Plasma concentration and vascular effect of ?-endorphin in spontaneously hypertensive and Wistar Kyoto rats

In order to find out whether beta-endorphin (beta-E) is involved in the development of hypertension, we performed two series of experiments. Firstly, spontaneously hypertensive rats (SHR) and their normotensive Wistar Kyoto controls (WKY) were submitted to ether stress. Plasma concentrations of beta-endorphin-like immunoreactivity (beta-EI), adrenocorticotropin (ACTH) and alpha-melanotropin (alpha-MSH) were measured by radioimmunoassay. The basal concentration of beta-EI was similar in WKY and SHR, whereas WKY had higher levels of ACTH and lower levels of alpha-MSH than SHR. In both strains acute stress enhanced the plasma concentration of beta-EI to the same extent and with a similar time-course. The increase of plasma beta-EI coincided with a rise in ACTH but not alpha-MSH. Gel chromatography of beta-EI revealed that plasma extracts contain similar amounts of beta-lipotropin- (beta-LPH) and beta-E-sized immunoreactive components, and that acute stress elevated both forms of beta-EI. Secondly, isolated tail arteries of SHR and WKY were perfused and field stimulated with two pulses at 1 Hz. beta-E depressed stimulation-evoked vasoconstriction with the same potency in both strains. Thus, basal and stress-induced levels of beta-EI did not differ in SHR and WKY. Moreover, in the tail artery of both strains the sensitivity of presynaptic opioid receptors towards beta-E was almost identical. If the beta-E sensitivity of these receptors in other arteries of WKY and SHR is also similar a major role of the circulating peptide in the development of hypertension is rather unlikely.