Symposium 32: Cardiac and vascular strategies of lower vertebrates. Chair: P. Davie (New Zealand)

S32-1

Cardiac functioning in fish Davie, Peter S. Comparative Physiology and Anatomy, Massey University, Palmerston North, New Zealand. Fish adjust functioning of the heart when faced with conditions such as low oxygen tension or the demands of exercise. Athletic fishes which have high cardiac work outputs often have tetrahedral shaped ventricles. This paper addresses how the superficial myocardium contributes to contraction of isolated in situ trout hearts working under different conditions. The contractile behaviour of the trout ventricle was described from video images while simultaneously measuring intraventricular pressure and cardiac performance. Edges of the tetrahedral ventricles shortened on average by between 3.3 (dorsal edge, control flow rates) - 15.6%,(ventral edge, maximum flow or power output). Ventricles accommodated large stroke volumes by increased diastohc dimensions with little if any reduction in systolic dimensions. Greater output pressure increased both diastolic and systolic dimensions. Plots of intraventricular pressure versus edge- lengths gave loops whose enclosed areas were strongly correlated with cardiac work output. Slopes of the pressure-edge-length loop areas with cardiac work output were steeper for increasing afterload than for increasing preload suggesting that for a given change in myocardiai power output, the change in loop area is greater for pressure increases than for flow increases. How different edges behave allows an analysis of effects on cardiac work of shifting the ventricular apex with respect to the centre of the base.

S32-2

Do hagfishes exhibit “primitive” features in their cardiovascular control systems? Forster ME and Olson KR Deptartment of Zoology, University of Canterbury, Christchurch, New Zealand and Indiana University School of Medicine, South Bend Center for Medical Education, University of Notre Dame, Notre Dame, IN 46556, USA

With their early separation from the vertebrate lineage, hagfishes might be expected to exhibit unique properties of their blood systems. However, what is suprising is how much conformity there seems to be amongst the vertebrates, at least with respect to actions of vasoactive drugs. The actions of catecholamines, natriuretic peptides, arginine vasotocin, etc. must have been established in ancestral chordates, and the properties of receptors changed little over evolutionary time. In vitro rings of dorsal aorta from Eptatretus cirrhatus can remain contracted in anoxic saline for periods exceeding 12 h, and then relax and react to drugs when reoxygenated. In general, vasodilatation of vascular beds in response to local hypoxia is seen as adaptive, allowing increased blood flow and an improved supply of oxygen. Vasoconstriction in the pulmonary arterial bed of mammals is considered to be a mechanism allowing blood to bypass poorly perfused sectors of the lung. What then is the significance of the vasoconstriction of the hagfish dorsal aorta, which seems to be an intrinsic property, being unaffected by drugs and other agents which effect the response in the mammalian lung?

S32-3

Gill blood flow regulation in fish Sundin L and Nilsson GE, Dept. of Zoology, University of Gijteborg, Gothenburg, Sweden and Division of General Physiology, University of Oslo, Oslo, Norway.

In response to different physiological stimuli such as hypoxia and exercise, fish can regulate the size of the functional gill surface area by altering the number of lamellae being perfused and adjusting the pattern of blood flow within each lamella. This can be achieved by alterations m ventral and dorsal aortic blood pressures, but also by changes in the vascular resistances between different parts of the gill microvasculature. Simultaneous measurements of cardiovascular parameters and in vivo observation of the microvasulature in the primary filament and the secondary lamellae, have made it possible to identify specific target areas in the gill microvasculature for a number of putative regulatory substances, such as acetylcholine, serotonin, adenosine and endothelin. The agonist-induced changes observed range from an impact on the whole filament circulation down to variations of blood distribution within each individual lamella. The variations in blood flow patterns are due to constriction of filament arteries and arterioles, and to contraction of pillar cells. All of these changes may greatly influence the different tasks that the gills carry out, but perhaps most importantly gas exchange.

S32-4

Cardiovascular dynamics during rest and under water exercise in the estuarine crocodile

‘Franklin C.E.. ‘Axelsson M. & 3Butler P.J. ‘Department of Zoology, The University of Queensland, Brisbane, QLD 4072, Australia; *Department of Zoology, University of GGteborg, PO Box 463,405 30 GGteborg, Sweden; School of Biological Sciences, University of Birmingham, Edgbaston, Birmingham B 15 2TT, England.

Crocodilians at “rest” have a remarkable range of heart rates from 30-40 bpm while basking in the sun to 5-8 bpm during submergence. Consideration of blood flows and pressures further complicates the definition of “resting” conditions, with the capacity of the crocodilian heart to shunt blood away from the lungs. Pulmonary-to-systemic shunts have been recorded during diving but also in calm, breathing animals. Previous studies have concentrated on resting animals, in this study, estuarine crocodiles, Crocodylusporosus were swum in a 3.6 m long water-flume in order to investigate the effect of exercise on cardiovascular dynamics. Although the animals had free access to air, they spent most of their time under water when swimming. During short-term exercise (~25 minutes at round 30 cm s-l) there was an increase in heart rate (30 to 51 bpm) and systemic and pulmonary blood pressures (7.5 to 8.3 and 1.5 to 2.3 kPa respectively). At the same time blood flow to the lungs was elevated by 37% and systemic blood flow was increased by 31% while left aortic blood flow was unchanged. Arterial PO2 was 13.5 kPa at rest and decreased to 7.0 kPa at the end of the exercise period. A more detailed analysis of the results will be presented and discussed.