Stefan Nilsson

The Circulatory System

Profound changes in the demands on the circulatory system have occurred during the evolution of the vertebrates from the aquatic forms to the more advanced terrestrial forms. These changes reflect a variety of anatomical and functional alterations, and also the adjustment from aquatic life at zero gravity to the demands of terrestrial life. The best documented driving force in the evolution of the vertebrate cardiovascular functions is the need for an efficient transport of respiratory gases between the gas exchanger (skin, gill, lung) and the tissues (Johansen and Burggren 1980, Johansen 1982).

The Alimentary Canal

The functions of the alimentary canal of the vertebrates are basically to (1) receive and act as a reservoir for ingested food and fluid; (2) process the food chemically and physically; (3) absorb water and nutrients and (4) dispose of the wastes. These functions of the gut are controlled by a multitude of systems which regulate the muscular activity of the gut wall, the secretion of digestive juices into the lumen of the gut, and the blood flow in the mucosa. The control systems include the coordinated reflexes of the enteric nervous system and some reflexes that also include extrinsic autonomic pathways (vagal, splanchnic and pelvic). In addition paracrine cells release substances that affect the adjacent cells, and endogenous endocrine systems exert control over the gut motility and, especially, the secretory activity of the gastric and intestinal glands (Larsson 1980).

Bombesin-, gastrin/CCK-, 5-hydroxytryptamine-, neurotensin-, somatostatin-, and VIP-like immunoreactivity and catecholamine fluorescence in the gut of the elasmobranch, Squalus acanthias

SummaryThe presence of peptides and 5-hydroxytryptamine (5-HT) in neurons and endocrine cells in the gastrointestinal tract of the spiny dogfish, Squalus acanthias, was investigated by means of immunohisto-chemistry, and the distribution of catecholamines by use of the Falck-Hillarp fluorescence-histochemical technique. Bombesin-like immunore-activity was present in numerous nerves in all layers and all parts of the gut, and also in endocrine cells in the mucosa throughout the stomach, rectum and intestine. VIP-like immunoreactivity occurred in an abundance of nerve fibres and in nerve cell bodies in all parts of the gut except the oesophagus, while 5-HT-like immunoreactivity was found sparsely in nerve fibres and more frequently in endocrine cells throughout the gut. Gastrin/CCK-like immunoreactivity was present in numerous nerve fibres in the rectum, but only in scattered fibres in the other parts of the gut. Endocrine cells showing gastrin/CCK-like immunoreactivity were present in the intestine only. Somatostatin-like immunoreactivity occurred in both nerve fibres and endocrine cells of the stomach and intestine, but only in nerves in the rectum. Neurotensin-like immunoreactivity was confined to endocrine cells of the intestine. Falck-Hillarp fluorescence histochemistry revealed 5-HT in endocrine cells and catecholamines in nerve fibres (and possibly also in endocrine cells) throughout the gut. Bombesin-, VIP-, gastrin/CCK- and somatostatin-like immunoreactivities and catecholamine fluorescence were present in nerve fibres of the rectal gland and, with the exception of gastrin/CCK-like immunoreactivity, also in nerve bundles in the walls of the coeliac and mesenteric arteries. The findings of the present study form an anatomical basis for the assumption that several of the neuropeptides and amines could function as neurotransmitters or neuromodulators in the gut of Squalus.

Sympathetic nervous control of adrenaline release from the head kidney of the cod, gadus morhua

Abstract 1. The release of catecholamines from the chromaffin tissue by different kinds of stimulation has been studied in the perfused head kidney of the cod, and in intact and operated fish in vivo. 2. Fluorimetric analysis of the perfusate shows a dose-dependent release of adrenaline and noradrenaline by acetylcholine in the dose-range 10−9−10−7 mol. Similarly, both acetylcholine and nicotine produce an increased outflow of label from the head kidney pre-loaded with 3H-adrenaline. 3. Electrical stimulation of the nervous supply to the left cardinal vein produces release of endogenous catecholamines. In head kidneys pre-loaded with 3H-adrenaline, an increase in the outflow of label is elicited by electrical stimulation with an optimal frequency of 20 Hz for a 30 sec stimulation period. 4. The release of label from the head kidney pre-loaded with 3H-adrenaline produced by either acetylcholine or electrical stimulation is inhibited by hexamethonium (3 × 10−5−10−4 M). 5. Stress, induced by keeping the fish in air for 15 min, increases the plasma levels of adrenaline and noradrenaline >4-fold. This effect is prevented by bilateral sectioning of the 1st to 4th spinal nerves with their preganglionic sympathetic outflow. 6. It is concluded that the chromaffin tissue of the cod head kidney is under sympathetic nervous control by medullated fibres which are probably preganglionic and cholinergic.

Adrenergic control of the cardio-vascular system of the Atlantic cod,Gadus morhua, during “stress”

SummaryVentral and dorsal aortic blood pressure, heart rate and plasma concentrations of adrenaline and noradrenaline have been measured in Atlantic cod before and after “stress”. The “stress” was induced by lowering the water level in the tank, which forced the animals to lie on their side struggling to regain the normal posture. The effects of “stress” were studied in fish in which the nerve supply to the head kidney was sectioned, using sham-operated animals as controls. In control animals, there was an increase in the ventral aortic blood pressure and plasma levels of both catecholamines as a result of “stress” while the dorsal aortic blood pressure remains constant and heart rate, if anything, decreases. The effect on the heart rate can be blocked by atropine, indicating a vagal reflex. In fish where catecholamine release from chromaffin tissue was strongly reduced by sectioning the nerve supply to the head kidney, the dorsal aortic blood pressure was lower before “stress” by comparison to controls, and decreased further following “stress”. No significant changes in ventral or dorsal aortic blood pressure and heart rate were observed in another group of fish where the sympathetic innervation of the gills had been sectioned, when compared to sham-operated controls before or after “stress”. It is concluded that circulating catecholamines released from the head kidney play a major role in the control of branchial vascular resistance after “stress”, counter-acting the effect of a non-adrenergic constrictory innervation of the gills. Circulating catecholamines may also be of importance in the control of systemic vascular resistance after “stress”.

Autonomic nervous control of blood pressure and heart rate during hypoxia in the cod, Gadus morhua

SummaryThe autonomic nervous and possible adrenergic humoral control of blood pressure and heart rate during hypoxia was investigated in Atlantic cod. The oxygen tension in the water was reduced to 4.0–5.3 kPa (i.e.. PwO2=30–40 mmHg), and the fish responded with an immediate increase in ventral and dorsal aortic blood pressure (PvaPda), as well as a slowly developing bradycardia. The plasma concentrations of circulating catecholamines increased during hypoxia with a peak in the plasma level of noradrenaline occurring before the peak for adrenaline. Bretylium was used as a chemical tool to differentiate between neuronal and humoral adrenergic control of blood pressure and heart rate (fH) during hypoxia. The increase in Pva and Pda in response to hypoxia was strongly reduced in bretylium-treated cod, which suggests that adrenergic nerves are responsible for hypoxic hypertension. In addition, a small contribution by circulating catecholamines to the adrenergic tonus affecting Pva during hypoxia was suggested by the decrease in Pva induced by injection of the α-adrenoceptor antagonist phentolamine. The cholinergic and the adrenergic tonus affecting heart rate were estimated by injections of atropine and the β-adrenoceptor antagonist sotalol. The experiments demonstrate an increased cholicholinergic as well as adrenergic tonus on the heart during hypoxia.

Cardiovascular and ventilatory control during hypoxia

The cardiovascular and respiratory physiology of fish has been an area of major interest in comparative physiology for several decades, but the systems that control these functions and the nature of the nervous reflexes that regulate cardiovascular and respiratory functions have received relatively little interest. Research on cardiovascular control systems has been largely restricted to adrenergic vasomotor control - adrenergic neurones and circulating catecholamines - and of course also the double antagonistic cholinergic and adrenergic innervation of the teleost heart. In addition, recent studies have discussed whether catecholamines are involved in ventilatory control in teleosts (Aota et al., 1990; Playl et al., 1990; Kinkead and Perry, 1990,1991; Perry et al., 1991).

Nervous control of the branchial vascular resistance of the Atlantic cod,Gadus morhua

SummaryThe effects on branchial vascular resistance of electrical stimulation of the nervous supply to the gills of the Atlantic cod were studied in constant pressure perfused gill preparations.Stimulation of the right sympathetic chain immediately anterior to the coeliac ganglion produces either a β-adrenoceptor mediated decrease in branchial vascular resistance of the gill arches on the right side, or an α-adrenoceptor mediated increase which is reversed by phentolamine to a β-adrenoceptor mediated decrease in branchial vascular resistance.Stimulation of the entire ‘vago-sympathetic’ nerve trunk to the third isolated gill arch produces an increase in branchial vascular resistance, which in some preparations can be reversed by atropine to a β-adrenoceptor mediated decrease. A second type of constrictory innervation of vagal origin (non-adrenergic, non-cholinergic) may be concluded from the lack of blocking capacity of cholinergic and adrenergic antagonists.It is concluded that the branchial vascular bed of the cod is controlled by both sympathetic (dilatory and sometimes also constrictory) and parasympathetic (constrictory) fibres. The site of action of the nerve supply on the various effectors of the complex vasculature of the gills is not known. An autonomic innervation with its direct, rapid and restricted effects may reinforce the more general effects of circulating vaso-active substances.

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.

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.