Michael Axelsson

Aerobic scope fails to explain the detrimental effects on growth resulting from warming and elevated CO2 in Atlantic halibut

As a consequence of increasing atmospheric CO2, the worlds oceans are becoming warmer and more acidic. Whilst the ecological effects of these changes are poorly understood, it has been suggested that fish performance including growth will be reduced mainly as a result of limitations in oxygen transport capacity. Contrary to the predictions given by the oxygen- and capacity-limited thermal tolerance hypothesis, we show that aerobic scope and cardiac performance of Atlantic halibut (Hippoglossus hippoglossus) increase following 14–16 weeks exposure to elevated temperatures and even more so in combination with CO2-acidified seawater. However, the increase does not translate into improved growth, demonstrating that oxygen uptake is not the limiting factor for growth performance at high temperatures. Instead, long-term exposure to CO2-acidified seawater reduces growth at temperatures that are frequently encountered by this species in nature, indicating that elevated atmospheric CO2 levels may have serious implications on fish populations in the future.

Gut blood flow in fish during exercise and severe hypercapnia

This paper reviews the effects of exercise and hypercapnia on blood flow to the splanchnic circulation. Brief struggling behaviours are known to decrease blood flow to the gut (GBF). Likewise, prolonged swimming in unfed fish has been shown to reduce GBF in proportion to the increased oxygen uptake. Therefore, the normal postprandial increase in GBF theoretically should be impaired whenever fish are active. However, indirect evidence suggests that GBF is spared to some degree when fed fish swim continuously but at a cost (10-15%) to their critical swimming speed. Severe respiratory acidosis can be created by the new intensive aquaculture settings that use oxygen injection into re-circulated water. The only study so far to examine the effects of severe hypercapnia on GBF and its regulation showed that routine GBF and alpha-adrenergic control of GBF remained normal in unfed white sturgeon (Acipenser transmontanus). However, severe hypercapnia produced a hyperactive state and increased sensitivity of GBF to struggling. As a result, routine GBF was maintained for a short period of time. Thus, environmental changes such as severe hypercapnia can indirectly impact GBF through altered struggling behaviour, but the implications of the overall reduction in GBF to food assimilation have yet to be established.

Respiratory and cardiovascular responses to hypoxia in the Australian lungfish.

Simultaneous measurements of pulmonary blood flow (qPA), coeliacomesenteric blood flow (qCoA), dorsal aortic blood pressure (PDA), heart rate (fH) and branchial ventilation frequency (fv) were made in the Australian lungfish, Neoceratodus forsteri, during air breathing and aquatic hypoxia. The cholinergic and adrenergic influences on the cardiovascular system were investigated during normoxia using pharmacological agents, and the presence of catecholamines and serotonin in different tissues was investigated using histochemistry. Air breathing rarely occurred during normoxia but when it did, it was always associated with increased pulmonary blood flow. The pulmonary vasculature is influenced by both a cholinergic and adrenergic tonus whereas the coeliacomesenteric vasculature is influenced by a beta-adrenergic vasodilator mechanism. No adrenergic nerve fibers could be demonstrated in Neoceratodus but catecholamine-containing endothelial cells were found in the atrium of the heart. In addition, serotonin-immunoreactive cells were demonstrated in the pulmonary epithelium. The most prominent response to aquatic hypoxia was an increase in gill breathing frequency followed by an increased number of air breaths together with increased pulmonary blood flow. It is clear from the present investigation that Neoceratodus is able to match cardiovascular performance to meet the changes in respiration during hypoxia.

Cholinergic and Adrenergic Tones in the Control of Heart Rate in Teleosts. How Should They be Calculated

Cholinergic and adrenergic tones were calculated for three different teleost fish species: Gadus morhua, Labrus bergylta, and Sparus aurata using atropine as a muscarinic receptor antagonist and either sotalol or propranolol as β-adrenoceptor antagonists. Depending on the order of administration of atropine and the two β-adrenoceptor antagonists, it was observed that propranolol but not sotalol enhanced cholinergic tone. Thus, if propranolol is used to determine autonomic cardiac influences, it has to be injected after atropine and not before. Differences in intrinsic heart rate were observed between treatments in two of the three species studied, suggesting the activity of a non-cholinergic non-adrenergic mechanism in heart rate control in fish. Different models to calculate cholinergic and adrenergic tones are discussed. The additive model described by other authors is appropriate provided that no interaction exists between cholinergic and adrenergic influences. We demonstrate no interaction in the species studied in this experiment. Finally, a modification of the additive model that uses R-R interval instead of heart rate in the computation is proposed. This is justified with a computer simulation in terms of the linearity of the response given the reciprocal relationship between R-R interval and heart rate. comp biochem physiol 118A;1:131-139, 1997.

Gastrointestinal blood flow in the red Irish lord, Hemilepidotus hemilepidotus: long-term effects of feeding and adrenergic control

Abstract Cardiac output, blood flow to the coeliac and mesenteric arteries, dorsal aortic blood pressure and heart rate were recorded simultaneously at rest and postprandial for 6 days in a teleost, the red Irish lord (Hemilepidotus hemilepidotus). We anticipated that gastrointestinal blood flow would increase postprandially, supported by an increase in cardiac output. However, we had no predictions for either the exact time-course of this response, or for the regional distribution of blood flow between to the two major arteries comprising the splanchnic circulation. In resting, unfed animals, blood flow to the coeliac artery and mesenteric artery was 4.1 ± 0.6 ml min−1 kg−1 and 4.9 ± 1.3 ml min−1 kg−1, respectively (mean ± SEM, n=7), which together represented 34% of cardiac output. Feeding increased blood flow to the coeliac and mesenteric arteries in a time-dependent manner. The increase in coeliac artery blood flow preceded that in the mesenteric artery, a finding that is consistent with the coeliac artery supplying blood to the liver and stomach, while the mesenteric artery supplies blood to the stomach and intestine. Coeliac blood flow had increased by 84 ± 18% after 1 day and had a peak increase of 112 ± 40% at day 4 postprandial. Mesenteric blood flow was not significantly elevated at day 1, but had increased by 94 ± 19% at day 4 postprandial. Cardiac output also increased progressively, increasing by a maximum of 90 ± 30% at day 4. Because the increase in cardiac output was adequate to meet the postprandial increase in gut blood flow, the postprandial decreases in vascular resistance for the coeliac and mesenteric circulations mirrored the increases in blood flow. Intra-arterial injections of adrenaline and noradrenaline into resting fish more than doubled coeliac and mesenteric vascular resistances, and blood flow decreased proportionately. This adrenergic vasoconstriction was totally abolished by pretreatment with the α-adrenoceptor antagonist phentolamine, which in itself approximately halved coeliac and mesenteric vascular resistances. These observations indicate a significant α-adrenergic tone in the gastrointestinal circulation of the red Irish lord, the loss of which could not entirely account for the postprandial increase in gastrointestinal blood flow. Other control mechanisms are suggested.

Ventricular haemodynamics in Python molurus: separation of pulmonary and systemic pressures.

SUMMARY Vascular pressure separation by virtue of a two-chambered ventricle evolved independently in mammals and birds from a reptilian ancestor with a single ventricle, and allowed for high systemic perfusion pressure while protecting the lungs from oedema. Within non-crocodilian reptiles, ventricular pressure separation has only been observed in varanid lizards and has been regarded as a unique adaptation to an active predatory life style and high metabolic rate. The systemic and pulmonary sides of the ventricle in Python molurus are well separated by the muscular ridge, and a previous study using in situ perfusion of the heart revealed a remarkable flow separation and showed that the systemic side can sustain higher output pressures than the pulmonary side. Here we extend these observations by showing that systemic blood pressure Psys exceeded pulmonary pressure Ppul almost seven times (75.7±4.2 versus 11.6±1.1 cm H2O). The large pressure difference between the systemic and pulmonary circulation persisted when Psys was altered by infusion of sodium nitroprusside or phenylephrine. Intraventricular pressures, measured in anaesthetised snakes, showed an overlap in the pressure profile between the pulmonary side of the ventricle (cavum pulmonale) and the pulmonary artery, while the higher pressure in the systemic side of the ventricle (cavum arteriosum) overlapped with the pressure in the right aortic arch. This verifies that the pressure differences originate within the ventricle, indicating that the large muscular ridge separates the ventricle during cardiac contraction.

Gastrointestinal blood flow and postprandial metabolism in swimming sea bass Dicentrarchus labrax.

In trout and salmon, the metabolic costs of exercise and feeding are additive, which would suggest that gastrointestinal blood flow during exercise is maintained to preserve digestive and absorptive processes related to the specific dynamic action (SDA) of food. However, in most published studies, gastrointestinal blood flow drops during swimming, hypoxia, and general stress. To test whether gastrointestinal blood flow is spared during exercise after feeding, sea bass were instrumented with flow probes to measure cardiac output and celiacomesenteric blood flow while swimming in a respirometer before and after feeding. Swimming at 2 body lengths per second (bl s−1) increased metabolic rate considerably more than did feeding (208% vs. 32% increase, respectively, relative to resting), and a similar pattern was observed for cardiac output. In unfed fish, resting gastrointestinal blood flow was 13.8 ± 0.5 mL min−1 kg−1. After feeding, resting gastrointestinal blood flow increased by 82% but then decreased progressively with increasing swimming speeds. At 2 bl s−1, gastrointestinal blood flow in fed fish was not significantly different compared with that in unfed swimming fish, and, therefore, the data do not support the gastrointestinal sparing hypothesis. The magnitude of the SDA was maintained despite the decrease in gastrointestinal blood flow and the consequent reduction in oxygen supply to the gut. An estimate of maximal oxygen flow to the gastrointestinal tract after feeding yielded 2.6 mmol O2 h−1 kg−1, but this amount is not able to cover the oxygen demand of 3.16 mmol O2 h−1 kg−1. Therefore, the SDA must reflect metabolic processes in tissues other than those directly perfused by the celiacomesenteric artery.

Baroreflex mediated control of heart rate and vascular capacitance in trout

SUMMARY The baroreflex was triggered by altering branchial blood pressure with pre- and post-branchial occlusions for 30 s in rainbow trout Oncorhynchus mykiss. The cardiac limb of the baroreflex was monitored by continuous heart rate (fH) measurements. Responses of venous capacitance vessels were assessed, immediately following either occlusion, by measuring mean circulatory filling pressure (MCFP). Arterial responses were evaluated as the change in dorsal aortic blood pressure (Pda) before and after pre-branchial occlusion. In untreated fish pre-branchial occlusion resulted in tachycardia (62.4±2.4 to 69.1±1.7 beats min–1), decreased venous capacitance reflected as an increase in MCFP (0.17±0.03 to 0.27±0.03 kPa) and increased Pda (4.0±0.2 kPa compared to 3.2±0.1 kPa before occlusion). Post-branchial occlusion somewhat reversed the responses since fH decreased (62.4±2.4 to 53.0±3.1 beats min–1), whereas MCFP remained unaltered. Treatment with the α-adrenergic blocker prazosin (1 mg kg–1) increased resting MCFP to 0.33±0.03 kPa and appeared to abolish both venous and arterial responses to branchial occlusion. Subsequent atropine treatment (1.2 mg kg–1) abolished all chronotropic responses. We present for the first time ample evidence for baroreflex-mediated control of cardiovascular homeostasis, including both the chronotropic and the vascular limb of the baroreflex in an unanaesthetized fish. Furthermore, a novel technique to cannulate and occlude the dorsal aorta, using a Fogarty thru-lumen embolectomy catheter, is explained.

Cardiac preload and venous return in swimming sea bass (Dicentrarchus labrax L.)

SUMMARY Cardiac preload (central venous pressure, Pcv), mean circulatory filling pressure (MCFP), dorsal aortic blood pressure (Pda) and relative cardiac output (Q̇) were measured in sea bass (Dicentrarchus labrax) at rest and while swimming at 1 and 2 BL s-1. MCFP, an index of venous capacitance and the upstream venous pressure driving the return of venous blood to the heart, was measured as the plateau in Pcv during ventral aortic occlusion. Compared with resting values, swimming at 1 and 2 BL s-1 increased Q̇ (by 15±1.5 and 38±6.5%, respectively), Pcv (from 0.11±0.01 kPa to 0.12±0.01 and 0.16±0.02 kPa, respectively), MCFP (from 0.27±0.02 kPa to 0.31±0.02 and 0.40±0.04 kPa, respectively) and the calculated pressure gradient for venous return (ΔPv, from 0.16±0.01 kPa to 0.18±0.02 and 0.24±0.02 kPa, respectively), but not Pda. In spite of an increased preload, the increase in Q̇ was exclusively mediated by an increased heart rate (fh, from 80±4 beats min-1 to 88±4 and 103±3 beats min-1, respectively), and stroke volume (Vs) remained unchanged. Prazosin treatment (1 mg kg-1 Mb) abolished pressure and flow changes during swimming at 1 BL s-1, but not 2 BL s-1, indicating that other control systems besides an α-adrenoceptor control are involved. This study is the first to address the control of venous capacitance in swimming fish. It questions the generality that increased Q̇ during swimming is regulated primarily through Vs and shows that an increased cardiac filling pressure does not necessarily lead to an increased Vs in fish, but may instead compensate for a reduced cardiac filling time.

On the cardiac control in the south american lungfish lepidosiren paradoxa

Abstract 1. 1. The mechanisms behind cardiac control were investigated in the South American lungfish, Lepidosiren paradoxa , using fish with chronically implanted cannulae and electromagnetic flow probes. In addition, a preliminary study was made of the cardiovascular events associated with air breathing. 2. 2. The study suggests that the heart of Lepidosiren is controlled by cholinergic vagal fibres which, in some animals, exert a tonic influence in the resting fish. Cyclic changes in heart rate in association with air breaths is due to modulation of this cholinergic tonus. 3. 3. In addition to the variable cholinergic tonus, there appears to be a relatively stable adrenergic tonus on the heart, which causes an elevated heart rate. The adrenergic tonus is likely to be due to local release of catecholamines from endogenous chromaffin cells within the atrium. 4. 4. Preliminary results suggest that pulmonary arterial flow increases by about 50% immediately following an air breath. The mechanism behind this increase probably involves both an elevation of the heart rate and a redistribution of blood flow into the pulmonary circuit.