Different blood pressure responses to opioids in 3 rat hypertension models: role of the baseline status of sympathetic and renin-angiotensin systems.

Opioids interact with sympathetic and renin-angiotensin systems in control of mean arterial pressure (MAP). Our earlier finding that biphalin, a synthetic enkephalin analogue, decreased MAP in anaesthetized spontaneously hypertensive rats (SHR) prompted us to further explore this action, to get new insights into pathogenesis of various forms of hypertension. Biphalin effects were studied in SHR, uninephrectomized rats on a high-salt diet (HS/UNX), and rats with angiotensin-induced hypertension (Ang-iH). Besides MAP, renal and iliac blood flows (RBF, IBF) and vascular resistances were measured. In anaesthetized and conscious SHR, biphalin (300 μg·h-1·kg-1 i.v.) decreased MAP by ∼10 and ∼20 mm Hg, respectively (P < 0.001). In anaesthetized HS/UNX and normotensive rats, MAP increased by ∼6-7 mm Hg (P < 0.02); without anaesthesia, only transient decreases occurred. MAP never changed in Ang-iH rats. Morphine (1.5 mg·h-1·kg-1 i.v.) decreased MAP in HS/UNX but only transiently so without anaesthesia; such anaesthesia dependence of response was also seen in normotensive rats. Ang-iH rats never responded to morphine. Hypotensive effect in SHR only depends primarily on the reduction by biphalin of vascular responsiveness to increased sympathetic stimulation; such increase is well documented for SHR. No MAP response to biphalin or morphine in Ang-iH could depend on angiotensin-induced alterations of the vascular wall morphology and function.