Abstract
We contrasted the forced diving bradycardia between two genetically similar (inbred) rat strains (Fischer and Buffalo), compared to that of outbred rats (Wistar). The animals were habituated to forced diving for 4 weeks. Each animal was then tested during one 40 s dive on each of 3 days. The heart rate (fH) was measured before, during, and after each dive. Fischer and Buffalo exhibited marked difference in dive bradycardia (Fischer: 120.9 ± 14.0 beats min−1 vs. Buffalo: 92.8 ± 12.8 beats min−1, P < 0.05). Outbred rats showed an intermediate response (103.0 ± 30.9 beats min−1) but their between-animal variability in mean dive fH and pre-diving resting fH were higher than the inbred strains (P < 0.05), which showed no difference (P > 0.05). The decreased variability in fH in inbred rats as compared with the outbred group indicates that reduced genetic variability minimizes variability of the diving bradycardia between individuals. Heritability within strains was assessed by the repeatability (R) index and was 0.93 ± 0.05 for the outbred, 0.84 ± 0.16 for Buffalo, and 0.80 ± 0.12 for Fischer rats for fH during diving. Our results suggest that a portion of the mammalian diving bradycardia may be a heritable trait.
Highlights
The heart and brain are organs that are vitally dependent upon a continuous supply of oxygen
DIVE TRAINING Initially, most rats struggled during the forced diving but at the end of the training period most appeared calm during the submergence
Our data are the first to suggest a genetic component of the universal diving bradycardia and we have shown that genetically distinct populations of rats demonstrate divergent heart rate responses during diving
Summary
The heart and brain are organs that are vitally dependent upon a continuous supply of oxygen. Interruption of oxygen supply for only a short time, as occurs during sleep apnea, stroke, or heart disease, can cause permanent damage or death. The response is observed in both aquatic (Butler and Jones, 1997), and semi-aquatic (rat and musk rat; Signore and Jones, 1995, 1996; Mcculloch et al, 2010a; Panneton et al, 2010a,b) mammals, preventing permanent damage of heart and brain during prolonged apnea. Even fish show bradycardia when removed from water, raising the question whether bradycardia is a pre-adaptation to hypoxia or asphyxia (Davis et al, 2004) or an evolved adaptation driven by selection across evolutionary time (Scholander, 1963)
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