Objectives. The aims of this study were to determine whether hypertensive patients showed increased endogenous opioid tone and to find a possible correlation between beta-endorphin levels and 24-h ambulatory blood pressure. We also investigated whether circulating beta-endorphin levels were associated with pain perception at rest. Background. Experimental studies suggest an involvement of the endogenous opioid system in cardiovascular control mechanisms. Methods. We determined baseline beta-endorphin plasma levels by radioimmunoassay in 81 consecutive subjects (48 hypertensive, 33 normotensive) after a 30-min rest and before 24-h ambulatory blood pressure monitoring. In 72 of 81 subjects with a dental formula suitable for the pulpar test (graded increase of test current -0 to 0.03 mA applied to three healthy teeth), pain perception was also investigated. Results. Hypertensive patients showed higher beta-endorphin plasma levels than normotensive subjects (p <0.002). Circulating endogenous opioid levels correlated with 24-h diastolic blood pressure (p <0.01), whereas the relation with systolic pressure did not reach statistical significance. When 24-h blood pressure recordings were divided into daytime and nighttime values, and blood pressure loads (percent of measurements ≤140 mm Hg for systolic blood pressure and ≤90 mm Hg for diastolic pressure) were calculated, a significant correlation was found between beta-endorphin levels and diastolic pressures and load. Similarly, presampling diastolic blood pressure was significantly correlated with beta-endorphin levels. Of the 72 subjects tested, hypertensive patients showed a lower pain sensitivity than normotensive subjects. A positive correlation was found between pain threshold and circulating beta-endorphin levels (p <0.05). Conclusions. Sustained arterial pressure is probably involved in the tonic activation of cardiovascular mechanisms linked to endogenous opioid tone. Circulating plasma endorphins may account, at least in part, for the pain perception pattern relating to blood pressure levels at rest.
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