Distribution, half-life, and excretion of ¹⁴C- and ³H-labeled l-Proly-l-Leucyl-Glycinamide in the rat

H-Pro-Leu-Gly-NH₂, a potentinhibitor of melanocyte stimulating hormone (MSH) release [MSH-release inhibiting factor (MIF)], was labeled with ¹⁴C-leucine or ³H-proline and injected i.v. into rats. H-Pro-¹⁴C-Leu-Gly-NH₂ was found to have a half-life of approximately 9 min and to be distributed in a space greater than that of the plasma volume. Relatively little radioactivity and no intact H-Pro-Leu-Gly-NH₂ could be found in the urine 1 h afte.r administration. ³H- and ¹⁴C-labeled-H-Pro-Leu-Gly-NH₂ accumulated in the pineal, pituitary, kidney, liver, and adrenals; the elevated tissue to plasma ratio in the pineal, along with the identification of unchanged H-Pro-Leu-Gly-NH₂ there, suggests the possibility of a direct influence of the hypothalamus upon the pineal gland.