Physiological Responses of Fish to Exercise

Fish 511/Fall 2014

I. Effect of exercise on the metabolic demand for oxygen:

A. Several measures of oxygen demand are in use: •Standard (= basal) metabolic rate. Animal is at complete rest and in post-absorptive state. Fish confined in a respirometer are usually not totally inactive, so the standard rate may

be estimated by extrapolation of a VO2 vs. activity regression to zero activity. •Routine metabolic rate. Includes effect of "normal" activity; hence, not well-defined.

•Active metabolic rate. Usually standardized as the max. VO2 sustainable for 30 min (or 1 h).

B. The difference between the standard and active rates is the scope for activity or metabolic scope. (The metabolic scope is affected by temperature–the maximum metabolic scope has been shown to coincide with the preferred temperature of a number of species.)

C. Oxygen delivery is adequate to meet the O2 demand of the tissues at exercise levels up to the maximum sustainable swimming speed. Higher speeds are possible, but the extra cost is met by anaerobic metabolism, with accumulation of lactic acid. Anaerobic metabolism is inefficient: as swimming speed increases, the time to exhaustion decreases rapidly (Figure).

Time to exhaustion Swimming speed Rainbow trout Jack mackerel 4 lengths/s 1- 6 h indef. 5 lengths/s 1 m (note change!) -- 8 lengths/s < 20 s ("sprint") 6 h 10 lengths/s < 3 s ("burst") 3 m

"Burst" or "sprint" swimming activity is followed by minutes or even hours of recovery with elevated gill ventilation (hyperventilation) and disturbed acid-base balance. Some fishes (e.g. upstream-migrating adult salmon) may perform at 75% or more of the active metabolic rate for weeks.

II. Comparison of metabolic rates of fish with other vertebrates:

A. Aerobic metabolism: Fish and other poikilotherms have much lower standard, routine and active metabolic rates than homiotherms. Active rates for fish are 10% of rates for medium to large mammals and only 1% of rates for small mammals and birds. These differences are apparently related to differences in the pumping capability of the hearts of warm- and cold-blooded animals (see conclusion re cardiac output of fish heart as limiting factor, page 2).

B. Anaerobic metabolism. The instantaneous metabolic energy demand during burst swimming of fish (largely anaerobic) may exceed the maximum sustainable rate

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(aerobic) by 10-fold. Differences between burst metabolic rates for poikilotherms and homiotherms are much less than differences between active metabolic rates.

III. Immediate physiological responses to exercise include changes in:

1) Gill ventilation rate 2) Cardiac output 3) Venous oxygen saturation (“venous reserve”).

Gill ventilation rate (Vg): one study with rainbow trout (Table below) showed that at

maximum sustained swimming speed VO2 was increased 7.8x over the resting rate and Vg was increased 9x (from 200 to 1800 mL water/kg/minute) over the resting rate. The increase in gill ventilation rate was, however, considerably less than seen in trout in response to hypoxia (18X over resting; up to 3600 ml/kg/minute).

Many larger species capable of fast, sustained swimming switch over to ram ventilation at higher speeds (ram ventilation is not important for small fishes because they can not cruise at an adequately high speed). . Cardiac output (Vb): Both heart rate (fh) and stroke volume (SV) increase during exercise. In the previously mentioned study with trout, fh increased 1.4x and SV increased 2.2x, for a total increase in cardiac output of about 3 x.

The increase in SV is primarily a consequence of an increase in circulating catecholamines, mediating increased contractility of cardiac muscle (inotropic effect) and increased venous return (increasing cardiac filling and contractility according to Starling's Law). The increase in fh (of lesser importance than increased SV) is mediated by decreased vagal (cholinergic) tone.

O2 saturation of blood: Arterial PO2 (PaO2) remains high and O2 content (Ca O2) remains near saturation at all swimming speeds, but venous O2 saturation may decrease; e.g., in the previously referenced study on trout, venous O2 saturation declined from 70% at rest down to 15% at critical swimming speed. Hence, the delivery of O2 per unit volume of blood to the working muscle may increase several-fold as the "venous O2 reserve" is reduced (Figure).

Important conclusion: during exercise, the gill ventilation rate does not increase to the extent possible (i.e. to the extent seen in hypoxia); nevertheless, arterial O2 remains near saturation, even at the maximum sustainable swimming speed. Hence, O2 transfer across the gill surface does not seem to be limiting maximum O2 uptake: the limiting factor for aerobic oxygen transport must be cardiac output.

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Changes in the oxygen transport system of rainbow trout during maximum sustained exercise (from Kiceniuk and Jones 1977)

Resting Active Oxygen uptake VO2 =1 7.8x

Blood Vb =1 3x SATA,O2 100% 5% SATV,O2 70% 15% O2 Utilization 30% 80% Water Vg 1 9x CI,O2 10 mg/l 10 mg/l CE,O2 5 mg/l 6.5 mg/l O2 Extraction 50% 35%