Irena Duka

Vasoactive Potential of the B1 Bradykinin Receptor in Normotension and Hypertension

Abstract— The B1 type receptor of bradykinin (Bk B1R) is believed to be physiologically inert but highly inducible by inflammatory mediators and tissue damage. To explore the potential participation of the Bk B1R in blood pressure (BP) regulation, we studied mice with deleted Bk B2R gene with induced experimental hypertension, either salt-dependent (subtotal nephrectomy with 0.5% NaCl as drinking water) or renin/angiotensin-dependent (renovascular 2-kidney–1-clip). Compared with the wild-type controls, the B2R gene knockout mice had a higher baseline BP (109.7±1.1 versus 101.1±1.3 mm Hg, P =0.002), developed salt-induced hypertension faster (in 19.3±2.3 versus 27.7±2.4 days, P =0.024), and had a more severe end point BP (148±3.7 versus 133±3.1 mm Hg, P <0.05). On the contrary, renovascular hypertension developed to the same extent (149.7±4.3 versus 148±3.6 mm Hg) and in the same time frame (14±2.2 versus 14±2.1 days). A bolus infusion of a selective B1R antagonist at baseline produced a significant hypertensive response (by 11.4±2 mm Hg) in the knockout mice only. Injection of graded doses of a selective B1R agonist produced a dose-dependent hypotensive response in the knockout mice only. Assessment of tissue expression of B1R and B2R genes by reverse transcription–polymerase chain reaction techniques revealed significantly higher B1R mRNA levels in the B2R knockout mice at all times (normotensive baseline and hypertensive end points). At the hypertensive end points, there was always an increase in B1R gene expression over the baseline values. This increase was significant in cardiac and renal tissues in all hypertensive wild-type mice but only in the clipped kidney of the renovascular knockout mice. The B2R gene expression in the wild-type mice remained unaffected by experimental manipulations. These results confirm the known vasodilatory and natriuretic function of the Bk B2R; they also indicate that in its absence, the B1R can become upregulated and assume some of the hemodynamic properties of the B2R. Furthermore, they indicate that experimental manipulations to produce hypertension also induce upregulation of the B1R, but not the B2R, in cardiac and renal tissues.

Angiotensin-Converting Enzyme Inhibition After Experimental Myocardial Infarct: Role of the Kinin B1 and B2 Receptors

We sought to define the contribution of each of the 2 kinin receptors (bradykinin 1 receptor [B1R] and bradykinin 2 receptor [B2R]) to the cardioprotection of angiotensin-converting enzyme (ACE) inhibition after acute myocardial infarct. Wild-type mice and gene knockout mice missing either B1R or B2R were submitted to coronary ligation with or without concurrent ACE inhibition and had evaluation of left ventricular systolic capacity by assessment of fractional shortening (FS). Baseline FS was similar in all of the animals and remained unchanged in sham-operated ones. At 3 weeks after myocardial infarct, in the wild-type group there was a 27% reduction of FS (P<0.5) without ACE inhibition and 8% with ACE inhibition; in the B1R−/− groups the FS was reduced by 24% and was no different (at 28%) with ACE inhibition; in the B2R−/− groups, however, the FS was decreased by 39% and with ACE inhibition was decreased further by 52%. Analysis of bradykinin receptor gene expression in hearts showed that when one receptor was missing, the other became significantly upregulated; but the B1R remained highly overexpressed in the B2R−/− mice throughout, whereas the overexpressed B2R became significantly suppressed in the B1R−/− mice in a manner quantitatively and directionally similar to that of wild-type mice. We conclude that both bradykinin receptors contribute to the cardioprotective bradykinin-mediated effect of ACE inhibition, not only the B2R as believed previously; but, whereas with potentiated bradykinin in the absence of B1R, the upregulation of B2R is simply insufficient to provide full cardioprotection, in the absence of B2R, the upregulated B1R actually seems to inflict further tissue damage.

Role of the postsynaptic α2-adrenergic receptor subtypes in catecholamine-induced vasoconstriction

Catecholamines induce direct vasoconstriction mediated by postsynaptic alpha-adrenergic receptors (alpha-ARs) of both the alpha(1) and alpha(2) type. To evaluate the contribution of each alpha(2)-AR subtype (alpha(2A), alpha(2B), and alpha(2C)) to this function, we used groups of genetically engineered mice deficient for the gene to each one of these subtypes and compared their blood pressure (BP) responses to their wild-type counterparts. Blood pressure responses to a bolus of norepinephrine (NE) were assessed before and after sequential blockade of alpha(1)-ARs with prazosin and alpha(2)-ARs with yohimbine. The first NE bolus elicited a brief 32 to 44 mm Hg BP rise (p < 0.001 from baseline) in all six groups. Prazosin decreased BP by 23 to 33 mm Hg in all groups, establishing a new lower baseline. Repeat NE at that point elicited lesser but still significant (p < 0.001) brief pressor responses between 32% and 45% of the previous BP rise in five of the six groups. Only the alpha(2A)-AR gene knockouts differed, responding instead with a 20-mm Hg fall in BP, a significant change from baseline (p < 0.001) and different from the pressor response of their wild-type counterparts (p < 0.001). The addition of yohimbine produced no further BP change in the five groups, but it did produce a small 7. 5-mm Hg fall (p < 0.05) in the alpha(2A)-AR knockouts. Norepinephrine bolus during concurrent alpha(1) and alpha(2)-AR blockade produced significant (p < 0.001) hypotensive responses in all subgroups, presumably attributable to unopposed stimulation of beta(2)-vascular wall ARs. We conclude that the alpha(2)-AR-mediated vasoconstriction induced by catecholamines is attributable to the alpha(2A)-AR subtype because mice deficient in any one of the other subtypes retained the capacity for normal vasoconstrictive responses. However, the alpha(1)-ARs account for the major part (as much as 68%) of catecholamine-induced vasoconstriction.

Mechanisms Mediating the Vasoactive Effects of the B1 Receptors of Bradykinin

Abstract—Bradykinin normally exerts its vasodilatory effect via the B2 receptor (B2R), but in this receptor’s absence, the B1 receptor becomes expressed and activated. To explore the mechanism of B1R-mediated vasodilation, 8 groups of B2R gene–knockout mice received a 2-week infusion of a B1R antagonist (300 &mgr;g · kg−1 · d−1) or vehicle (groups 1 and 2), B1R antagonist or vehicle plus NO inhibition with N&ohgr;-nitro-l-arginine methyl ester (groups 3 and 4), B1R antagonist or vehicle plus cyclooxygenase inhibition with indomethacin (groups 5 and 6), or B1R antagonist or vehicle plus blockade of vasoconstricting prostaglandin (PG) H2 and thromboxane A2 (TxA2) with SQ29548 (groups 7 and 8). The B1R antagonist produced significant (P <0.05) blood pressure increases of 17.7±3.1 mm Hg in group 1 and 10.4±3 mm Hg in group 3, whereas their vehicle-treated respective control groups 2 and 4 had no significant blood pressure changes. Indomethacin abolished the capacity of the B1R antagonist to raise blood pressure, as did blockade of the receptors of PGH2 and TxA2. Injection with the B1R agonist produced a hypotensive response (12±1.3 mm Hg), which was further accentuated by TxA2 blockade (21.7±4.1 mm Hg). Analysis of B1R gene expression by reverse transcription–polymerase chain reaction (PCR) in cardiac and renal tissues revealed marked expression at baseline, with further upregulation by 1.5- to 2-fold after various manipulations. Expression of the TxA2 receptor gene in renal tissue by quantitative real-time PCR was significantly lower in mice treated with the B1R antagonist, consistent with increased levels of agonist for this receptor. The data confirm that the B1R becomes markedly expressed in the absence of B2R and suggest that it contributes to vasodilation by inhibiting a vasoconstricting product of the arachidonic acid cascade acting via the PGH2/TxA2 receptor.