Vasoactive effects of soluble matrix proteins and integrin-binding peptides on arterioles are mediated by αvβ3 and α5β1 integrins. To examine the underlying mechanisms, we measured L-type Ca2+ channel current in arteriolar smooth muscle cells in response to integrin ligands. Whole-cell, inward Ba2+ currents were inhibited after application of soluble cyclic RGD peptide, vitronectin (VN), fibronectin (FN), either of two anti–β3 integrin antibodies, or monovalent β3 antibody. With VN or β3 antibody coated onto microbeads and presented as an insoluble ligand, current was also inhibited. In contrast, beads coated with FN or α5 antibody produced significant enhancement of current after bead attachment. Soluble α5 antibody had no effect on current but blocked the increase in current evoked by FN-coated beads and enhanced current when applied in combination with an appropriate IgG. The data suggest that αvβ3 and α5β1 integrins are differentially linked through intracellular signaling pathways to the L-type Ca2+ channel and thereby alter control of Ca2+ influx in vascular smooth muscle. This would account for the vasoactive effects of integrin ligands on arterioles and provide a potential mechanism for wound recognition during tissue injury.

Arteriolar vasodilation and the resultant increase in blood flow are characteristic vascular responses to tissue injury. The dilatory mediators signaling these responses are incompletely understood. We show that integrin-binding peptides containing the Arg-Gly-Asp (RGD) tripeptide sequence cause immediate and, in some instances, sustained vasodilation when applied to isolated rat cremaster arterioles. The vasodilation is dependent on interaction of the soluble RGD sequence with the alpha v beta 3 integrin expressed by smooth muscle cells in the arteriolar wall. Possible in vivo sources of soluble RGD sequences are fragments of extracellular matrix proteins that are generated after tissue injury. Indeed, protease-generated fragments of denatured collagen type I (a major source of RGD sequences) also cause cremaster arteriolar vasodilation through the alpha v beta 3 integrin. Thus, extracellular matrix protein fragments containing the RGD sequence may act as vascular wound recognition signals to regulate blood flow to injured tissue.

The ability of an integrin-binding Arg-Gly-Asp-Asn (RGDN)-containing peptide to influence vascular tone by interacting with the ␣ 5 ␤ 1 integrin was studied using rat skeletal muscle arterioles. After blockade of ␤ 3 integrin function, isolated arterioles with spontaneous tone showed concentration-dependent vasoconstrictions to topical application of GRGDNP, a peptide that shows a greater ability to interact with ␣ 5 ␤ 1 than with ␣ v ␤ 3 . The constriction to GRGDNP (2.1 mM) was inhibited by blocking ␣ 5 integrin function, and was intensified by blocking ␤ 3 integrin function. In contrast, GRGDSP, a peptide that interacts better with ␣ v ␤ 3 , was unable to induce sustained constrictions. Removal of the endothelium abolished the vasoconstriction in response to GRGDNP, suggesting that the response was due to release of an endothelium-dependent factor.

It has previously been shown that synthetic peptides containing the sequence arginine-glycine-aspartic acid (RGD) cause vasodilation by activation of alpha(v)beta3-integrin present on vascular smooth muscle (VSM) cells. The purpose of this study was to determine whether this dilatory effect is mediated by a reduction in VSM cytosolic Ca2+ concentration ([Ca2+]i). First-order arterioles from the rat cremaster were isolated, cannulated, and pressurized. [Ca2+]i was quantitated from the ratio of emitted fluorescence intensity during alternate excitation of fura 2-loaded vessels at 340 and 380 nm. Cyclo(-Arg-Gly-Asp-D-Phe-Val) (cycloRGD; 0.21-210 microM) produced a concentration-dependent dilation of arterioles that had developed basal myogenic tone. Over the entire concentration range tested, [Ca2+]i decreased from 91 +/- 6 to 27 +/- 4 nM (69.7 +/- 5.0% reduction). In association with the decrease in [Ca2+]i, arteriolar lumen diameter increased from 89 +/- 8 to 184 +/- 8 pm (89.8 +/- 1.8% dilation). At intermediate concentrations, cycloRGD induced rhythmic spiking of Ca2+ superimposed on the concentration-dependent lowering of basal [Ca2+]i. These data directly link integrin activation with alterations in Ca2+ regulation, the net effect of which is a reduction in [Ca2+]i. These data further suggest that integrins, through their role in mediating cellular attachment to the extracellular matrix and in cellular signaling involving Ca2+, could provide a logical link to mechanotransduction and myogenic phenomena.

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