Drug induced changes in cardio-vascular parameters in the Atlantic cod,Gadus morhua

Atlantic cod (Gadus morhua L.) in situ cardiac performance at cold temperatures: long-term acclimation, acute thermal challenge and the role of adrenaline

The resting and maximum in situ cardiac performance of Newfoundland Atlantic cod (Gadus morhua L.) acclimated to 10, 4 and 0°C were measured at their respective acclimation temperatures, and when acutely exposed to temperature changes: i.e. hearts from 10°C fish cooled to 4°C, and hearts from 4°C fish measured at 10°C and 0°C. Intrinsic heart rate (fH) decreased from 41 beats min-1 (bpm) at 10°C to 33 bpm at 4°C and to 25 bpm at 0°C. However, this degree of thermal dependency was not reflected in maximal cardiac output. Qmax values were ~44, ~37 and ~34 ml min-1 kg-1 at 10, 4 and 0°C, respectively. Further, cardiac scope showed a slight positive compensation between 4 and 0°C (Q10 = 1.7), and full, if not a slight over compensation between 10 and 4°C (Q10 = 0.9). The maximal performance of hearts exposed to an acute decrease in temperature (i.e. from 10°C to 4°C and 4°C to 0°C) was comparable to that measured for hearts from 4 and 0°C acclimated fish, respectively. In contrast, 4°C acclimated hearts significantly out-performed 10°C acclimated hearts when tested at a common temperature of 10°C (in terms of both Qmax and power output). Only minimal differences in cardiac function were seen between hearts stimulated with basal (5 nM) vs. maximal (200 nM) levels of adrenaline, the effects of which were not temperature dependant. These results: 1) show that maximum performance of the isolated cod heart is not compromised by exposure to cold temperatures; and 2) support data from other studies which show that, in contrast to salmonids, cod cardiac performance/myocardial contractility is not dependent upon humoral adrenergic stimulation.

In situ cardiac function in Atlantic cod (Gadus morhua): effects of acute and chronic hypoxia

Recent in vivo experiments on Atlantic cod (Gadus morhua) acclimated to chronic hypoxia (6-12 weeks at 10°C; PwO2 ~8-9 kPa) revealed a considerable decrease in the pumping capacity of the heart. To examine whether this diminished cardiac performance was due to the direct effects of chronic moderate hypoxia on the myocardium (as opposed to alterations in neural and/or hormonal control), we measured the resting and maximum in situ function of hearts from normoxia- and hypoxia-acclimated cod: (1) when initially perfused with oxygenated saline; (2) at the end of a 15 min exposure to severe hypoxia (PO2 ~0.6 kPa); and (3) 30 min after the hearts had been reperfused with oxygenated saline. Acclimation to hypoxia did not influence resting (basal) in situ cardiac performance during oxygenated or hypoxic conditions. However, it caused a decrease in maximum cardiac output () under oxygenated conditions (from 49.5 to 40.3 ml min−1 kg−1; by 19%), that was due to diminished values for maximum stroke volume (VS) and scope for VS. Severe hypoxia reduced in both groups to ~20 ml min−1 kg−1, yet, the hearts of hypoxia-acclimated fish were better able to sustain this level of under hypoxia, and the recovery of (as compared with initial values under oxygenated conditions) was significantly improved (94% vs 83%). These data show that acclimation to hypoxia has a direct effect on cod myocardial function and/or physiology, and suggest that the cod heart shows some adaptations to prolonged hypoxia.

Oxygen limitation of thermal tolerance in cod,Gadus morhuaL., studied by magnetic resonance imaging and on-line venous oxygen monitoring

The hypothesis of an oxygen-limited thermal tolerance due to restrictions in cardiovascular performance at extreme temperatures was tested in Atlantic cod, Gadus morhua (North Sea). Heart rate, changes in arterial and venous blood flow, and venous oxygen tensions were determined during an acute temperature change to define pejus (“getting worse”) temperatures that border the thermal optimum range. An exponential increase in heart rate occurred between 2 and 16°C (Q10= 2.38 ± 0.35). Thermal sensitivity was reduced beyond 16°C when cardiac arrhythmia became visible. Flow-weighted magnetic resonance imaging (MRI) measurements of temperature-dependent blood flow revealed no exponential but a hyperbolic increase of blood flow with a moderate linear increase at temperatures >4°C. Therefore, temperature-dependent heart rate increments are not mirrored by similar increments in blood flow. Venous Po2(PvO2), which reflects the quality of oxygen supply to the heart of cod (no coronary circulation present), followed an inverse U-shaped curve with highest PvO2levels at 5.0 ± 0.2°C. Thermal limitation of circulatory performance in cod set in below 2°C and beyond 7°C, respectively, characterized by decreased PvO2. Further warming led to a sharp drop in PvO2beyond 16.1 ± 1.2°C in accordance with the onset of cardiac arrhythmia and, likely, the critical temperature. In conclusion, progressive cooling or warming brings cod from a temperature range of optimum cardiac performance into a pejus range, when aerobic scope falls before critical temperatures are reached. These patterns might cause a shift in the geographical distribution of cod with global warming.

The regulation and importance of glucose uptake in the isolated Atlantic cod heart: rate-limiting steps and effects of hypoxia

This study investigated the regulation of glucose uptake in Atlantic cod(Gadus morhua) hearts. Isolated hearts were perfused with or without glucose in the medium, under either normoxic or severely hypoxic conditions. Working at basal levels, hearts did not require extracellular glucose to maintain power under aerobic conditions. However, cardiac performance was significantly reduced without exogenous glucose under oxygen-limiting conditions. The addition of the glucose transporter inhibitor cytochalasin B caused hypoxic hearts to fail early, and hearts perfused with a glucose analogue, 2-deoxyglucose (2-DG), increased glucose uptake 3-fold under hypoxia. The uptake of 2-DG was only partially inhibited when cytochalasin B was added to the medium. Isolated ventricle strips were also incubated in the presence of 2-DG and the extracellular marker mannitol. Glucose uptake(glucose transport plus intracellular phosphorylation) was assessed by measuring the initial rate of 2-deoxyglucose-6-phosphate (2-DG-6-P)accumulation. At 1 mmol l-1 2-DG, the rate of 2-DG uptake remained linear for 60 min, and 2-DG-6-P, but not free 2-DG, accumulation was increased. The fact that intracellular 2-DG did not increase indicates that glucose transport is the rate-limiting step for glucose utilization in non-stimulated cardiac tissue. Replacement of Na+ by choline in the incubation medium did not affect 2-DG uptake, providing evidence that Na+-coupled glucose transport is absent in cod cardiac tissue. Similar to cytochalasin B, glucose uptake was also inhibited by phloridzin,suggesting that facilitated, carrier-mediated glucose transport occurs in cod hearts. Under the conditions employed in these experiments, it is clear that(1) activation of glucose transport is required to support hypoxic performance, (2) the rate-limiting step for glucose utilization is glucose transport rather than glucose phosphorylation, (3) 2-DG uptake accurately reflects glucose transport activity and (4) glucose uptake in cod hearts does not involve an Na+-dependent mechanism.

Cyclooxygenase-derived products, rather than nitric oxide, are endothelium-derived relaxing factor(s) in the ventral aorta of carp (Cyprinus carpio)

In some fish blood vessels, the existence of a NO (nitric oxide) system has been reported. We examined the possibility that this NO system acts as an endothelium-derived relaxing factor (EDRF) in carp aorta using the carp aorta alone and in a combined carp-rat aorta donor-detector system. Use of the typical NO stimulating agent in mammal acetylcholine (ACh) only induced constriction of the carp aorta. This response was not modified by denudation or by NO synthesis inhibition with N-nitro-L-arginine methyl ester. Neither the indirect NO stimulating agents bradykinin and histamine nor the direct NO releasers sodium nitroprusside (SNP) and SIN-1 induced vasorelaxation. Both SNP and ACh elevated the cGMP concentration in rat aorta, but not in carp aorta. In the aorta combination set-up, where carp served as a NO donor and rat aorta served as a NO detector, no relaxation of the rat aorta was observed. The calcium ionophore A23187, a known EDRF producer in mammals, induced relaxation of carp aorta through an endothelium- and cyclooxygenase-dependent mechanism. These results indicate that carp aorta does not produce NO as an EDRF nor does it respond to exogenously supplied NO. The major EDRF in carp is apparently a product(s) of cyclooxygenase metabolism.

Effects of hypoxia on blood pressure and heart rate in three marine teleosts

Ventral aortic blood pressure (Pva) and heart rate (HR) responses to rapidly (within 1 min) induced hypoxia (PWO2=4-5.3 kPa) were investigated in vivo in three species of marine teleosts (shorthorn sculpin, Myoxo-cephalus scorpius; eel-pout, Zoarces viviparus; and five-bearded rockling, Ciliata mustela). Fish were exposed to hypoxia for 4 min (M.scorpius) or 5 min (Z.viviparus and C.mustela).Pva was unaffected in M.scorpius, decreased in Z.viviparus and increased in C.mustela in response to hypoxia.Untreated M.scorpius and Z. viviparus responded with a characteristic bradycardia during hypoxia, whereas C.mustela developed no bradycardia. Injection of atropine followed by the β-adrenoceptor antagonist sotalol in M.scorpius and Z. viviparus, revealed that both the inhibitory (cholinergic) and the excitatory (adrenergic) influence on the heart increase during hypoxia. The inhibitory influence dominates, resulting in the observed bradycardia.

Regulation of cardiac output and gut blood flow in the sea raven,Hemitripterus americanus

Coeliac artery blood flow (Fca) before and after feeding was recorded in the sea raven. To obtain basic information about the scope of cardiovascular adjustment in the sea raven, a separate series of experiments was performed, in which ventral (Pva), and dorsal (Pda) aortic blood pressure, heart rate (HR) and cardiac output (jaz) were monitored during rest and encouraged exercise.Measurements of coeliac artery flow showed that visceral blood flow is substantial, particularly after feeding, and variations in the visceral vascular conductance affect Pda directly. Simultaneous recordings of intestinal and dorsal aortic blood pressures showed no measurable difference in the two arterial pressures, refuting the idea of a vascular control at the level of the main coeliac artery. Thus, in the sea raven, the adrenergic tonus affecting the visceral vasculature presumably acts at the arteriolar level.Sea ravens encouraged to exercise increased theirjaz by 64%; 32% through HR and 25% through stroke volume. The increase injaz during encouraged exercise was sufficient to produce an elevation of both Pva and Pda, despite an increase of systemic vascular conductance, β-adrenoceptor blockade with sotalol, however, severely impaired the increase injaz during exercise, and the change in Pda was reversed.During rest there were both an adrenergic and a cholinergic tonus affecting the HR, as revealed by the effects of injected pharmacological antagonists. Swimming activity decreased the cholinergic tonus, while the adrenergic tonus increased.

Cholinergic constriction in the general circulation and its role in coronary artery spasm

The release of acetylcholine from autonomic nerves in those tissues that receive a cholinergic innervation is widely believed to dilate blood vessels. Exogenously administered acetylcholine in vivo does dilate vascular beds and produce hypotension; however, this latter effect is indirect and probably the result of liberation of endothelium-derived relaxing factor (EDRF) from endothelial cells. Some blood vessels contain a substantial population of medial constrictor receptors for acetylcholine, and the implications of this presence for vascular control systems has been largely ignored, although it needs to be considered. A survey of the evolution of vasomotor control systems indicates that acetylcholine serves principally as an excitatory transmitter to blood vessels. Neurally mediated cholinergic constriction and not dilation is found in fish, amphibians, reptiles, and birds, with responses initiated by medial muscarinic receptors. Acetylcholine constricts many vascular preparations from these lower animals, but some vessels relax, reflecting the emergence of an EDRF responsive to acetylcholine. An examination of cholinergic responses in mammalian vessels reveals that cholinergic (neurogenic) dilation is limited to a very few vascular beds and to only a few species. Both experimental evidence and evolutionary considerations support the likelihood that cholinergic (neural) constriction operates in some vascular regions in mammals and, in particular, in the coronary circulation of some species, including humans. In fact, constriction, and not dilation, may be the dominant vascular response to activation of the cholinergic axis in most mammals, including humans. The complications and contradictions introduced by the simultaneous presence of both EDRF and a cholinergic constrictor innervation involving medial muscarinic receptors are discussed. A variety of evidence is also presented that implicates cholinergic constriction in at least some instances of coronary artery spasm and sudden death.

Adrenergic responses of the cardiovascular system of the eel, Anguilla australis, in vivo

Heart output, arterial pressures, and heart rate were measured directly in conscious unrestrained eels (Anguilla australis) and responses to intra-arterial injection of adrenaline monitored. Adrenaline increased systemic vascular resistance, heart output, and cardiac stroke volume in all animals. In some cases small transient decreases in stroke volume and hence heart output were seen at the peak of the pressor response: These probably reflect a passive decrease in systolic emptying due to increased afterload on the heart. In most cases, adrenaline produced tachycardia; but two animals showed consistent and profound reflex bradycardia, which was accompanied by a concomitant increase in stroke volume such that heart output was maintained or increased slightly. The interaction of changes in heart output and systemic vascular resistance produced complex and variable changes in arterial pressure. There was no consistent pattern of changes in branchial vascular resistance. Atropine treatment in vivo revealed vagal cardio-inhibitory tone in some animals and always blocked the reflex bradycardia seen during adrenaline induced hypertension. In some animals, adrenaline injection after atropine pretreatment led to the establishment of cyclic changes in arterial pressure with a period of about 1 min (Mayer waves).

Gill O2 consumption in a teleost fish, Gadus morhua

Stimulated by the many physiological functions of fish gills this study reports on perfusion studies of gill arches and head preparations from the Atlantic cod, Gadus morhua, with the objective of determining the oxygen requirement of gill tissues in situ. An additional objective was to assess what fraction of the gill tissue O2 consumption is taken up directly from ambient water without intervention of internal gill perfusion. At 15 degrees C and air-saturated ambient water about 58% of the total gill O2 requirement was taken up from the perfusate while 42% entered gill tissues directly from ambient water. In relation to total O2 uptake of intact cod fish the gill tissue requires 6.65% giving gill tissues a weight-specific O2 requirement of 95.2 microliter O2 . g body wt-1 . h-1 or nearly twice that of the intact fish (54.5 microliter O2 . g body wt-1 . h-1). It is discussed how the gill tissue O2 requirement influences the employment of the Fick principle for calculations of cardiac output in fishes.