Effect of bradykinin metabolism inhibitors on evoked hypotension in rats: rank efficacy of enzymes associated with bradykinin-mediated angioedema.

BACKGROUND AND PURPOSE: Inhibition of bradykinin metabolizing enzymes (BMEs) can cause acute angioedema, as demonstrated in a recent clinical trial in patients administered the antihypertensive, omapatrilat. However, the relative contribution of specific BMEs to this effect is unclear and confounded by the lack of a predictive pre-clinical model of angioedema. EXPERIMENTAL APPROACH: Rats were instrumented to record blood pressure and heart rate; inhibitors were infused for 35 min and bradykinin was infused during the last 5 min to elicit hypotension, as a functional marker of circulating bradykinin and relative angioedema risk. KEY RESULTS: In the presence of omapatrilat bradykinin produced dose-dependent hypotension, an effect abolished by B(2) blockade. In the presence of lisinopril (ACE inhibitor), but not candoxatril (NEP inhibitor) or apstatin (APP inhibitor), bradykinin also elicited hypotension. Lisinopril-mediated hypotension was unchanged with concomitant blockade of NEP or NEP/DPPIV (candoxatril+A-899301). However, hypotension was enhanced upon concomitant blockade of APP and further intensified in the presence of NEP inhibition to values not different from omapatrilat alone. CONCLUSIONS AND IMPLICATIONS: We demonstrated that bradykinin is degraded in vivo with an enzyme rank-efficacy of ACE>APP>>NEP or DPPIV. These results suggest the effects of omapatrilat are mediated by inhibition of three BMEs, ACE/APP/NEP. However, dual inhibition of ACE/NEP or ACE/NEP/DPPIV elicits no increased risk of angioedema compared to ACE inhibition alone. Thus, novel BME inhibitors must display no activity against APP to avoid angioedema risk due to high prevalence of ACE inhibitor therapy in patients with diabetes and cardiovascular disease.

Spinal pharmacology of antinociception produced by microinjection of mu or delta opioid receptor agonists in the ventromedial medulla of the rat.

This study examined the role of spinal GABAergic, serotoninergic and alpha(2) adrenergic receptors in the antinociception produced by the microinjection of equi-antinociceptive doses of selective opioid receptor agonists in the nucleus raphe magnus (NRM) or the nucleus reticularis gigantocellularis pars alpha (NGCpalpha) of the rat. Rats were pretreated with intrathecal administration of either the GABA(A) receptor antagonist bicuculline, the GABA(B) receptor antagonist CGP35348, the serotonin(1/2) receptor antagonist methysergide, the alpha(2) adrenergic receptor antagonist yohimbine or saline. Ten minutes later, either the delta(1) opioid receptor agonist [D-Pen(2,5)]enkephalin (DPDPE), delta(2) opioid receptor agonist [D-Ala(2),Glu(4)]deltorphin (DELT) or mu opioid receptor agonist [D-Ala(2),NMePhe(4),Gly-ol(5)]enkephalin (DAMGO) was microinjected into the NRM, NGCpalpha or sites in the medulla outside these two regions. The increase in tail-flick latency produced by microinjection of DPDPE into the NRM or NGCpalpha was antagonized by intrathecal pretreatment with either methysergide or yohimbine. Intrathecal pretreatment with CGP35348 antagonized the antinociception produced by microinjection of DPDPE in the NRM, whereas bicuculline antagonized the antinociception produced by microinjection of DPDPE in the NGCpalpha. The increase in tail-flick latency produced by microinjection of DELT into the NGCpalpha, but not the NRM was antagonized by intrathecal pretreatment with yohimbine or CGP35348. Intrathecal pretreatment with methysergide or bicuculline did not antagonize the antinociception produced by microinjection of DELT into either the NRM or the NGCpalpha. The increase in tail-flick latency produced by microinjection of DAMGO in the NRM was antagonized by intrathecal pretreatment with methysergide or CGP35348, but not by bicuculline or yohimbine. Taken together, these results support the hypothesis that the antinociception produced by activation of delta(1), delta(2) or mu opioid receptors in the rostral ventromedial medulla is mediated by different neural substrates.

Supraspinal and spinal delta(2) opioid receptor-mediated antinociceptive synergy is mediated by spinal alpha(2) adrenoceptors.

Concurrent administration of low doses of [D-Ala(2), Glu(4)]deltorphin (DELT) in the spinal cord and rostral ventromedial medulla of the rat produces a synergistic antinociception in the tail-flick test. It was postulated that the synergistic antinociception results from an interaction of the intrathecally-administered DELT with norepinephrine released in the spinal cord as a result of the microinjection of DELT in the rostral ventromedial medulla. Three approaches were taken to test this hypothesis. The first experiment determined that microinjection of DELT in the rostral ventromedial medulla produced an increase in tail-flick latency that was partially attenuated by intrathecal administration of the alpha(2)-adrenoceptor antagonist yohimbine. These data indicated that microinjection of DELT in the medulla causes a release of norepinephrine in the spinal cord. The second experiment determined that intrathecal co-administration of DELT with the alpha(2)-adrenoceptor agonist dexmedetomidine in a 2:1 fixed dose ratio produced a synergistic antinociception in the tail-flick test. The final experiment determined that the antinociception produced by concurrent medullary and intrathecal administration of DELT was completely antagonized by intrathecal administration of yohimbine. Taken together, these findings support the hypothesis that the synergistic antinociception produced by concurrent activation of medullary and spinal delta(2) opioid receptors is mediated, in part, by endogenous norepinephrine release in the spinal cord. The norepinephrine, acting at alpha(2)-adrenoceptors, interacts in a synergistic manner with intrathecally administered DELT, acting at spinal delta(2) opioid receptors, to produce antinociception.