The Effects of Manganese on the Rabbit Anterior Mesenteric–Portal Vein

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Abstract
The effects of manganese and lanthanum were tested on the contractile responses of the rabbit anterior mesenteric–portal vein (A.M.V.). Since these ions have been shown to block calcium fluxes in other tissues, examining their effects may give information concerning the importance of calcium influx for contractions of the A.M.V. The effects of the two cations on the A.M.V. were found to be similar except that lanthanum was more potent and its inhibitory effects were not readily reversible. Spontaneous action potentials and their associated contractions were abolished by the addition of 0.5 mM Mn2+. This inhibition could be rapidly reversed by adding EDTA to chelate the manganese. Manganese inhibited the responses to all agonists tested (noradrenaline, histamine, serotonin, procaine, and KCl). The inhibition of responses to all agonists but procaine could be partially reversed by raising the external calcium concentration. The interaction between calcium and manganese follows competitive kinetics. Adding manganese during the tonic phase of a contraction evoked by 10−6 g/ml noradrenaline resulted in a relaxation to a decreased, but sustained, tension. The inhibitory effects of manganese add further evidence that an influx of calcium from the extracellular fluid is required for spontaneous activity and for both the initiation and maintenance of contractions to agonists in the rabbit A.M.V.

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