41
Chapter two: Oxygen stores and diving
order to breathe. Isolated dive hole studies have been conducted only in McMurdo Sound,
Antarctica—with Weddell seals and emperor penguins (Ponganis et al. 1997b; Kooyman
et al. 1980, 1983). The studies with captive or trained animals have included bottlenose
dolphins, beluga whales (Delphinapterus leucas), Baikal seals, and California sea lions
(Ponganis et al. 1997a; Williams et al. 1999). Blood samples have been obtained through
indwelling venous or arterial catheters in pinnipeds, and through needle venipuncture of
tail fluke vessels in the cetaceans.
Blood or plasma lactate concentrations were originally measured through standard
spectrophotometric enzyme assays. Lactate analyzers are now available commercially;
these devices are based on the enzymatically linked generation of an electrical current.
A typical reaction involves the generation of an electrical current on a platinum electrode
by hydrogen peroxide, the production of which is linked to lactate by the enzyme, lactate
oxidase. Although such analyzers simplify and streamline the analysis process, the technical bottleneck in lactate determinations of ADLs remains obtaining the blood sample in
the post-dive period.
If all reactions that consume or generate lactate could be immediately inhibited in
a blood sample, it is possible that a remote blood sampling device (Hill 1986; Ponganis
et al. 2009) could collect samples during short trips to sea. Lactate determinations could
then potentially be performed days later. The limitation with this approach is the size
of the device, the limited number of samples obtained, and the need for a guaranteed
quick return of the animal (both for sample analysis and the removal of the device and
catheter).
Intravascular blood lactate sensors have been developed and used under experimental
conditions in other animals (Baker and Gough 1995). This specific approach has involved
the detection of lactate with an oxygen electrode enveloped by an outer membrane containing lactate oxidase. The resultant reaction consumes O 2 inside the sensor envelope,
decreasing the partial pressure of O 2 within the outer membrane and decreasing the current output of the O 2 electrode. A second O 2 electrode (with no lactate oxidase) provides
a reference O 2 current, thus providing a difference between the two electrode currents
that is proportional to the blood lactate concentration. Such an approach holds promise
to measure blood lactate concentrations and determine ADLs in more species of marine
mammals. However, there are many technical difficulties to be evaluated, including prevention of thrombus formation, pressure and temperature effects on the reactions, the
lifespan of the embedded lactate oxidase, and the sensitivity and accuracy of the technique
under conditions of low and changing blood P O2 during and after a dive. Despite the testing and development needed for this approach in marine mammals, it may hold the most
promise for providing blood lactate concentration profiles and subsequent determination
of an ADL.
2.3.6 Summary
It should now be clear that there are a number of assumptions and potential sources of
error in determining the O 2 storage of any particular species. The measurement of heart
rate and the ADL also have significant challenges. It should come as no surprise that this
area of diving physiology is still an active area of research as new measurement techniques
are developed and new measurements are made. Consequently, what we know about O 2
storage in marine mammals is subject to frequent revision and our understanding of how
marine mammals can make extended dives continues to improve.
Chapter two: Oxygen stores and diving
order to breathe. Isolated dive hole studies have been conducted only in McMurdo Sound,
Antarctica—with Weddell seals and emperor penguins (Ponganis et al. 1997b; Kooyman
et al. 1980, 1983). The studies with captive or trained animals have included bottlenose
dolphins, beluga whales (Delphinapterus leucas), Baikal seals, and California sea lions
(Ponganis et al. 1997a; Williams et al. 1999). Blood samples have been obtained through
indwelling venous or arterial catheters in pinnipeds, and through needle venipuncture of
tail fluke vessels in the cetaceans.
Blood or plasma lactate concentrations were originally measured through standard
spectrophotometric enzyme assays. Lactate analyzers are now available commercially;
these devices are based on the enzymatically linked generation of an electrical current.
A typical reaction involves the generation of an electrical current on a platinum electrode
by hydrogen peroxide, the production of which is linked to lactate by the enzyme, lactate
oxidase. Although such analyzers simplify and streamline the analysis process, the technical bottleneck in lactate determinations of ADLs remains obtaining the blood sample in
the post-dive period.
If all reactions that consume or generate lactate could be immediately inhibited in
a blood sample, it is possible that a remote blood sampling device (Hill 1986; Ponganis
et al. 2009) could collect samples during short trips to sea. Lactate determinations could
then potentially be performed days later. The limitation with this approach is the size
of the device, the limited number of samples obtained, and the need for a guaranteed
quick return of the animal (both for sample analysis and the removal of the device and
catheter).
Intravascular blood lactate sensors have been developed and used under experimental
conditions in other animals (Baker and Gough 1995). This specific approach has involved
the detection of lactate with an oxygen electrode enveloped by an outer membrane containing lactate oxidase. The resultant reaction consumes O 2 inside the sensor envelope,
decreasing the partial pressure of O 2 within the outer membrane and decreasing the current output of the O 2 electrode. A second O 2 electrode (with no lactate oxidase) provides
a reference O 2 current, thus providing a difference between the two electrode currents
that is proportional to the blood lactate concentration. Such an approach holds promise
to measure blood lactate concentrations and determine ADLs in more species of marine
mammals. However, there are many technical difficulties to be evaluated, including prevention of thrombus formation, pressure and temperature effects on the reactions, the
lifespan of the embedded lactate oxidase, and the sensitivity and accuracy of the technique
under conditions of low and changing blood P O2 during and after a dive. Despite the testing and development needed for this approach in marine mammals, it may hold the most
promise for providing blood lactate concentration profiles and subsequent determination
of an ADL.
2.3.6 Summary
It should now be clear that there are a number of assumptions and potential sources of
error in determining the O 2 storage of any particular species. The measurement of heart
rate and the ADL also have significant challenges. It should come as no surprise that this
area of diving physiology is still an active area of research as new measurement techniques
are developed and new measurements are made. Consequently, what we know about O 2
storage in marine mammals is subject to frequent revision and our understanding of how
marine mammals can make extended dives continues to improve.