Disappearance and excretion of labeled α-MSH in man

Despite the considerable evidence for the CNS actions of α-melanocyte-stimulating hormone (α-MSH) in man, little attention has been given to its half-time disappearance from plasma and urinary excretion in normal individuals. In the first experiment, a healthy man was given 15 μCi of tritiated (³H)-α-MSH as a rapid IV injection. A plot of the disappearance time in plasma was characteristic of a multiexponential curve, the linear components of which were resolved by the subtraction method and half-time disappearance calculated directly from the slope of the regression line. The half-time disappearance was 1 min for the first component and 25 min for the second component. After the IV administration of 50 μCi of ¹²⁵I-α-MSH in the second experiment, the two components showed halftime disappearances of 1 min and 4.8 min respectively. These times were not changed by precipitation of the plasma with 10% trichloroacetic acid. Thirty-eight percent and 42% of the label appeared in the urine 4 hr after the injection of either ³H-α-MSH or ¹²⁵I-α-MSH. The results suggest that the persistence of high levels of α-MSH in the blood after injection in man may be too short to fully explain the CNS effects of α-MSH.