EFFECT OF MAGNESIUM-IONS ON RABBIT DETRUSOR CONTRACTILITY AND INTRACELLULAR FREE CALCIUM

Magnesium (Mg2+) is one of the most abundant ions in the body. In the human body, Mg2+ plays important roles including cofactors in many cru cial enzyme systems, especially those involving energy transfer, stora ge and utilization. Alteration of the concentration of Mg2+ may cause neuromuscular hyperactivity, psychiatric disturbances, calcium/potassi um abnormalities, and overactivity of cardiac muscle. Most information on the effect of Mg2+ on muscle contraction has been obtained from st udies on cardiac, skeletal, and vascular muscle; much less is known ab out the effect of Mg2+ in other smooth muscle systems. In the current study, we investigated the effect of Mg2+ on the contraction and intra cellular free calcium of rabbit urinary bladder detrusor muscle in res ponse to carbachol and transmural field stimulation (FS). The results can be summarized as follows: (1) Reduction of the concentration of ma gnesium [Mg2+] from normal Tyrode's solution enhanced the spontaneous basal activity, whereas addition of Mg2+ gradually abolished this spon taneous activity. (2) Muscle contraction induced by FS or carbachol wa s enhanced in Mg2+-free Tyrode's solution. Addition of Mg2+ inhibited the response to both forms of stimulation in a dose-dependent manner. (3) Inhibitory effects of Mg2+ were potentiated when the Ca2+ concentr ation in the Tyrode's solution was reduced to 0.6 mM, whereas increasi ng the extracellular concentration of Ca2+ (5.4 mM) reduced the inhibi tory effects of Mg2+. (4) Using FURA-2 to monitor intracellular free c alcium simultaneous with contractile tension, we demonstrated that the alterations in the contractile responses observed at different concen trations of extracellular Mg2+ correlated with similar changes in intr acellular free calcium. We conclude from this study that Mg2+ can sign ificantly alter the contractile responses to FS and muscarinic stimula tion by alteration of calcium influx during contraction.