only occasionally, evidence was found for the presence of surfactant protein in lamellar bodies. Anguilla anguilla; Perca fluviatilis

Schonweger, G., T. Schwerte, and B. Pelster. Temperature-dependent development of cardiac activity in unrestrained larvae of the minnow Phoxinus phoxinus. Am J Physiol Regulatory Integrative Comp Physiol 279: R1634-R1640, 2000.--The minnow (Phoxinus phoxinus) was raised up to the stage of swim bladder inflation at temperatures between 10 [degrees] C and 25 [degrees] C, and the time of development significantly decreased at higher temperatures. Accordingly, initiation of cardiac activity was observed at day 2 in 25 [degrees] C animals and at day 4 in 12.5 [degrees] C animals. Only a minor increase in body mass was observed during the incubation period, and, at the end of the incubation period, animals raised at 25 [degrees] C did not have a significantly lower body mass compared with animals raised at 15 [degrees] C. Metabolic activity, determined as the rate of oxygen consumption of a larva, increased from 3.3 to 19.5 nmol/h during development at 15 [degrees] C and from 5.6 to 47.6 nmol/h during development at 25 [degrees] C. Heart rate showed a clear correlation to developmental stage as well as to developmental temperature, but at the onset of cardiac activity, diastolic ventricular volume and also stroke volume were higher at the lower temperatures. Furthermore, stroke volume increased with development, except for the group incubated at 12.5 [degrees] C, in which stroke volume decreased with development. Initial cardiac output showed no correlation to incubation temperature. Although metabolic activity increased severalfold during development from egg to the stage of swim bladder inflation at 15 [degrees] C and at 25 [degrees] C, weight-specific cardiac output increased only by -40% with proceeding development. At 12.5 [degrees] C, cardiac output remained almost constant until opening of the swim bladder. The data support the notion that oxygen transport is not the major function of the circulatory system at this stage of development. The changes in heart rate with temperature appear to be due to the intrinsic properties of the pacemaker; there was no indication for a regulated response. cardiovascular function; ontogeny; heart; temperature acclimation