| pA2
online © Copyright 2003 The British Pharmacological Society |
003P
University
of Manchester Autumn Meeting September 2003 |
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Suppression of aqueous humour formation by bradykinin occurs via nitric oxide and cyclic GMP
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Arterial perfusion of the isolated bovine eye allows a resumption of aqueous humour (AH) formation by the ciliary body. We have previously shown that azide decreases AH formation in the perfused eye by a cyclic GMP-dependent mechanism (Shahidullah and Wilson, 1999). We have now studied the effect of bradykinin (BK), which in vascular tissues acts by releasing endogenous nitric oxide, in turn generating cyclic GMP.
Bovine eyes were perfused through a ciliary artery at 37°C with Krebs solution gassed with O2 containing 5% CO2 and comprising (mM): NaCl, 118; KCl, 4.7; CaCl2, 2.5; KH2PO4, 1.2; MgSO4, 1.2; NaHCO3, 25; glucose, 11.5; ascorbate, 0.05 (Wilson et al., 1993). Arterial flow rate was 2.5 ml min-1. AH formation was estimated by fluorescein dilution in a circuit that recycled AH from the cannulated anterior chamber via a fluorimeter. The slope of the regression line (loge [fluorescein] vs time) was taken as the rate constant for AH formation (Kout min-1).
Once
fluorescence values had reached steady state, the starting value for AH
formation rate (Kout
min-1 x104)
was calculated over the first 50 min. These values were somewhat variable
among eyes, but for a given eye, remained stable for at least 120 min
(see Table 1); thus each eye acted as its own control. Drugs were added
to the arterial perfusate reservoir at 40 min: BK alone or with ODQ (30
nM) to inhibit soluble guanylate cyclase or L-NAME (100µM) to block
nitric oxide production.
Data were subjected to one-way ANOVA and are expressed as mean ± s.e. mean (n = 8 to 13).
Table 1. Effects of BK in the presence of ODQ or L-NAME. *** P<0.001, indicates significant difference from starting rate.
|
[BK]
(nM)
|
Kout
min-1x104
0-50 min (control) |
(mean
±
s.e.mean)
70-120 min (BK) |
Inhibitor
|
|
nil
|
34
±
1
|
33
±
3
|
bbb
|
|
0.3
|
24
±
3
|
21
±
4
|
bbb
|
|
1
|
35
±
3
|
23
±
3 ***
|
bbb
|
|
3
|
37
±
2
|
21
±
2 ***
|
bbb
|
|
1
|
29
±
4
|
26
±
3
|
ODQ
|
|
1
|
13
±
2
|
14
±
3
|
L-NAME
|
Addition of BK at 40 min produced a concentration-dependent suppression of AH formation, which was slow in onset (hence control readings were continued till 10 min after BK addition). BK at 1 nM inhibited AH formation by 30% but when ODQ or L-NAME were perfused simultaneously with BK (1 nM), this inhibition fell to insignificant levels.
We conclude that in the bovine eye, BK inhibits AH production by stimulating nitric oxide synthesis, which in turn activates soluble guanylate cyclase. It is therefore possible that AH production is modulated by endogenous BK.
Shahidullah,
M. et al. (1999).Br.J.Pharmacol.,127,1438-1446.
Wilson, W.S.
et al. (1993). Curr. Eye Res. 12, 609-620.