Malcolm E. Forster

The aerobic scope of an antarctic fish, Pagothenia borchgrevinki and its significance for metabolic cold adaptation

SummaryResting weight-specific oxygen consumption of the cryopelagic Antarctic nototheniid Pagothenia borchgrevinki at 0°C was 39.6 ml kg-1 · h-1 for a 50 g fish, with oxygen consumption being described by the regression equation: log10 VO2(ml/h)=−1.104+0.825 log10 Mb (g). These values are considerably below those raported by Wohlschlag (1964a,b). VO2 max. in forced swimming was described by the regression equation: log10 VO2 max = −0.507+0.823 log10 Mb. Despite low basal metabolism, factorial aerobic scope is similar to that reported for most other teleost fish, as is the cost of net transport. Myotomal muscles were used only at the highest swimming speeds and once they were recruited the fish fatigued rapidly. After swimming, oxygen debt was repaid rapidly, with a half-time of 20 min.

Cardiac chymase converts rat proAngiotensin-12 (PA12) to angiotensin II: effects of PA12 upon cardiac haemodynamics

AIMS The aim of this study was to observe the direct physiological and biochemical cardiac effects in response to a newly identified putative component of the renin-angiotensin system, proangiotensin-12 (PA12); and investigate whether PA12 can serve as a substrate for Angiotensin II (AngII) generation. METHODS AND RESULTS The direct cardiac actions of PA12 and its role as a substrate for chymase-dependent AngII generation were investigated in Sprague-Dawley rats using an isolated heart model of cardiac ischaemia-reperfusion injury. PA12 potently constricted coronary arteries with no significant effect on left-ventricular contractility. PA12 impaired recovery from global ischaemia, maintaining coronary constriction and markedly increasing release of creatine kinase and troponin I (TnI), indicating greater myocardial injury. Analysis of perfusate collected after transcardiac passage revealed a marked increase in AngII production from hearts infused with PA12. Cardiac AngII production was not blocked by angiotensin-converting enzyme inhibitors, whereas inhibition of chymase with chymostatin significantly reduced AngII production and attenuated PA12-induced vasoconstriction and myocardial damage following ischaemia. Furthermore, Angiotensin II type 1 receptor (AT(1)R) blockade abolished PA12 activity. In vitro, PA12 was efficiently and precisely converted to AngII as assessed on reverse phase-high performance liquid chromatography coupled to tandem mass spectrometry. This conversion was blocked by chymostatin. CONCLUSION PA12 may act as a circulating substrate for cardiac chymase-mediated AngII production, in contrast to ACE-mediated AngII production from AngI.

Recovery from exhausting exercise in an Antarctic fish, Pagothenia borchgrevinki

SummaryPagothenia borchgrevinki, an Antarctic teleost fish was swum to exhaustion and changes in lactic acid levels and blood haematocrit were monitored during recovery. The fish did not perform well at high swimming speeds due to an inability of the white myotomal muscle to produce ATP by anaerobic glycolysis. Consequently, low levels of lactic acid were produced which were fairly rapidly broken down. Haematocrit values were low in non-exercised fish, and these increased by over 100% during exercise, falling back to control levels over many hours. This is probably related to the increased oxygen demand during exercise.

Cardiovascular responses to hypoxia in the hagfish, Eptatretus cirrhatus

Simultaneous measurements of cardiac output (Q), blood pressures and blood gases were made in the hagfish, Eptatretus cirrhatus, during exposure to hypoxia. The partial pressure of oxygen in the medium (PIO2) was reduced from 20.7 kPa to 8.0 kPa and then lowered to 5.3 kPa. At a PIO2 of 5.3 kPa there was a 40% increase in Q. Part of the increase may have been due to the increased activity of the animal at low PIO2. In recovery, when the animals were inactive, Q continued to rise to 160% of the control values. At 5.3 kPa, oxygen consumption (as determined by the Fick principle) fell to 29% of the normoxic value and was associated with increased branchial vascular resistance (Rg) and an increased diffusion limitation of the gills (Ldiff). Adrenaline increased heart rate and aortic blood pressures. Increased Rg and Ldiff could be provoked in normoxia by the injection of the beta-adrenergic blocking drug propranolol. We suggest that catecholamines may be involved in the tonic control of gill vasomotor tone.

Confirmation of the low metabolic rate of hagfish

Abstract 1. 1. In Eplatretus cirrhatus (Forster) basal oxygen consumption (VO2) at 11°C was 6.93 ± 0.23 ml kg−1 h−1 (0.31 ± 0.01 μmoles kg−1 h−1), confirming the low metabolic rate of eptatreti hagfish. 2. 2. There was an initial settlement period, but no evidence for a circadian rhythm in oxygen usage. 3. 3. In closed-box respirometry hagfish maintained VO2 down to a critical PO2 (PwO2, cr) of ca. 45 mm Hg, which is not unusually low when compared to many teleost fish. 4. 4. The animals vigorous attempts at escape at PO2s below 35 mmHg suggest that E. cirrhatus does not willingly withstand severe hypoxic conditions.

The effects of fish anaesthetics (MS222, metomidate and AQUI-S) on heart ventricle, the cardiac vagus and branchial vessels from Chinook salmon (Oncorhynchus tshawytscha)

The acute effects of 3 fish anaesthetics (MS222, metomidate and AQUI-S) were investigated on 3 parts of the cardiovascular system of Chinook salmon (Oncorhynchus tshawytscha). All 3 anaesthetics reduced the contractile force of paced strips of ventricular myocardium. MS222 reduced the contractility by almost 75%, and was more potent than metomidate and AQUI-S, which reduced the contractility by about 25%. MS222 blocked vagal nerve transmission to the heart at the normally applied anaesthetic concentration (NAAC) for Chinook salmon, whereas metomidate and AQUI-S required 100 times their NAACs to have the same effect. Using myography, MS222 and AQUI-S caused a maximal 30–40% dilation of EBAs at 10% NAAC, whereas the equivalent effect with metomidate was only seen at 100% NAAC. MS222 again caused the greatest dilation of the ABAs. AQUI-S dilated the ABAs at up to 50% NAAC, but this was reversed so that there was no dilation at 100% NAAC. Metomidate did not affect the ABAs. These data from in vitro and in situ experiments, which generally show inhibitory effects, are used to suggest possible cardiovascular outcomes in anaesthetised Chinook salmon.

Cardiovascular responses to acute handling stress in the Antarctic fish Trematomus bernacchii are not mediated by circulatory catecholamines.

Cannulated specimens of the benthic Antarctic teleost Trematomus bernacchii were subjected to a five min period of handling stress, following which blood pressures were measured and then blood samples taken for measurement of plasma catecholamine concentrations. There were two control groups of fish, one group of undisturbed fish at Scott Base, Antarctica and the other group of undisturbed fish measured in Christchurch.Plasma catecholamine concentrations were not changed following 5 min of acute handling stress. However, heart rate, ventral aortic blood pressure and haematocrit were elevated following handling, suggesting increased sumpathetic nerve activity. The low plasma catecholamine concentrations measured in all three groups of fish are similar to those found in other species of teleost at rest.The resting heart rates found in this series of experiments were approximately double those recorded in previous work on this species and this difference may be correlated with the length of time that the fish are held in captivity. The fish transported to Christchurch also had higher mean cell haemoglobin concentrations (MCHC) and lower plasma chloride concentrations.

Catecholamine release in heat-stressed Antarctic fish causes proton extrusion by the red cells

Abstract Two species of Antarctic fish were stressed by moving them from seawater at −1 °C to seawater at 10 °C and holding them for a period of 10 min. The active cryopelagic species Pagothenia borchgrevinki maintained heart rate while in the benthic species Trematomus bernacchii there was an increase in heart rate. Blood pressure did not change in either species. Both species released catecholamines into the circulation as a consequence of the stress. P. borchgrevinki released the greater amounts, having mean plasma concentrations of 177 ± 54 nmol · l−1 noradrenaline and 263 ± 131 nmol · l−1 adrenaline at 10 min. Plasma noradrenaline concentrations rose to 47 ± 14 nmol · l−1 and adrenaline to 73 ± 28 nmol · l−1 in T. bernacchii. Blood from P. borchgrevinki was tonometered in the presence of isoprenaline. A fall in extracellular pH suggests the presence of a Na+/H+ antiporter on the red cell membrane, the first demonstration of this in an Antarctic fish. Treatment with the β-adrenergic antagonist drug sotalol inhibited swelling of red blood cells taken from temperature-stressed P. borchgrevinki, suggesting that the antiporter responds to endogenous catecholamines.

Cardiac Function in the New Zealand Hagfish, Eptatretus cirrhatus

Evidence from respiratory and circulatory studies suggests that the hagfish genus Eptatretus is more adapted to an aerobic life and is more active than Myxine. This study aims to compare the cardiac dynamics of the two genera. The ECG of E. cirrhatus differed from that of fish in showing both a clear V wave and a PT deflection, the first caused by depolarization of the sinus venosus and the second by the repolarization of the atrium. Atrial and ventricular pressures were low; the ventral aortic average pressure was 16.3 ± 3.5 cm H₂O. Various cardiac parameters of the two hagfish are compared, and it is concluded that their hearts are very alike but differ from those of teleost fish in the slow rate of conduction in and contraction of the myocardium. These findings are discussed with reference to its anaerobic metabolism, its absence of a coronary circulation and T tubules, and the uncertain development of gap junctions between the myocardial cells.

Cardiovascular responses to swimming in eels

Abstract o 1. Blood pressures and cardiac output were measured in short-finned eels ( Anguilla australis schmidtii , Phillips) swimming at speeds up to 35 cm sec −1 . 2. At speeds above 22 cm sec −1 there was an increase in branchial vascular resistance. 3. The pressure differential across the gill increased due to elevated ventral aortic pressure. 4. It is suggested that cessation of swimming, commonly observed in fish with high ventra aortic pressures, might have a protective function for the gills.