Short Communication Contractile Performance of Cephalopod Hearts under Anoxic Conditions

The cephalopod circulatory system includes an efferent systemic ventricle and two afferent branchial hearts. Branchial hearts lack coronary arteries and thus must be nourished by only venous blood. During routine metabolism in normoxic water, oxygen availability to branchial hearts is only a small percentage of that to the systemic heart; during hypoxic excursions the venous oxygen content may, at least transiently, approach zero (Johansen, Brix & Lykkeboe, 1982; Houlihan, Innes, Wells & Wells, 1982). The extreme difference in oxygen delivery to these tissues suggests that branchial hearts are better able to meet their energetic demands through anaerobic metabolism than are systemic hearts. This hypothesis was tested by following isometric force development of the cardiac tissue of the cuttlefish, Sepia officinalis, during cyanide poisoning. Experiments were also conducted with the systemic heart of Octopus vulgaris, in order to allow a comparison between a decapod and an octapod. Specimens of Sepia officinalis Lam. (193 +29g; JV = 7) and Octopus vulgaris Cuvier (1550 +. 367 g; N = 6) were maintained in running sea water. Animals were anaesthetized in 3 % ethanol, decapitated and the hearts excised. Sepia systemic ventricles were sectioned along the median line and each half utilized. Octopus ventricles were further sectioned transversely, allowing four preparations per animal. Each Sepia branchial heart was used for a single preparation. Tissues were spread flat to expose the interior surface, one end firmly fixed with a plastic clip, and the other end tied with surgical silk to a Harvard 363 isometric tension transducer interfaced with a Biotronex BL recorder. Hearts were forced to contract via field stimulation from two parallel silver electrodes, one on each side of the tissue. Square wave pulses of 20 ms duration with the minimum voltage necessary to produce the maximal mechanical response were generated from a Grass SD9 stimulator. After positioning, tissues were allowed a 30min rest period and thereafter paced at 12 beats min". Following 5 min of contraction, NaCN (1 mmol 1~' final concentration) was added to the bathing media in one half of the preparations and the recording of force continued for a further 25 min. Preliminary experiments showed that 2 mmol 1~ NaCN did not have any greater effect than 1 mmol I" NaCN, a concentration which inhibits