Early upregulation of kinin B1 receptors in retinal microvessels of the streptozotocin-diabetic rat

1 Retinal microvessel responses to kinin B-1 and B-2 receptor agonists and antagonists were investigated in streptozotocin (STZ)-diabetic rats and age-matched controls. In addition, quantitative in vitro autoradiography was performed on retinas from control and STZ-diabetic rats with radioligands specific for B-2 ([I-125]HPP-Hoe 140), and B-1 receptors ([I-125]HPP-[des-Arg(10)]-Hoe 140). 2 In control rats, the B-2 receptor agonist bradykinin (BK, 0.1 - 50 nm) vasodilated retinal vessels in a concentration and time-dependent manner. This effect was completely blocked by the B-2 receptor antagonist Hoe140 (1 muM). In contrast, the B, receptor agonist des-Arg(9)-BK (0.1 -50 nM) was without effect. 3 Des-Arg(9)-BK was able to produce a concentration-dependent vasodilatation as early as 4 days after STZ injection, and the effect of 1 nM des-Arg(9)-BK was inhibited by the B-1 receptor antagonist des-Arg(10)-Hoe140 (1 muM). Low-level B-1 receptor binding sites were detected in control rats, but densities were 256% higher in retinas from 4- to 21-day STZ-diabetic rats. 4 In control rats, the vasodilatation in response to I nm BK involved neither calcium influx nor nitric oxide (NO) as GdCl3 and L-NAME were without effect. However, the vasodilatation did involve intracellular calcium mobilization as well as products of the cyclooxygenase-2 (COX-2) pathway as 2.5-di-t-butylhydroquinone (BHQ), cADP ribose and L-745 337 inhibited this response. The vasodilatation response was blocked by trans-2-phenyl cyclopropylamine (TPC) demonstrating that prostacyclins mediate this response. 5 In STZ-diabetic rats, the vasodilatation in response to des-Arg(9)-BK involved both calcium influx and intracellular calcium mobilization from stores both IP3 sensitive and non-IP3 sensitive. Indeed, the effect was blocked by GdCl3, BHQ and cADP ribose. Furthermore, NO production and products of the COX-2 pathway including prostacyclin are involved as the response was inhibited by L-NAME, L-745 377 and TPC. 6 Vasodilatation in response to either 1 nM BK or 1 nM des-Arg(9)-BK were blocked by NF023 demonstrating that a G(o)/G(i) G-protein transduces both these effects. 7 This is the first report on the retinal circulation which provides evidence for vasodilator B-2 receptors and the upregulation of B-1 receptors very early following induction of diabetes with STZ rats. These results suggest that kinin receptors may be potential targets for therapeutics to treat retinopathies.