Influence of magnesium on norepinephrine-and histamine-induced contractions of pulmonary vascular smooth muscle.

We examined the effects of magnesium on contractile responses of the rabbit pulmonary artery. The contractile force was determined, after applications of norepinephrine or histamine, in a normal or Ca++-free solution containing 0 mM Mg++ or 1.2 mM Mg++. In a normal solution, Mg++ increased the EC50 value for histamine, but did not alter the EC50 value for norepinephrine or the maximum force induced by norepinephrine or histamine. Contractile responses to norepinephrine and histamine were equally reduced by a Ca++-free, Mg++-free solution, and were further reduced by a Ca++-free solution containing Mg++, but with a greater reduction in the response to histamine than in the response to norepinephrine. The results indicate that in the pulmonary artery, Mg++ alters the sensitivity to histamine but not to norepinephrine, and may differentially inhibit bound Ca++ release by norepinephrine and histamine.