Cardiovascular responses to hypoxia in the hagfish, Eptatretus cirrhatus

Regulation of heart rate in vertebrates during hypoxia: A comparative overview

Acute exposure to low oxygen (hypoxia) places conflicting demands on the heart. Whilst an increase in heart rate (tachycardia) may compensate systemic oxygen delivery as arterial oxygenation falls, the heart itself is an energetically expensive organ that may benefit from slowing (bradycardia) to reduce work when oxygen is limited. Both strategies are apparent in vertebrates, with tetrapods (mammals, birds, reptiles, and amphibians) classically exhibiting hypoxic tachycardia and fishes displaying characteristic hypoxic bradycardia. With a richer understanding of the ontogeny and evolution of the responses, however, we see similarities in the underlying mechanisms between vertebrate groups. For example, in adult mammals, primary bradycardia results from the hypoxic stimulation of carotid body chemoreceptors that are overwhelmed by mechano-sensory feedback from the lung associated with hyperpnoea. Fish-like bradycardia prevails in the mammalian foetus (which, at this stage, is incapable of pulmonary ventilation), and in fish and foetus alike, the bradycardia ensues despite an elevation of circulating catecholamines. In both cases, the reduced heart rate may primarily serve to protect the heart. Thus, the comparative perspective offers fundamental insight into how and why different vertebrates regulate heart rate in different ways during periods of hypoxia.

Ventilatory sensitivity to ammonia in the Pacific hagfish (Eptatretus stoutii), a representative of the oldest extant connection to the ancestral vertebrates

Ventilatory sensitivity to ammonia occurs in teleosts, elasmobranchs and mammals. Here, we investigated whether the response is also present in hagfish. Ventilatory parameters (nostril flow, pressure amplitude, velar frequency and ventilatory index, the last representing the product of pressure amplitude and frequency), together with blood and water chemistry, were measured in hagfish exposed to either high environmental ammonia (HEA) in the external sea water or internal ammonia loading by intra-vascular injection. HEA exposure (10 mmol l^-1 NH₄HCO₃ or 10 mmol l^-1 NH₄Cl) caused a persistent hyperventilation by 3 h, but further detailed analysis of the NH₄HCO₃ response showed that initially (within 5 min) there was a marked decrease in ventilation (80% reduction in ventilatory index and nostril flow), followed by a later 3-fold increase, by which time plasma total ammonia concentration had increased 11-fold. Thus, hyperventilation in HEA appeared to be an indirect response to internal ammonia elevation, rather than a direct response to external ammonia. HEA-mediated increases in oxygen consumption also occurred. Responses to NH₄HCO₃ were greater than those to NH₄Cl, reflecting greater increases over time in water pH and P _NH3  in the former. Hagfish also exhibited hyperventilation in response to direct injection of isotonic NH₄HCO₃ or NH₄Cl solutions into the caudal sinus. In all cases where hyperventilation occurred, plasma total ammonia and P _NH3  levels increased significantly, while blood acid-base status remained unchanged, indicating specific responses to internal ammonia elevation. The sensitivity of breathing to ammonia arose very early in vertebrate evolution.

The effects of salinity and hypoxia exposure on oxygen consumption, ventilation, diffusive water exchange and ionoregulation in the Pacific hagfish (Eptatretus stoutii)

Hagfishes (Class: Myxini) are marine jawless craniate fishes that are widely considered to be osmoconformers whose plasma [Na⁺], [Cl^-] and osmolality closely resemble that of sea water, although they have the ability to regulate plasma [Ca²⁺] and [Mg²⁺] below seawater levels. We investigated the responses of Pacific hagfish to changes in respiratory and ionoregulatory demands imposed by a 48-h exposure to altered salinity (25 ppt, 30 ppt (control) and 35 ppt) and by an acute hypoxia exposure (30 Torr; 4 kPa). When hagfish were exposed to 25 ppt, oxygen consumption rate (MO₂), ammonia excretion rate (J_amm) and unidirectional diffusive water flux rate (J_H2O, measured with ³H₂O) were all reduced, pointing to an interaction between ionoregulation and gas exchange. At 35 ppt, J_H2O was reduced, though MO₂ and J_amm did not change. As salinity increased, so did the difference between plasma and external water [Ca²⁺] and [Mg²⁺]. Notably, the same pattern was seen for plasma Cl^-, which was kept below seawater [Cl^-] at all salinities, while plasma [Na⁺] was regulated well above seawater [Na⁺], but plasma osmolality matched seawater values. MO₂ was reduced by 49% and J_H2O by 36% during hypoxia, despite a small elevation in overall ventilation. Our results depart from the "classical" osmorespiratory compromise but are in accord with responses in other hypoxia-tolerant fish; instead of an exacerbation of gill fluxes when gas transfer is upregulated, the opposite happens.

Introducing a novel mechanism to control heart rate in the ancestral Pacific hagfish

Although neural modulation of heart rate is well established among chordate animals, the Pacific hagfish (Eptatretus stoutii) lacks any cardiac innervation, yet it can increase its heart rate from the steady, depressed heart rate seen in prolonged anoxia to almost double its normal normoxic heart rate, an almost fourfold overall change during the 1-h recovery from anoxia. The present study sought mechanistic explanations for these regulatory changes in heart rate. We provide evidence for a bicarbonate-activated, soluble adenylyl cyclase (sAC)-dependent mechanism to control heart rate, a mechanism never previously implicated in chordate cardiac control.

Characterizing the metabolic capacity of the anoxic hagfish heart

Pacific hagfish, Eptatretus stoutii, can recover from 36 h of anoxia at 10°C. Such anoxia tolerance demands the mobilization of anaerobic fuels and the removal of metabolic wastes--processes that require a functional heart. The purpose of this study was to measure the metabolic response of the excised, cannulated hagfish heart to anoxia using direct calorimetry. These experiments were coupled with measurements of cardiac pH and metabolite concentrations, at multiple time points, to monitor acid-base balance and anaerobic ATP production. We also exposed hagfish to anoxia to compare the in vitro responses of the excised hearts with the in vivo responses. The calorimetry results revealed a significant reduction in the rate of metabolic heat production over the first hour of anoxia exposure, and a recovery over the subsequent 6 h. This response is likely attributable to a rapid anoxia-induced depression of aerobic ATP-production pathways followed by an upregulation of anaerobic ATP-production pathways such that the ATP production rate was restored to that measured in normoxia. Glycogen-depletion measurements suggest that metabolic processes were initially supported by glycolysis but that an alternative fuel source was used to support the sustained rates of ATP production. The maintenance of intracellular pH during anoxia indicates a remarkable ability of the myocytes to buffer/regulate protons and thus protect cardiac function. Altogether, these results illustrate that the low metabolic demand of the hagfish heart allows for near-routine levels of cardiac metabolism to be supported anaerobically. This is probably a significant contributor to the hagfish's exceptional anoxia tolerance.

Characterizing the metabolic capacity of the anoxic hagfish heart

Pacific hagfish, Eptatretus stoutii, can recover from 36 h of anoxia at 10°C. Such anoxia tolerance demands the mobilization of anaerobic fuels and the removal of metabolic wastes, processes that require a functional heart. The purpose of this study was to measure the metabolic response of the excised, cannulated hagfish heart to anoxia using direct calorimetry. These experiments were coupled with measurements of cardiac pH and metabolite concentrations, at multiple time points, to monitor acid-base balance and anaerobic ATP-production. We also exposed hagfish to anoxia to compare the in vitro responses of the excised hearts with the in vivo responses. The calorimetry results revealed a significant reduction in the rate of metabolic heat production over the first hour of anoxia exposure, and a recovery over the subsequent 6 h. This response was likely attributable to a rapid anoxia-induced depression of aerobic ATP-production pathways followed by an up-regulation of anaerobic ATP-production pathways such that the ATP production rate was restored to that measured in normoxia. Glycogen-depletion measurements suggest that metabolic processes were initially supported by glycolysis but that an alternate fuel source was used to support the sustained rates of ATP production. The maintenance of intracellular pH during anoxia indicates a remarkable ability of the myocytes to buffer/regulate protons and thus protect cardiac function. Altogether, these results illustrate that the low metabolic demand of the hagfish heart allows for near-routine levels of cardiac metabolism to be supported anaerobically. This is likely a significant contributor to the hagfish's exceptional anoxia tolerance.

Characterizing the metabolic capacity of the anoxic hagfish heart

Pacific hagfish, Eptatretus stoutii, can recover from 36 h of anoxia at 10°C. Such anoxia tolerance demands the mobilization of anaerobic fuels and the removal of metabolic wastes, processes that require a functional heart. The purpose of this study was to measure the metabolic response of the excised, cannulated hagfish heart to anoxia using direct calorimetry. These experiments were coupled with measurements of cardiac pH and metabolite concentrations, at multiple time points, to monitor acid-base balance and anaerobic ATP-production. We also exposed hagfish to anoxia to compare the in vitro responses of the excised hearts with the in vivo responses. The calorimetry results revealed a significant reduction in the rate of metabolic heat production over the first hour of anoxia exposure, and a recovery over the subsequent 6 h. This response was likely attributable to a rapid anoxia-induced depression of aerobic ATP-production pathways followed by an up-regulation of anaerobic ATP-production pathways such that the ATP production rate was restored to that measured in normoxia. Glycogen-depletion measurements suggest that metabolic processes were initially supported by glycolysis but that an alternate fuel source was used to support the sustained rates of ATP production. The maintenance of intracellular pH during anoxia indicates a remarkable ability of the myocytes to buffer/regulate protons and thus protect cardiac function. Altogether, these results illustrate that the low metabolic demand of the hagfish heart allows for near-routine levels of cardiac metabolism to be supported anaerobically. This is likely a significant contributor to the hagfish's exceptional anoxia tolerance.

The beat goes on: Cardiac pacemaking in extreme conditions

In order for an animal to survive, the heart beat must go on in all environmental conditions, or at least restart its beat. This review is about maintaining a rhythmic heartbeat under the extreme conditions of anoxia (or very severe hypoxia) and high temperatures. It starts by considering the primitive versions of the protein channels that are responsible for initiating the heartbeat, HCN channels, divulging recent findings from the ancestral craniate, the Pacific hagfish (Eptatretus stoutii). It then explores how a heartbeat can maintain a rhythm, albeit slower, for hours without any oxygen, and sometimes without autonomic innervation. It closes with a discussion of recent work on fishes, where the cardiac rhythm can become arrhythmic when a fish experiences extreme heat.

Evolutionary origins of the blood vascular system and endothelium

Every biological trait requires both a proximate and evolutionary explanation. The field of vascular biology is focused primarily on proximate mechanisms in health and disease. Comparatively little attention has been given to the evolutionary basis of the cardiovascular system. Here, we employ a comparative approach to review the phylogenetic history of the blood vascular system and endothelium. In addition to drawing on the published literature, we provide primary ultrastructural data related to the lobster, earthworm, amphioxus, and hagfish. Existing evidence suggests that the blood vascular system first appeared in an ancestor of the triploblasts over 600 million years ago, as a means to overcome the time-distance constraints of diffusion. The endothelium evolved in an ancestral vertebrate some 540-510 million years ago to optimize flow dynamics and barrier function, and/or to localize immune and coagulation functions. Finally, we emphasize that endothelial heterogeneity evolved as a core feature of the endothelium from the outset, reflecting its role in meeting the diverse needs of body tissues.

Exceptional cardiac anoxia tolerance in tilapia (Oreochromis hybrid)

Anoxic survival requires the matching of cardiac ATP supply (i.e. maximum glycolytic potential, MGP) and demand (i.e. cardiac power output, PO). We examined the idea that the previously observed in vivo downregulation of cardiac function during exposure to severe hypoxia in tilapia (Oreochromis hybrid) represents a physiological strategy to reduce routine PO to within the heart's MGP. The MGP of the ectothermic vertebrate heart has previously been suggested to be ∼70 nmol ATP s(-1) g(-1), sustaining a PO of ∼0.7 mW g(-1) at 15°C. We developed an in situ perfused heart preparation for tilapia (Oreochromis hybrid) and characterized the routine and maximum cardiac performance under both normoxic (>20 kPa O(2)) and severely hypoxic perfusion conditions (<0.20 kPa O(2)) at pH 7.75 and 22°C. The additive effects of acidosis (pH 7.25) and chemical anoxia (1 mmol l(-1) NaCN) on cardiac performance in severe hypoxia were also examined. Under normoxic conditions, cardiac performance and myocardial oxygen consumption rate were comparable to those of other teleosts. The tilapia heart maintained a routine normoxic cardiac output (Q) and PO under all hypoxic conditions, a result that contrasts with the hypoxic cardiac downregulation previously observed in vivo under less severe conditions. Thus, we conclude that the in vivo downregulation of routine cardiac performance in hypoxia is not needed in tilapia to balance cardiac energy supply and demand. Indeed, the MGP of the tilapia heart proved to be quite exceptional. Measurements of myocardial lactate efflux during severe hypoxia were used to calculate the MGP of the tilapia heart. The MGP was estimated to be 172 nmol ATP s(-1) g(-1) at 22°C, and allowed the heart to generate a PO(max) of at least ∼3.1 mW g(-1), which is only 30% lower than the PO(max) observed with normoxia. Even with this MGP, the additional challenge of acidosis during severe hypoxia decreased maximum ATP turnover rate and PO(max) by 30% compared with severe hypoxia alone, suggesting that there are probably direct effects of acidosis on cardiac contractility. We conclude that the high maximum glycolytic ATP turnover rate and levels of PO, which exceed those measured in other ectothermic vertebrate hearts, probably convey a previously unreported anoxia tolerance of the tilapia heart, but a tolerance that may be tempered in vivo by the accumulation of acidotic waste during anoxia.

Structure and function of the velar muscle in the New Zealand hagfish Eptatretus cirrhatus: response to temperature change and hypoxia

The rate of velar movement in Eptatretus cirrhatus, as determined by electromyography, increased with Q(10) 3·2 during exposure to temperatures between 7 and 19° C and increased 3·9 fold during exposure to hypoxia (oxygen partial pressure = 6·67 kPa). This confirms the role of the velum in generating respiratory currents and modification of its activity in response to changes in metabolic demand or environmental oxygen availability. The maximum velar rate observed was 168 beats min(-1) , higher than that recorded in any hagfish species to date. Fibres of musculus craniovelaris were exclusively small, red (slow-twitch) fibres, consistent with a high aerobic capacity required by fibres involved in rhythmic, ongoing activity.

Cardiac responses to anoxia in the Pacific hagfish, Eptatretus stoutii

In the absence of any previous study of the cardiac status of hagfishes during prolonged anoxia and because of their propensity for oxygen-depleted environments, the present study tested the hypothesis that the Pacific hagfish Eptatretus stoutii maintains cardiac performance during prolonged anoxia. Heart rate was halved from the routine value of 10.4±1.3 beats min⁻¹ by the sixth hour of an anoxic period and then remained stable for a further 30 h. Cardiac stroke volume increased from routine (1.3±0.1 ml kg⁻¹) to partially compensate the anoxic bradycardia, such that cardiac output decreased by only 33% from the routine value of 12.3±0.9 ml min⁻¹ kg⁻¹. Cardiac power output decreased by only 25% from the routine value of 0.26±0.02 mW g⁻¹. During recovery from prolonged anoxia, cardiac output and heart rate increased to peak values within 1.5 h. Thus, the Pacific hagfish should be acknowledged as hypoxic tolerant in terms of its ability to maintain around 70% of their normoxic cardiac performance during prolonged anoxia. This is only the second fish species to be so classified.

Anoxic survival of the Pacific hagfish (Eptatretus stoutii)

It is not known how the Pacific hagfish (Eptatretus stoutii) can survive extended periods of anoxia. The present study used two experimental approaches to examine energy use during and following anoxic exposure periods of different durations (6, 24 and 36 h). By measuring oxygen consumption prior to anoxic exposure, we detected a circadian rhythm, with hagfish being active during night and showing a minimum routine oxygen consumption (RMR) during the daytime. By measuring the excess post-anoxic oxygen consumption (EPAOC) after 6 and 24 h it was possible to mathematically account for RMR being maintained even though heme stores of oxygen would have been depleted by the animal's metabolism during the first hours of anoxia. However, EPAOC after 36 h of anoxia could not account for RMR being maintained. Measurements of tissue glycogen disappearance and lactate appearance during anoxia showed that the degree of glycolysis and the timing of its activation varied among tissues. Yet, neither measurement could account for the RMR being maintained during even the 6-h anoxic period. Therefore, two independent analyses of the metabolic responses of hagfish to anoxia exposure suggest that hagfish utilize metabolic rate suppression as part of the strategy for longer-term anoxia survival.

Different sensitivities of arteries and veins to vasoactive drugs in a hagfish, Eptatretus cirrhatus

We used myography on five different arteries and three veins of the hagfish, Eptatretus cirrhatus, to test the response of vessels to vasoactive drugs. Concentration-response curves were generated for carbachol, endothelin-1, arginine vasotocin and the adrenergic agonists, phenylephrine and isoprenaline. pEC50 values indicated that veins were more sensitive to endothelin-1 than were arteries, but the arteries were more sensitive to the cholinergic agonist, carbachol. Segmental arteries did not react to arginine vasotocin, but all other vessels did, and on a molar basis it was the most potent agonist tested. That ventral and dorsal aortas were more sensitive to arginine vasotocin than smaller vessels might indicate that this neurohypophysial peptide has the potential to exert a profound influence on branchial vascular resistance and cardiac output in hagfishes. The results also demonstrate the potential for a variety of endogenous peptides to contribute to central venous tone.

Oxygen dependency of hydrogen sulfide-mediated vasoconstriction in cyclostome aortas

Hydrogen sulfide (H2S) has been proposed to mediate hypoxic vasoconstriction (HVC), however, other studies suggest the vasoconstrictory effect indirectly results from an oxidation product of H2S. Here we examined the relationship between H2S and O2 in isolated hagfish and lamprey vessels that exhibit profound hypoxic vasoconstriction. In myographic studies, H2S (Na2S) dose-dependently constricted dorsal aortas (DA) and efferent branchial arteries (EBA) but did not affect ventral aortas or afferent branchial arteries; effects similar to those produced by hypoxia. Sensitivity of H2S-mediated contraction in hagfish and lamprey DA was enhanced by hypoxia. HVC in hagfish DA was enhanced by the H2S precursor cysteine and inhibited by amino-oxyacetate, an inhibitor of the H2S-synthesizing enzyme,cystathionine β-synthase. HVC was unaffected by propargyl glycine, an inhibitor of cystathionine λ-lyase. Oxygen consumption(ṀO2) of hagfish DA was constant between 15 and 115 mmHg PO2 (1 mmHg=0.133 kPa), decreased when PO2 &lt;15 mmHg, and increased after PO2 exceeded 115 mmHg. 10 μmol l–1 H2S increased and ⩾100μmol l–1 H2S decreased ṀO2. Consistent with the effects on HVC, cysteine increased and amino-oxyacetate decreased ṀO2. These results show that H2S is a monophasic vasoconstrictor of specific cyclostome vessels and because hagfish lack vascular NO, and vascular sensitivity to H2S was enhanced at low PO2, it is unlikely that H2S contractions are mediated by either H2S–NO interaction or an oxidation product of H2S. These experiments also provide additional support for the hypothesis that the metabolism of H2S is involved in oxygen sensing/signal transduction in vertebrate vascular smooth muscle.

The heart as a working model to explore themes and strategies for anoxic survival in ectothermic vertebrates

Most vertebrates die within minutes when deprived of molecular oxygen (anoxia), in part because of cardiac failure, which can be traced to an inadequate matching of cardiac ATP supply to ATP demand. Cardiac power output (PO; estimated from the product of cardiac output and central arterial pressure and an indirect measure of cardiac ATP demand) is directly related to cardiac ATP supply up to some maximal level during both normoxia (ATP supply estimated from myocardial O(2) consumption) and anoxia (ATP supply estimated from lactate production rates). Thus, steady state PO provides an excellent means to examine anoxia tolerance strategies among ectothermic vertebrates by indicating a matching of cardiac glycolytic ATP supply and demand. Here, we summarize in vitro measurements of PO data from rainbow trout, freshwater turtles and hagfishes to provide a reasonable benchmark PO of 0.7 mW g(-1) for maximum glycolytic potential of ectothermic hearts at 15 degrees C, which corresponds to a glycolytic ATP turnover rate of about 70 nmol ATP g(-1) s(-1). Using this benchmark to evaluate in vivo PO data for hagfishes, carps and turtles, we identify two cardiac survival strategies, which in conjunction with creative waste management techniques to reduce waste accumulation, allow for long-term cardiac survival during anoxia in these anoxia-tolerant species. Hagfish and crucian carp exemplify a strategy of evolving such a low routine PO that routine cardiac ATP demand lies within the range of the maximum cardiac glycolytic potential. Common carp and freshwater turtles exemplify an active strategy of temporarily and substantially decreasing cardiac and whole body metabolism so that PO is below maximum cardiac glycolytic potential during chronic anoxia despite being quite close to this potential under normoxia.

Effects of salinity manipulations on blood pressures in an osmoconforming chordate, the hagfish, Eptatretus cirrhatus

Arterial and venous pressures were measured in hagfishes subjected to acute changes in salinity. The osmotic pressure of the seawater (SW) was increased or decreased by approximately 10%. Sixty minutes after the change in medium osmolarity the osmotic pressure of the blood corresponded with that of the medium. Following transfer to 90% SW all measured parameters changed as predicted for a passive increase in blood volume, apart from the pressure in the posterior cardinal vein (PCV) which fell. By 2 h dorsal aortic (DA) pressure and pressure in the PCV and supraintestinal vein had returned to pre-change values. In contrast, following exposure to 110% SW, pressures fell and apart from the supraintestinal vein they remained low at 120 min. At 24 h, DA pressure was lower than pre-change values for both groups. The data are consistent with the concept of central venous tone being regulated in hagfishes, which cope better with volume expansion than volume depletion.

Changes in plasma catecholamine concentration during salinity manipulation and anaesthesia in the hagfish Eptatretus cirrhatus

Plasma catecholamines were measured following surgery under anaesthesia and after exposing hagfish to 90 and 110% sea water (SW). Plasma noradrenaline (NA) concentration increased from a resting value of 7 to 818 nM l(-1) on anaesthesia. Plasma adrenaline (AD) did not change. NA concentrations also increased during volume depletion (110% SW), but to much lower values (26 nM l(-1 )at 100 min). AD concentrations were increased at 20 min, then fell. During volume loading (90% SW) NA fell, and AD increased to a maximum concentration of 511 nM l(-1) at 40 min (resting concentration 24 nM l(-1)). The data are consistent with a vasoconstrictory role for NA on central veins when venous pressures fall and a vasodilatory role for AD on volume expansion.

The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste

The fish gill is a multipurpose organ that, in addition to providing for aquatic gas exchange, plays dominant roles in osmotic and ionic regulation, acid-base regulation, and excretion of nitrogenous wastes. Thus, despite the fact that all fish groups have functional kidneys, the gill epithelium is the site of many processes that are mediated by renal epithelia in terrestrial vertebrates. Indeed, many of the pathways that mediate these processes in mammalian renal epithelial are expressed in the gill, and many of the extrinsic and intrinsic modulators of these processes are also found in fish endocrine tissues and the gill itself. The basic patterns of gill physiology were outlined over a half century ago, but modern immunological and molecular techniques are bringing new insights into this complicated system. Nevertheless, substantial questions about the evolution of these mechanisms and control remain.

Effects of acute hypoxia on the energy status and antioxidant defense system in the blood of carp - Cyprinvs carpio L

The influence of acute hypoxia on glucose, pyruvate, lipid peroxide (LP) reduced glutathione (GSH) concentrations and lactate level in the whole blood of carp (Cyprinus carpio L) under aquarium conditions were studied. The activities of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), the concentrations of ATP and ADP and ATP/ADP ratio in the red blood cells (RBCs) were analyzed. Glutathione-S-transferase (GST) activity was determined in the plasma. Our experiments showed that short-term and long-term hypoxia causes significant changes of all examined haema-tological parameters. Increased concentration of LP and increased SOD CAT and GST activities, as well as a decreased GSH-Px activity showed that under hypoxic conditions oxidative stress and RBCs damage were produced.

Blood and extracellular fluid volume in whole body and tissues of the Pacific hagfish, Eptatretus stouti

Whole-body and 20 individual-tissue (51)Cr-RBC (red cell space; RCS) and (99)Tc-diethylenetriaminepentaacetic acid (extracellular space; ECS) spaces were measured in seven unanesthetized Pacific hagfish (Eptatretus stouti). Volume indicators were administered via a dorsal aortic cannula implanted the previous day. Blood samples were collected at 6, 12, 18, and 24 h after injection. Tissues were removed at 24 h and radioactivity was measured; tissue water content (percent of wet weight) was determined by desiccation at 95 degrees C for 48 h. Mixing rates of both indicators were identical and were essentially complete by 12 h, indicating that blood convection is the rate-limiting process. At 24 h, the whole-body RCS was 19.3+/-2.1 mL kg(-1) body weight, and the ECS was 338.5+/-15.2 mL kg(-1) body weight. Blood volume estimated from the 24-h RCS and the mean central hematocrit (14%) was 137.9 mL kg(-1) body weight. Liver RCS (118.6+/-30.5 microL g(-1) tissue weight) was twice that of any other tissue and was also the most variable, ranging from 59 to 263 microL g(-1), whereas liver ECS (406.0+/-34.3 microL g(-1)) was in the range of other tissues, and water content (66.9%+/-3.5%) was low. Gill RCS (55.9+/-5.7 microL g(-1)), ECS (415.3+/-37.7 microL g(-1)), and percent water (83.1%+/-0.8%) were higher than most other tissues. RCS, ECS, and percent water were consistently lowest in ovum (1.1+/-0.02 microL g(-1), 111.1+/-4.3 microL g(-1), 51.3%+/-3.5%, respectively). Tongue, notocord, and myotome had generally lower RCS (2.1+/-0.4, 2.2+/-0.5, 7.1+/-0.1 microL g(-1), respectively) and ECS (121.2+/-7.0, 246.3+/-17.4, 185.3+/-16.7 microL g(-1), respectively), although their water content was in the midrange (74.7+/-0.5, 81.2+/-1.6, 74.4%+/-0.6%, respectively). Skin had a low RCS (6.8+/-1.1) and midrange ECS (387.5+/-28.0) but very low water content (61.2%+/-2.1%). These findings confirm that hagfish blood volume is at least twice as large as other fish, whereas our estimate of extracellular fluid volume is larger than previously reported and more in line with the predicted interstitial volume. RCS, ECS, and water content vary, often independently, between tissues, which may perhaps be indicative of specific tissue needs or functions. A distinct spleen is lacking in hagfish, and the liver appears to serve this function by sequestering red cells. To our knowledge, this is the first report of tissue ECS in Myxiniformes.

Hypoxic vasoconstriction of cyclostome systemic vessels: the antecedent of hypoxic pulmonary vasoconstriction?

Hypoxic vasoconstriction (HV) is an intrinsic response of mammalian pulmonary vascular smooth muscle (VSM). In the present study, HV was examined by myography of vessel rings from three primitive vertebrates: New Zealand hagfish (NZH), Pacific hagfish (PH), and sea lamprey (SL). Hypoxia dilated pre-gill arteries (ventral aorta, afferent branchial) from all species, whereas it contracted systemic arteries [dorsal aorta (DA), efferent branchial, celiacomesenteric]. DA HV was reproducible over several days, and it could be sustained in NZH for 8 h without adverse effects. Tension was proportional to PO(2), and half-maximal HV was obtained at PO(2) (mmHg) of 4.7 +/- 0. 2 (NZH), 0.8 +/- 0.1 (PH), and 10.7 +/- 1.9 (SL). HV did not require preconditioning (preexisting contractile stimulus) and was unaffected by elevated extracellular potassium (200 mM NZH; 80 mM SL); removal of the endothelium (NZH); or inhibitors of cyclooxygenase, lipoxygenase, cytochrome P-450 or antagonists of alpha-adrenergic, muscarinic, nicotinic, purinergic, or serotoninergic receptors. These results show that HV is an intrinsic feature of systemic VSM in cyclostomes and suggest that HV has been in the repertoire of VSM responses, since the origin of vertebrates. The exceptionally hardy HV in cyclostome DA may provide a useful model with which to examine both the phylogeny and mechanisms of this response.