Rhythmic contractility in the hepatic portal "corkscrew" vein of the rat snake

Terrestrial, but not aquatic, species of snakes have hepatic portal veins with a corkscrew morphology immediately posterior of the liver. Relatively large volumes of venous blood are associated with this region, and the corkscrew vein has been proposed to function as a bidirectional valve that impedes gravitational shifts of intravascular volume. To better understand the functional significance of the corkscrew anatomy, we investigated the histology and contractile mechanisms in isolated corkscrew segments of the hepatic portal vein of a yellow rat snake (Pantherophis obsoletus). Morphologically, the corkscrew portal vein is here shown to have two distinct layers of smooth muscle--an inner circular layer, and an outer longitudinal layer, separated by a layer of collagen--whereas only a single circular layer of smooth muscle is present in the adjacent posterior caval vein. Low frequency (approximately 0.3 cycles*min(-1)) spontaneous and catecholamine-induced rhythms were observed in 11% and 89% of portal vein segments, respectively, but neither spontaneous nor agonist-induced cycling was observed in adjacent posterior (non-corkscrew) caval veins. Catecholamines, angiotensin II, or stretch increased the amplitude and/or frequency of contractile cycles. Ouabain, verapamil or indomethacin, but not tetrodotoxin, alpha-, or ss-adrenergic receptor antagonists, inhibited cyclical contractions indicating a dependence of these cycles on Na+/K+ ATPase, extracellular Ca2+ and prostanoid(s). These data suggest that the rhythmic contractility of the corkscrew segment of the ophidian portal vein may act in conjunction with its morphological features to improve venous return and to prevent retrograde shifts of blood that might otherwise pool in posterior veins.