A Physiological Comparison of Bivalve Mollusc Cerebro-visceral Connectives With and Without Neurohemoglobin. II. Neurohemoglobin Characteristics

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Abstract
Several bivalve mollusc species possess hemoglobin in their nervous systems whereas most species do not. The function of this neurohemoglobin was investigated in situ in cerebro-visceral connectives of Tellina alternata and Spisula solidissima. Both neurohemoglobins, located in glial cells, exhibit high oxygen affinities and relatively high Hill numbers. The rate of oxygen diffusion into the connective begins to fall below the consumption rate near the PO2 at which each neurohemoglobin begins to unload oxygen, assuming the perineural sheath presents an effective barrier to oxygen diffusion. The neurohemoglobin could thus act as an oxygen store during periods of low PO2. Oxygen unloading from the neurohemoglobin proceeds for a considerable length of time at a constant rate. The long duration may be attributed to the geometry of the connective and to the perineural sheath, whose primary function may be to retain oxygen within the connective during anoxic conditions. The constant unloading rate may be attributed to neurohemoglobin cooperativity in situ because the driving force of unloading remains nearly constant at the P50 of each neurohemoglobin. An oxygen supply at a constant rate for an extended period of time would be useful to an animal requiring aerobic nervous function during anoxic conditions.