Beta-adrenergic effects of catecholamines on ventilation in fishes

In this thesis, the suggestion that the catecholamines, adrenaline (AD) and/or noradrenaline (NA) may have a role in the control of ventilation in fish was tested. This idea was developed from the observations that catecholamines could modify ventilation in eels, and that situations that cause changes in ventilation are usually associated with raised plasma catecholamine levels. Specifically, the focus of this thesis was to investigate the possible role of plasma NA and AD in 6-adrenergic stimulation of ventilation in fishes. The first study looked at whether it was possible for catecholamines present in the circulatory system to stimulate ventilation. Ventilation frequency, opercular pressure amplitude, heart rate, dorsal aortic pressure, arterial pH, arterial O₂ tension, and plasma catecholamines concentrations were recorded in rainbow trout, Oncorhynchus mykiss, during normoxia after injection of various dosages of catecholamines. This was to assess if catecholamines injected into fish could change ventilation. AD injection resulted in a depression of arterial O₂ tension, hypoventilation due to a drop in ventilation frequency, and a drop in heart rate, while dorsal aortic pressure increased. NA depressed ventilation frequency, but opercular amplitude increased, and dorsal aortic pressure increased. To eliminate the possibility that the ventilatory responses observed were a mixed response to both α- and β-adrenoceptors, I repeated the test using the α- antagonist phentolamine, and measured ventilation frequency, opercular amplitude, arterial pH, red blood cell pH, arterial O₂ tension and content, and haematocrit. AD injection after a-blockade had no significant effect on blood gas measurements, possibly because they had returned to resting levels by the time the blood sample was taken. NA injection after α-blockade led to a significant increase in opercular amplitude, but the other cardiovascular and ventilatory variables were unchanged. These results suggest that NA can modify ventilatory output from the respiratory centre. The possible role of catecholamines in the ventilatory response to acute external hypercapnia was studied in rainbow trout. The ventilatory response to hypercapnia of fish pre-treated with the β-adrenoceptor antagonist D,L-propranolol, was compared to fish pre-treated with D-propranolol (an isomer with minimal β-antagonistic activity) and saline pre-treated fish. A sustained 3.6 fold increase in gill ventilation volume was observed in the saline and D-propranolol treated groups during the 30 mi interval of hypercapnia. Fish pre-treated with D,L-propranolol displayed a blunted (1.9-fold increase) increase in gill ventilation volume. These fish could not maintain ventilation during hypercapnia. These results indicate that the β-component of an adrenergic response is involved in the usual hyperventilatory response to external hypercapnia. The possible role of catecholamines in the ventilatory response to exhaustive exercise was investigated, with and without β-blockade, in rainbow trout. In the control fish, both ventilation frequency and opercular amplitude increased, PHa dropped, pHi increased slightly, haematocrit and catecholamine levels increased. Nadolol itself has effects on blood oxygen status, which sets ventilatory effort to a different level than animals that are not so treated. Fish pre-treated with nadolol had an impaired ability to increase opercular amplitude, due to the higher level of oxygen present in the blood. However, there was also an inability of the nadolol treated fish to maintain ventilation to the same extent as the sham treated fish, which was not due to differences in blood oxygen status. These results demonstrate that catecholamines may have a role in the control of ventilation during recovery from exercise in fish. The ability of catecholamines to alter the activity of respiratory neurons in the medulla was studied in dogfish. First, however, it had to be established that catecholamines could cross the blood-brain barrier. To study this, dogfish were injected with[³H]-AD or[³H]-NA into the caudal vein to see if these compounds could be detected in the medulla and cerebellum. The medulla accumulated NA over AD, while the cerebellum accumulated equivalent amounts of both. This shows that catecholamines do cross the blood-brain barrier in dogfish, and that different areas of the brain accumulate catecholamines differently. When NA was applied to respiratory-related neurons, there were both excitatory and inhibitory effects on the activity of these respiratory-related neurons. The excitatory effects were blocked by propranolol, indicating that the excitatory effects are a β- adrenoceptor mediated response. This demonstrates that catecholamines can affect centrally located respiratory neurons. In conclusion, though the initial ventilatory responses to various environmental conditions are neural in origin (i.e.O₂-chemoreceptor mediated), there are situations when catecholamines are modulating ventilation, as ventilation could not be maintained in β-blocked fish exposed to hypercapnia, or in β-blocked fish during recovery from exhaustive exercise. NA is the most likely candidate for stimulating ventilation fish as it can cross the blood-brain barrier, and can modulate ventilation in fish by its effects on centrally located respiratory neurons via β-adrenergic receptors.Science, Faculty ofZoology, Department ofGraduat