Immunoreactive plasma concentrations of an endogenous antiopiate are higher in spontaneously hypertensive rats than in Wistar-Kyoto rats.

Tyrosine-MIF-1 (Tyr-Pro-Leu-Gly-NH2) is present in rat brain in varying concentrations throughout the day and can act as an opiate antagonist. Since altered sensitivity to pain is known to occur in hypertension, plasma and brain concentrations of Tyr-MIF-1--like immunoreactivity were measured in spontaneously hypertensive rats (SHR) and compared every 4 hours for 24 hours with the concentrations in control Wistar-Kyoto rats (WKY). The Tyr-MIF-1--like immunoreactivity in plasma was significantly higher in SHR than in the WKY at each interval; the mean difference was 62% (p less than 0.001). High-performance liquid chromatography demonstrated that peak immunoreactivity eluted in the same position as the synthetic tetrapeptide. Brain concentrations of the peptide were not reliably different between SHR and WKY. The diurnal rhythm was particularly evident in SHR: the highest concentrations of peptide in both brain and plasma occurred at 2000 hours. These results suggest the presence of another difference between SHR and WKY.