Alterations in Tyrosine Hydroxylase and Monoamine Oxidase Activity in Blood Vessels

Abstract

THE enzyme tyrosine hydroxylase¹ (TH), which has been reported as the rate limiting step in noradrenaline biosynthesis, can be modified by nerve stimulation, cold^2,3, exercise⁴, reser-pine, phenoxybenzamine, and monoamine oxidase inhibitors^5–7. These treatments affect not only the enzyme in vitro but also catecholamine synthesis in vivo. Much of this information has come from studies with heart, brain, adrenals and spleen, but we found that blood vessels contain appreciable concentrations of noradrenaline⁸ and synthesize it in vivo from its precursor tyrosine. We now report that blood vessels have higher tyrosine hydroxylase activity than the heart and that this activity can be modified by reserpine and L-dihydroxyphenylalanine (L-dopa). Furthermore, the activity of tyrosine hydroxylase in the blood vessels of a spontaneously hypertensive rat differs from that in its normotensive control. We also found that the activity of the enzyme monoamine oxidase in the vasculature was affected by drugs and changes in blood pressure.