Cardiorespiratory responses of the facultative air-breathing fish jeju, Hoplerythrinus unitaeniatus (Teleostei, Erythrinidae), exposed to graded ambient hypoxia R.D. Oliveira a , J.M. Lopes a , J.R. Sanches a , A.L. Kalinin a , M.L. Glass b , F.T. Rantin a, * a Department of Physiological Sciences, Federal University of Sa ˜o Carlos, Via Washington Luiz, Km 235, 13565-905, Sa ˜o Carlos, SP, Brazil b Department of Physiology, Faculty of Medicine of Ribeira ˜o Preto, University of Sa ˜o Paulo, Av. dos Bandeirantes, 3900, 14049-900, Ribeira ˜o Preto, SP, Brazil Received 8 June 2004; received in revised form 7 October 2004; accepted 12 October 2004 Abstract The jeju, Hoplerythrinus unitaeniatus , is equipped with a modified part of the swim bladder that allows aerial respiration. On this background, we have evaluated its respiratory and cardiovascular responses to aquatic hypoxia. Its aquatic O 2 uptake (V ˙ O 2 ) was maintained constant down to a critical P O 2 ( P cO 2 ) of 40 mm Hg, below which V ˙ O 2 declined linearly with further reductions of P iO 2 . Just below P cO 2 , the ventilatory tidal volume (V T ) increased significantly along with gill ventilation (V ˙ G ), while respiratory frequency changed little. Consequently, water convection requirement (V ˙ G /V ˙ O 2 ) increased steeply. The same threshold applied to cardiovascular responses that included reflex bradycardia and elevated arterial blood pressure ( P a ). Aerial respiration was initiated at water P O 2 of 44 mm Hg and breathing episodes and time at the surface increased linearly with more severe hypoxia. At the lowest water P O 2 (20 mm Hg), the time spent at the surface accounted for 50% of total time. This response has a character of a temporary emergency behavior that may allow the animal to escape hypoxia. D 2004 Elsevier Inc. All rights reserved. Keywords: Air-breathing fish; Hypoxia; Air-breathing organ; Respiration; Respiratory function; Cardiac function; Aerial respiration; Hoplerythrinus unitaeniatus 1. Introduction Descendants from Osteichthyes include Actinopterygii, the ray-finned fishes, and Sarcopterygii, the lobe-finned fishes, including the lungfish (Dipnoi) (Carroll, 1988). This latter group possesses lungs that are very much like those of amphibians (Perry et al., 2001). In particular, the lepidosir- enid lungfish have well-developed lungs and highly reduced gills (Johansen and Lenfant, 1967) and their basic respira- tory control shares features with that of tetrapod vertebrates (Milsom, 2002; Sanchez et al., 2001). Within teleosts, air-breathing organs (ABOs) have developed independently at least 60 times (Graham, 1997) based on an amazing number of different solutions. This raises questions concerning the control of bimodal respira- tion. Teleosts possess branchial O 2 -receptors that screen O 2 levels of blood and inspired water to adjust gill ventilation. These receptors reflect a highly O 2 -oriented respiratory control to respond to the ever-changing O 2 -availability of the aquatic environment. Nevertheless, some evidence favors the existence of some peripheral CO 2 /pH-receptors in teleosts, whereas central acid–base control seems absent (Milsom, 2002). The O 2 -capacitance coefficient in air is about 30 times that of water (Dejours, 1981). Therefore, it is not surprising that air-breathing has developed in various groups of actinopterygian fish. This raises questions about the possible range responses of the ABO to temperature, hypoxia and hypercarbia. Detailed studies are relevant, since various solutions can be compared as to structure, gas exchange 1095-6433/$ - see front matter D 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.cbpb.2004.10.011 * Corresponding author. Tel.: +55 16 260 8314; fax: +55 16 260 8328. E-mail address: ftrantin@power.ufscar.br (F.T. Rantin). Comparative Biochemistry and Physiology, Part A 139 (2004) 479– 485 www.elsevier.com/locate/cbpa