Rhythmic contractions in the portal “corkscrew” vein of a colubrid snake

Abstract

Terrestrial, and especially scansorial, species of snakes have portal veins with a corkscrew morphology immediately posteriorad of the liver. A corkscrew vein can act as a functional valve that impedes gravitational shifts of blood volume. In the corkscrew portal vein of a colubrid snake, spontaneous low frequency and low amplitude contractile rhythms were observed in 11 % of all vein rings. Catecholamines stimulated dose‐dependent cyclical contractions in 89 % of portal vein rings, but not in abdominal veins. Increased contractile amplitude and cycle frequency were stimulated by catecholamines, angiotensin II, and stretch. Near maximal agonist‐induced tension eliminated cyclical contractions, which resumed after removing agonists or adding acetylcholine, but were unaffected by adrenergic receptor antagonists. Treatment with ouabain, verapamil, or indomethacin but not tetrodotoxin rapidly inhibited cyclical contractions, indicating dependence on Na+‐K+ ATPase, extracellular Ca2+, and prostanoids. Histologically, portal vein rings had two smooth muscle layers (luminal, circular; adventitial, longitudinal) sandwiched on a collagen layer. These data suggest that portal vein rhythmic contractility and corkscrew morphology act in concert to improve venous return and to mitigate blood pooling in snakes. Supported by NSF IBN 91‐05247 (KRO), NASA 199‐14‐12‐04 (ARH), and NRC (SRA to HBL).