Effect of magnesium on isoproterenol-induced alpha and beta receptor responses of vascular smooth muscle.

The presence of 1.2 mM magnesium in normal Ringer's solution decreased the sensitivity (increased ED50) and increased the maximum tension of rabbit aortic strips developed in response to dl-isoproterenol. Magnesium in the medium enhanced the degree of reserpine-induced supersensitivity to isoproterenol in whole aortic strips. On the other hand, in adventitia-free aortae, supersensitivity to isoproterenol was absent in the presence of magnesium. Isoproterenol-induced relaxation responses mediated by beta receptors were not modified by magnesium. However, maximum contractile responses mediated by alpha receptors were increased in the presence of magnesium. The decrease in sensitivity to isoproterenol in the presence of magnesium might be the result of its effect on calcium movements at the membrane and/or at intracellular calcium binding sites. The potentiating effect on the contractile tension responses to isoproterenol by magnesium might be a result of an increase in the amount of available calcium or to conformational changes produced on the membrane leading to a change in the contractility of the cells.