Ultrastructure of 16 S substances obtained by reassociation using different combinations of three isolated subunits of scorpion hemocyanin (Androctonus australis garzonii)

Regulation of air and blood flow through the booklungs of the desert scorpion, Paruroctonus mesaensis

Injections of dye, latex and India ink were used to reveal the path of hemolymph circulation through the scorpion booklungs. Fine, branched arteries carry blood directly to muscle and other organs. The blood returns through venous channels to the ventral mesosoma where it passes laterally through the booklungs and into the pneumocardial veins just beneath the pleural cuticle. Blood flows dorsally through these veins to the pericardial sinus and heart. The scorpion has four pairs of booklungs located in the anterior segments of the ventral mesosoma. Each booklung has a spiracle which opens into an atrium enclosed by cuticular membrane. Air passes from the atrium into the booklung lamellae. Agitation of the animal or application of CO(2) causes retraction of the anterior and posterior atrial membrane. This expands the atrial chamber and allows gas exchange in the booklung lamellae. The posterior atrial membrane has a specialized region which forms a springy valve. This normally closes the spiracle unless pulled open by contraction of the attached poststigmaticus muscle. The pectens and receptors within the atrium may mediate the responses to CO(2). Slender hypocardial ligaments containing muscle fibers extend from the heart (dorsal mesosoma) to the booklungs in the ventral mesosoma. Heart movements thus cause dorso-ventral movement of the booklungs. The significance of these movements is as yet unclear. They may increase ventilation, help force blood to the heart and/or agitate the blood and booklung lamellae and thereby aid gas exchange. Passage of blood through the booklungs is regulated by dorsal and ventral muscles attached to the atrium at the lateral edge of the booklung. Contraction of the ventral atrial muscle closes the excurrent channel for passage of blood from the booklung into the pneumocardial vein. Electrical stimulation of the segmentai nerves from the subesophageal and first three abdominal ganglia causes spiracle opening and contraction of muscles attached to the atrial membrane. A previous study showed that these same segmental nerves also modulate heart activity. They thus provide a major pathway for regulation of the respiratory and circulatory systems.

Haemocyanins

About ten years ago, one of the authors participated in a review of haemocyanin structure and function (van Holde & van Bruggen, 1971). At that time, it was possible to describe the field in terms of a limited amount of exciting new structural information, and a long list of unanswered questions. While the stoichiometry of oxygen binding was understood, virtually nothing was known about the active site. Even the oxidation state of the copper was a matter of conjecture. The size of the haemocyanin polypeptide chains was the subject of intense debate, with very little substantive knowledge available. While the haemocyanins were known to be allosteric proteins, there were virtually no experimental studies of oxygen binding on a level that could be meaningfully interpreted in terms of extant theories.