The effectiveness of oxygen transfer during normoxia and hypoxia in the dogfish (Scyliorhinus canicula L.) before and after cardiac vagotomy

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

“…In teleosts, the early proposition that the hypoxic bradycardia enhances gill oxygen uptake 171 has been refuted by experiments that have surgically (vagotomy) or pharmacologically (atropine treatment) prevented bradycardia and recorded similar rates of oxygen consumption in normoxia and graded hypoxia 113,114,172 . Moreover, whilst oxygen transfer across the dogfish gill was reduced when the reflex bradycardia was abolished by atropine or vagotomy, 118,173 the measured gas transfer efficiency in the same species was unaffected by specific cardiac vagotomy that abolished the hypoxic bradycardia without affecting gill ventilation 117 . It is also noteworthy that the hypothesis that hypoxic bradycardia affects gas exchange across the gill would also not explain its purpose in the mammalian foetus.…”

“…Most adult fish (including cartilaginous and bony fishes, which given their broadly similar physiological responses are grouped together in this section, despite their phylogenetic disparity) display a characteristic hypoxic bradycardia that has intrigued comparative physiologists for decades. [110][111][112] The bradycardia is largely blocked by atropine or vagotomy [112][113][114][115][116][117][118] and some studies have directly measured increased efferent vagus nerve activity. [119][120][121] Hypoxic bradycardia has been reported in the vast majority of fish species studied, with several notable exceptions, including some Antarctic fishes, 122 which occupy habitually oxygen-saturated water, and some air-breathing fishes, [123][124][125] in which high blood PO 2 can be defended by aerial ventilation.…”

“…113,114,172 Moreover, whilst oxygen transfer across the dogfish gill was reduced when the reflex bradycardia was abolished by atropine or vagotomy, 118,173 the measured gas transfer efficiency in the same species was unaffected by specific cardiac vagotomy that abolished the hypoxic bradycardia without affecting gill ventilation. 117 It is also noteworthy that the hypothesis that hypoxic bradycardia affects gas exchange across the gill would also not explain its purpose in the mammalian foetus.…”

Effects of dissolved oxygen concentration on freshwater fish: A review

“…Because marked bradycardia is countered by an increase in stroke volume, cardiac output is maintained during hypoxia. Systemic resistance increases, increasing both dorsal and ventral aortic blood pressure (Short et al, 19979) [123] . Pulse pressure rises dramatically as a result of the increased heart-stroke volume and lower heart rate.…”

“…Hypoxia increases the number of circulating erythrocytes, which expand in some fish, resulting in a significant increase in haematocrit. In sandy dogfish (S. canicula), however, no rise in haematocrit has been seen in response to hypoxia (Butler et al, 1979) [124] . Hypoxia cause increased activity in vagal cholinergic fibres innervating the heart leads decrease in heart rate (Randall, 1982) [127] .…”