The bradykinin B1 receptor and the central regulation of blood pressure in spontaneously hypertensive rats

We evaluated if the brain bradykinin (BK) B1 receptor is involved in the regulation of blood pressure (BP) in conscious rats. Basal mean BP and HR were 115±2 and 165±3 mmHg and 345±10 and 410±14 beats min−1 in Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR), respectively. Intracerebroventricular (i.c.v.) injection of 1 nmol B1 receptor agonist Lys-desArg9-BK significantly increased the BP of WKY and SHR by 7±1 and 19±2 mmHg, respectively. One nmol Sar[D-Phe8]-desArg9-BK, a kininase-resistant B1 agonist, increased the BP of WKY and SHR by 19±2 and 17±2 mmHg, respectively and reduced HR in both strains. I.c.v. injection of 0.01 nmol B1 antagonists, LysLeu8-desArg9-BK or AcLys[D-βNal7,Ile8]-desArg9-BK (R715), significantly decreased mean BP in SHR (by 9±2 mmHg the former and 14±3 mmHg the latter compound), but not in WKY. In SHR, the BP response to R715 was associated to tachycardia. I.c.v. Captopril, a kininase inhibitor, increased the BP of SHR, this response being partially prevented by i.c.v. R715 and reversed into a vasodepressor effect by R715 in combination with the B2 antagonist Icatibant. I.c.v. antisense oligodeoxynucleotides (ODNs) targeted to the B1 receptor mRNA decreased BP in SHR, but not in WKY. HR was not altered in either strain. Distribution of fluorescein-conjugated ODNs was detected in brain areas surrounding cerebral ventricles. Our results indicate that the brain B1 receptor participates in the regulation of BP. Activation of the B1 receptor by kinin metabolites could participate in the pathogenesis of hypertension in SHR. Keywords: Brain, hypertension, genetic, kinins, DNA, fluorescence Introduction The kallikrein-kinin system is present in exocrine glands, kidney, cardiovascular system, brain, and pituitary and it has been implicated in a plethora of physiological functions such as the regulation of local and systemic haemodynamics, vascular permeability, neuronal activation, and passage of water and electrolytes across epithelia (see Clements, 1989; Margolius, 1995). Kinins are local hormones released by kallikrein from the substrate kininogen. They act on two receptor subtypes, B1 and B2, classified according to the relative potency of natural agonists (see Regoli & Barabe, 1980). While the B2 receptor is typically activated by bradykinin (BK) and Lys-bradykinin (Lys-BK), the B1 receptor is selectively sensitive to kinin metabolites without the C-terminal arginine residue, e.g. des-Arg9-BK (DABK) and Lys-des-Arg9-BK (Lys-DABK). Although most of the central and peripheral effects of kinins are known to be mediated by the B2 receptor, which is constitutively expressed in different cell types, recent reports suggest a potential role of the B1 receptor in pathophysiology. For instance, it might be implicated in inflammation, hyperalgesia, hyperthermia, and experimental diabetes (Dray & Perkins, 1993; Davis et al., 1994; Coelho et al., 1997; Zuccolo et al., 1996). In these conditions, the B1 receptor is apparently rapidly upregulated. Evidence exists that all the components of the kallikrein-kinin system are present in the rat brain (Correa et al., 1979; Chao et al., 1983; Perry & Snyder, 1984; Scicli et al., 1984; Simson et al., 1985; Lopes & Couture, 1997) and that activation of the brain B2 receptor by endogenous kinins exerts modulatory effects on systemic blood pressure (BP) (Lindsley et al., 1988; Yang et al., 1989; Madeddu et al., 1990; 1996; Alvarez et al., 1992; Priviteira et al., 1994; Khan et al., 1995; Caligiorne et al., 1996). In particular, inhibition of brain kininase II, one of the enzymes responsible for kinin degradation, increases the already elevated BP of spontaneously hypertensive rats (SHR) through the B2 receptor (Madeddu et al., 1990). Possible participation of the B1 receptor in this hypertensive effect has yet to be determined. Recently, Raidoo & Bhoola (1997) have demonstrated that the B1 receptor is localized on neurones of the human brain at the level of the thalamus, spinal cord and hypothalamus, providing an anatomical basis for a possible role of this subtype receptor in the central regulation of BP. However, functional studies performed so far did not reach consistent conclusions. Martins et al. (1991) reported that neither the B1 receptor agonist DABK nor the antagonist des-Arg9-Leu8-BK (DALBK) injected into the fourth cerebral ventricle altered the BP of Wistar Kyoto (WKY) or SHR, nor did the antagonist interfere with the BP response to i.c.v. BK. In contrast, Caligiorne et al. (1996) showed that DALBK, microinjected into the nucleus of tractus solitarii of WKY rats prevented the cardiovascular effects of exogenous BK, and Alvarez et al. (1992) reported that brain B1 receptor blockade lowers BP in SHR but not in normotensive rats. In some of the above reported studies, full understanding of the potential role of the B1 receptor might have been precluded by the pharmacokinetic properties and short half-life of first-generation analogue-antagonists. These limitations were overcome by the recent introduciton of second-generation B1 antagonists (Drapeau et al., 1993; Gobeil et al., 1996), characterized by enhanced selectivity and resistance to kininases, and by the availability of antisense oligodeoxynucleotides (ODNs), targeted to B1 receptor mRNA. In the present study, we applied these newly developed strategies to evaluate if kinins participate in the central regulation of BP through the activation of the B1 receptor. In addition, we determined whether this receptor plays a role in the acute hypertensive response to i.c.v. Captopril, a kininase inhibitor, in SHR.