Abstract
BackgroundATP is one of the principal sympathetic neurotransmitters which contracts vascular smooth muscle cells (SMCs) via activation of ionotropic P2X receptors (P2XRs). We have recently demonstrated that contraction of the guinea pig small mesenteric arteries evoked by stimulation of P2XRs is sensitive to inhibitors of IP3 receptors (IP3Rs). Here we analyzed contribution of IP3Rs and ryanodine receptors (RyRs) to [Ca2+]i transients induced by P2XR agonist αβ-meATP (10μM) in single SMCs from these vessels. MethodsThe effects of inhibition of L-type Ca2+ channels (VGCCs), RyRs and IP3Rs (5μM nicardipine, 100μM tetracaine and 30μM 2-APB, respectively) on αβ-meATP-induced [Ca2+]i transients were analyzed using fast x–y confocal Ca2+ imaging. ResultsThe effect of IP3R inhibition on the [Ca2+]i transient was significantly stronger (67±7%) than that of RyR inhibition (40±5%) and was attenuated by block of VGCCs. The latter indicates that activation of VGCCs is linked to IP3R-mediated Ca2+ release. Immunostaining of RyRs and IP3Rs revealed that RyRs are located mainly in deeper sarcoplasmic reticulum (SR) while sub-plasma membrane (PM) SR elements are enriched with type 1 IP3Rs. This structural peculiarity makes IP3Rs more accessible to Ca2+ entering the cell via VGCCs. Thus, IP3Rs may serve as an “intermediate amplifier” between voltage-gated Ca2+ entry and RyR-mediated Ca2+ release. ConclusionsP2X receptor activation in mesenteric artery SMCs recruits IP3Rs-mediated Ca2+ release from sub-PM SR, which is facilitated by activation of VGCCs. Sensitivity of IP3R-mediated release to VGCC antagonists in vascular SMCs makes this mechanism of special therapeutic significance.
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