Summary: Coleoid cephalopods (octopus, cuttlefish and squid) have
high metabolic rates and a relatively low oxygen-carrying
capacity in the blood, which implies large cardiac outputs by
the standards of invertebrate animals or fish (Wells, 1992).
Furthermore, during periods of exercise, the increased rate of
oxygen uptake (2.3-fold in Octopus vulgaris; Wells et al., 1983)
is met by an increased cardiac output, since there is little or no
scope for increasing oxygen extraction (already approximately
80% in resting conditions; Houlihan et al., 1986).
The high demand on the cardiac pump is also reflected in
the cellular characteristics of the systemic cardiac muscle.
Isolated cardiac muscle strips from Octopus vulgaris show a
regular twitch force development at stimulation frequencies at
which the cardiac muscle of other ectothermic vertebrates,
such as the rainbow trout, commonly fail (Gesser et al., 1997).
Furthermore, ryanodine strongly inhibits twitch-force
development and increases resting tension, suggesting that
excitationcontraction coupling is highly dependent on Ca2+
cycling via the sarcoplasmic reticulum (SR), as is known to
occur in the cardiac muscle of the rat.