Does respiratory sinus arrhythmia occur in fishes?

Area of Science:

• Comparative physiology
• Cardiovascular research
• Fish biology

Background:

• Respiratory sinus arrhythmia (RSA) is a known phenomenon in mammals, influencing heart rate variability (HRV).
• The extent to which non-mammalian vertebrates exhibit similar cardio-respiratory coupling remains largely unexplored.
• Antarctic and sub-Antarctic fish species offer a unique model due to differing environmental temperatures and oxygen supply selection pressures.

Purpose of the Study:

• To test the hypothesis that respiratory modulation of heart rate variability (HRV) is exclusive to mammals.
• To investigate vagally mediated control of heart rate in fish around the ventilatory cycle.
• To compare cardio-respiratory coupling in Antarctic versus sub-Antarctic fish species.

Main Methods:

• Studied eight fish species (four Antarctic, four sub-Antarctic) with similar genotypes/ecotypes.
• Measured intrinsic heart rate (fH) and respiration rate (fV).
• Utilized power spectral analysis to examine heart rate patterns relative to respiration.

Main Results:

• All fish species exhibited vagal activity that synchronized resting heart rate (fH) with respiration rate (fV).
• Temperate species showed cyclical bradycardia every 2-4 heartbeats, while Antarctic species displayed it at >4 heartbeats.
• This indicates varying selection pressures on cardio-respiratory coupling.

Conclusions:

• Vagally mediated control of heart rate around the ventilatory cycle is present in fish.
• The findings demonstrate that cardio-respiratory coupling mechanisms similar to mammalian RSA operate in non-mammalian vertebrates.
• Environmental temperature influences the degree of cardio-respiratory synchronization in fish.