Calcium and Contraction in the Rabbit Anterior Mesenteric–Portal Vein

Abstract

Studies were done measuring contractile activity in the isolated rabbit anterior mesenteric–portal vein (A.M.V.). The rabbit A.M.V. requires external calcium for both spontaneous activity and the initiation and maintenance of contractions to agonists.After bathing the tissues in calcium-free Krebs' solution containing 1 mM ethyleneglycoltetraacetic acid (EGTA) for 10 min, and then washing out the EGTA, the re-addition of calcium produced a contraction. The pre-addition of a small amount of calcium (0.1 mM) after EGTA treatment (while not inducing a contractile response) alters subsequent contractile responses to larger concentrations of calcium. This is interpreted as the ability of membrane-bound calcium to control membrane permeability to calcium.Other experiments involved bathing the tissues in calcium-free solution until no response was obtained to agonists, and then re-adding calcium. The time course of the responses to noradrenaline after calcium re-addition depended on the length of calcium incubation. This suggests that noradrenaline can release a loosely bound calcium fraction which may be involved in controlling calcium influx. This bound calcium, however, does not provide a major source of activator calcium in normal Krebs' solution.