CAVEY, M.J.*; WILKENS, J.L.: F-actin in the Amuscular Arteries of the American Lobster

Seven vessels leave the heart of the lobster Homarus americanus: the unpaired anterior median artery, sternal artery, and dorsal abdominal artery and the paired anterior lateral arteries and hepatic arteries. Superficially, these arteries have a common, relatively-simple organization, consisting of an elastic lamina next to the lumen, a multilayered endothelium, and an enveloping adventitia of collagen fibers. Patches of striated muscle cells occur in sectors of the dorsal abdominal artery. All of these arteries should serve as “capacitance” vessels, utilizing elastic elements to minimize the pulsatility of hemolymph flow from the heart. It has been proposed that lobster arteries, with the possible exception of the dorsal abdominal artery, cannot contribute to the differential distribution of hemolymph owing to the absence of diameter-regulating muscle (and, hence, the inability to control their individual resistances). Comparative ultrastructural studies have now revealed extensive networks of cytoplasmic filaments in the endothelial cells of the amuscular arteries. All vessels were prepared in tandem for immunohistochemistry for F-actin. They were slit lengthwise and opened onto microscope slides. The vascular spreads were fixed with formaldehyde, permeabilized with Triton-X 100, and stained with rhodamine phalloidin. Both sarcomeric myofilaments in the muscle cells of the dorsal abdominal artery and the aggregated filaments in the endothelial cells of the six amuscular arteries stain positively for F-actin. It thus appears that the muscular and amuscular arteries alike may have the properties of “resistance” vessels, possessing the contractile machinery by which to regulate their diameters and actively control hemolymph flow.