The role of joint nerves and mast cells in the alteration of vasoactive intestinal peptide (VIP) sensitivity during inflammation progression in rats

Abstract

The present study examined the peripheral effects of vasoactive intestinal peptide (VIP) on rat knee joint blood flow during acute and chronic inflammation. The involvement of joint nerves and synovial mast cells on these effects was also investigated. Prior to blood flow assessment, animals were deeply anaesthetised with ethyl carbamate (urethane; 2 mg kg(-1) i.p.). Local application of VIP (10(-13)-10(-9) mol) onto the capsular surface of normal rat knee joints caused a dose-dependent increase in synovial perfusion with an ED50 of 1.2 x 10(-11) mol. The dilator effect of the peptide was transient with the maximal response occurring approximately 1 min after drug administration. VIP-induced vasodilatation was blocked by co-administration of the VIP receptor antagonist VIP(6-28) (10(-9) mol). The inhibitory effect of the antagonist was consistent across the entire VIP dose range (P=0.01). The vasoresponsiveness to VIP was significantly attenuated in acutely inflamed joints; however, surgical denervation of acutely inflamed knees re-established the vasodilator effect of the neuropeptide. Topical application of VIP to 1- and 3-week adjuvant monoarthritic knees produced a hyperaemic response, which was not significantly different from normal (P=0.06 and 0.73 for 1- and 3-week adjuvant treated joints, respectively). Stabilisation of synovial mast cells by disodium cromoglycate (cromolyn) pretreatment did not alter the vasoresponsiveness to VIP in acute or chronically inflamed joints. The vasodilatatory effect of VIP is lost during acute knee joint inflammation and this abrogated effect is neurally dependent. In the chronic phase of knee joint inflammation, VIP-mediated hyperaemia recovers to normal levels. Synovial mast cells do not influence the vasomotor effects of exogenously applied VIP in inflamed knee joints.

Figure 1

Changes in knee joint diameter following arthritis induction. Compared to pretreated control, knee joint diameter increased significantly in acutely inflamed, 1-, and 3-week chronically inflamed joints (^*P=0.02; ^**P<0.005; ^***P<0.0005; paired Student's t-test). Data are means±s.e.m.

Figure 2

Representative single point perfusion measurement of VIP dilator response (a). Topical application of 10⁻⁹ mol VIP to the rat knee at 0 min resulted in a rapid increase in synovial blood flow with a maximal effect occurring 1 min after administration. Joint perfusion gradually returned to control levels approximately 4 min after VIP application. VIP caused a dose-dependent (P<0.0001, one-way ANOVA, n=15) vasodilatation of articular blood vessels (b), which was significantly attenuated (P=0.01; two-way ANOVA, n=6–15) in the presence of the VIP antagonist VIP_6–28. Data are means±s.e.m.

Figure 3

Vasomotor effects of VIP in normal joints compared to, acute, acute/denervated (a), 1 week (b) and 3 week (c) adjuvant chronically inflamed knees. The vasodilator effect of VIP in normal joints was significantly reduced (P=0.02, two-way ANOVA, n=6–10) by acute inflammation but re-established in surgically denervated acutely inflamed knee joints such that the vasoresponsiveness to the neuropeptide was not significantly different from normal. VIP-mediated vasodilatation in 1- and 3-week chronically inflamed knees was not statistically different from normal control animals. Mean basal perfusion values for the different animal groups were 441PU (normal), 710PU (acutely inflamed), 347PU (denervated acutely inflamed), 424PU (1-week chronically inflamed), and 776PU (3-week chronically inflamed). Data are means±s.e.m.

Figure 4

Topical application of 10⁻⁹ mol VIP_6–28 to normal rat knees caused basal perfusion to fall from 467PU to 414PU, whereas administration of the antagonist to acutely inflamed knees had no significant effect on joint blood flow. (^**P<0.005 paired Student's t-test; n=7. NS – not significantly different) Means±s.e.m. are shown.

Figure 5

Effect of the mast cell degranulator compound 48/80 on control articular perfusion (a) and following synovial mast cell stabilisation with cromolyn pretreatment (b). In control joints, compound 48/80 caused a noticeable increase in knee joint perfusion within 2 min after its topical application to the knee. Blood flow gradually recovered thereafter. Mast cell stabilisation with cromolyn prevented this hyperaemic response, which was maintained for at least 60 min. Data are presented as means±s.e.m.

Figure 6

Effect of mast cell stabilisation on VIP-mediated vasodilatation in normal (a), acute (b), and 3-week chronically inflamed (c) knee joints. The vasodilator effect of VIP in normal knee joints was significantly augmented in cromolyn treated animals (P=0.001, two-way ANOVA, n=7–15). Cromolyn pretreatment of acute and chronically inflamed joints had no significant effect on the vasoactivity profile of VIP observed in untreated inflamed joints. Data are shown as means±s.e.m.