Changes in cardiac output during swimming and aquatic hypoxia in the air-breathing Pacific tarpon

Abstract

Pacific tarpon (Megalops cyprinoides) use a modified gas bladder as an air-breathing organ (ABO). We examined changes in cardiac output (V̇_b) associated with increases in air-breathing that accompany exercise and aquatic hypoxia. Juvenile (0.49 kg) and adult (1.21 kg) tarpon were allowed to recover in a swim flume at 27 °C after being instrumented with a Doppler flow probe around the ventral aorta to monitor V̇_b and with a fibre-optic oxygen sensor in the ABO to monitor air-breathing frequency. Under normoxic conditions and in both juveniles and adults, routine air-breathing frequency was 0.03 breaths min^− 1 and V̇_b was about 15 mL min^− 1 kg^− 1. Normoxic exercise (swimming at about 1.1 body lengths s^− 1) increased air-breathing frequency by 8-fold in both groups (reaching 0.23 breaths min^− 1) and increased V̇_b by 3-fold for juveniles and 2-fold for adults. Hypoxic exposure (2 kPa O₂) at rest increased air-breathing frequency 19-fold (to around 0.53 breaths min^− 1) in both groups, and while V̇_b again increased 3-fold in resting juvenile fish, V̇_b was unchanged in resting adult fish. Exercise in hypoxia increased air-breathing frequency 35-fold (to 0.95 breaths min^− 1) in comparison with resting normoxic fish. While juvenile fish increased V̇_b nearly 2-fold with exercise in hypoxia, adult fish maintained the same V̇_b irrespective of exercise state and became agitated in comparison. These results imply that air-breathing during exercise and hypoxia can benefit oxygen delivery, but to differing degrees in juvenile and adult tarpon. We discuss this difference in the context of myocardial oxygen supply.

Introduction

The prevailing hypothesis is that air-breathing in fishes evolved to allow survival in hypoxic water (Gunther, 1871, Barrell, 1916, Carter and Beadle, 1931, Graham, 1997). An alternative hypothesis suggests that air-breathing evolved under normoxic conditions as a means of supplying oxygen to the myocardium and enhancing cardiac performance during exercise when venous oxygen return to the heart may otherwise become limiting (Farmer, 1997). Combining these two hypotheses, it is equally plausible that air-breathing evolved not only to survive in hypoxic water, but also to allow a good scope for activity under those conditions. Despite these possibilities, few studies have directly measured any cardiovascular variables of air-breathing fish during exercise (e.g. Burleson et al., 1998) and, as far as we are aware, no study of air-breathing fish has examined the interactive effects of exercise and hypoxia on cardiac output (V̇_b). Thus, the present study is the first to quantify V̇_b of an air-breathing fish during exercise in aquatic normoxia and hypoxia, and in doing so provides insight into some of the factors that may have influenced the evolutionary selection for air-breathing.

The present study utilises one of the most active species of air-breathing fish, the Pacific tarpon (Megalops cyprinoides). Adults of this species are found in the sea where spawning occurs in coastal waters. After hatching, the leptocephalus larvae migrate into freshwater rivers where they reside and mature for 2–4 years before returning to the sea (Coates, 1987). While adult fish in the open ocean may only rarely encounter extreme conditions of aquatic hypoxia, juvenile fish often experience severe seasonal hypoxia down to a water oxygen partial pressure (P_wO₂) of 2 kPa in freshwater rivers and ponds of northern Australia (Townsend, 1994, Wells et al., 2005). Under such field conditions, tarpon regularly rise to the surface of the water to exchange air in their modified gas bladder, which acts as an air-breathing organ (ABO). Air-breathing has also been documented in laboratory studies (Seymour et al., 2004, Seymour et al., 2007). The tarpon heart, like those of other air-breathing fishes, would obviously benefit from any oxygen removed from the ABO since the first organ perfused by oxygenated blood leaving the ABO is the heart.

The tarpon heart has remnants of a conus arteriosus and sets of conal valves (Parsons, 1930), the latter indicating that tarpon are an extant representative of an early teleost (Farrell, 2007a). Notably, the ventricular myocardium of tarpon has two muscle types: an outer compact myocardial layer supplied with a coronary circulation from the hypobranchial artery, and an inner trabecular (spongy) myocardium supplied by venous blood returning from the systemic circulation and being pumped by the heart (Farrell et al., 2007). This vascular arrangement provides a dual cardiac oxygen supply and has important implications for the two predominant hypotheses regarding the evolution of air-breathing in fishes. Given that there is no direct vascular connection between the two cardiac blood supplies (other than the coronary veins draining into the heart), the relative contribution of the coronary supply to myocardial oxygen demand should approximate the percentage of compact myocardium, which increases with body mass and reaches an impressive 50% in a 1.2 kg tarpon (Farrell et al., 2007).

Respiratory studies with juvenile tarpon indicate that they rarely breathe air when resting in normoxic water, and air-breathing increases only moderately when exercising under these conditions (Seymour et al., 2004). Thus, under resting and moderate exercise conditions, little oxygen is removed from the ABO during normoxia, and the ABO oxygen store (which may remain high; Seymour et al., 2007) has a minimal contribution to the cardiac oxygen demand. With normoxic exercise, it is expected that the increase in compact myocardial oxygen demand is met by an increase in coronary blood flow, as is the case in exercising salmonids (Farrell, 1987, Axelsson and Farrell, 1993, Gamperl et al., 1994, Gamperl et al., 1995). Furthermore, although venous oxygen return to the heart decreases with exercise because of increased oxygen extraction by the working skeletal muscles (Farrell and Clutterham, 2003), it has been suggested that oxygen delivery to trabecular myocardium should be sufficient as long as the venous oxygen tension remains above a threshold level of around 0.8 kPa (Farrell, 1987, Davie and Farrell, 1991).

Under hypoxic conditions and in contrast to normoxic exercise, oxygen delivery to the myocardium in tarpon is expected to change based on what is known for water-breathing fishes. With environmental hypoxia (P_wO₂ ∼ 11 kPa) in resting salmonids, oxygen content is reduced in the arterial and venous blood, and coronary blood flow and gill ventilation are increased as a compensation (Axelsson and Farrell, 1993, Gamperl et al., 1994). Juvenile tarpon are more tolerant of hypoxia than salmonids and begin air-breathing when exposed to P_wO₂ < 11.6 kPa while resting (Seymour et al., 2004), which should increase venous oxygen content of the blood returning to the heart and improve the oxygen supply to the trabecular myocardium. The degree to which air-breathing will benefit oxygen content of the coronary circulation will depend, among other things, on the amount of oxygen utilised by the trabecular myocardium prior to the blood reaching the gills, and on the extent to which transbranchial oxygen loss occurs from the gills to the hypoxic water (Graham, 1997). When tarpon exercise in hypoxia, cardiac oxygen delivery faces the dual problem of oxygen partial pressures decreasing in both venous and coronary arterial blood, unless air-breathing can fully compensate for these states. Air-breathing frequency and oxygen uptake rate from the ABO are certainly greatly enhanced during hypoxic exercise (Seymour et al., 2007), but it is unknown whether the heart secures a sufficient oxygen supply to maintain performance.

Preliminary experiments on tarpon attempted to implant polyethylene cannulae into the ventral and dorsal aortae to measure blood oxygen levels. However, these experiments revealed that the dorsal aorta cannot be cannulated by conventional methods used with other teleost fish because of an abrupt bend in this blood vessel at the point of cannulation (Fig. 1). Nevertheless, since fish hearts have limited anaerobic capacity, it can be assumed that a depression of cardiac performance is an indication of inadequate oxygen delivery to the myocardium, as has been shown previously (see Farrell and Stecyk, 2007). Consequently, by examining V̇_b in resting and exercising tarpon under conditions of normoxia and hypoxia, we can examine the possibility that air-breathing sufficiently supplements oxygen supply to the heart such that cardiac performance is maintained in the face of low aquatic oxygen availability.