The effects of varying Mg ion concentrations on contractions to the cotransmitters ATP and noradrenaline in the rat vas deferens.

The vas deferens responds to a single electrical pulse with a biphasic contraction caused by cotransmitters ATP and noradrenaline. Removing Mg (normally 1.2 mM) from the physiological salt solution (PSS) enhances the contraction. This study aimed to determine the effect of Mg concentration on nerve cotransmitter-mediated contractions. Rat vasa deferentia were sequentially bathed in increasing (0, 1.2, 3 mM) or decreasing (3, 1.2, 0 mM) Mg concentrations. At each concentration a single field pulse was applied, and the biphasic contraction recorded. Contractions to exogenous noradrenaline 10 μM and ATP 100 μM were also determined. The biphasic nerve-mediated contraction was elicited by ATP and noradrenaline as NF449 (10 μM) and prazosin (100 nM) completely prevented the respective peaks. Taking the contractions in normal PSS (Mg 1.2 mM) as 100%, lowering Mg to 0 mM enhanced the ATP peak to 170 ± 7% and raising Mg to 3 mM decreased it to 39 ± 3%; the noradrenaline peak was not affected by lowering Mg to 0 mM (97 ± 3%) but was decreased to 63 ± 4% in high Mg (3 mM). Contractions to exogenous ATP, but not noradrenaline, were increased in Mg 0 mM and both were inhibited with Mg 3 mM. Changing Mg concentration affects the contractions elicited by the cotransmitters ATP and noradrenaline. The greatest effects were to potentiate the contraction to ATP in Mg 0 mM and to inhibit the contraction to both ATP and noradrenaline in high Mg. Future publications should clearly justify any decision to vary the magnesium concentration from normal (1.2 mM) values.