Elementary calcium signaling in arterial smooth muscle

Figures & data

Figure 1. (A) Isolated mesenteric artery smooth muscle cell (VSMC). (B) Ca²⁺ fluorescence image of a Fluo-4-AM–loaded mesenteric VSMC. (C) Ca²⁺ fluorescence image of the same cell as in B during the occurrence of an elementary Ca²⁺ release event (Ca²⁺ spark). The Ca²⁺ fluorescence images were recorded with an inverted microscope (Nikon Eclipse Ti, oil immersion objective x40, numerical aperture 1.3). Details are as described in [6]. Magnification in A (10x ocular, 20x objective, numerical aperture 0.45). Bar, 2 µm.

Figure 2. Control of calcium sparks by voltage-dependent calcium channels. (A) Ca²⁺ entry from extracellular space is not required for calcium release in skeletal muscle. Charge movement within L-type channel Ca_v1.1 activates RyR1 channel via a direct physical interaction. Ca²⁺ efflux from SR through the opened RyR1 channel activates nearby RyR (RyR1 and RyR3) channels via calcium-induced calcium release (CICR). (B) In cardiomyocytes Ca_v1.2 mediates influx of extracellular Ca²⁺ into cytosol. Ca²⁺ then binds to and activates RyR2 channels via CICR. (C) Triggering of Ca²⁺ sparks is not controlled by rapid, direct cross-talk between Ca_v1.2 channels and RyR2 in arterial smooth muscle in contrast to cardiac and skeletal muscle cells. Instead, Ca_v1.2 channels contribute to global cytosolic [Ca²⁺], which in turn influences luminal SR calcium and thus calcium sparks. SERCA: sarco/endoplasmic reticulum calcium ATPase, BK: big conductance calcium-activated K⁺ channel. Modified from [29].

Figure 3. Ca²⁺ sparks in wild-type and smooth muscle myosin heavy chain (SMMHC)–Ryr2^−/- tibial artery smooth muscle cells (SMCs). (A) Ca²⁺ fluorescence image of a Fluo-4-AM–loaded control SMC. (B) Ca²⁺ fluorescence image of the same cell as in A during the occurrence of a Ca²⁺ spark. Two-dimensional images were recorded at a rate of 5/s using an inverted microscope (Nikon Eclipse Ti, oil immersion objective x40, numerical aperture 1.3;) as described in [6]. (C) Time course of Ca²⁺ fluorescence changes in cellular regions of interest (ROIs) without sparks (ROI a, left) and with sparks (ROI b, right). (D) Time course of Ca²⁺ fluorescence changes in an ROI (similar size as that in A) of a SMMHC-Ryr2^−/- SMC. (E) Percentage of SMCs with Ca²⁺ sparks. (F) Ca²⁺ spark frequencies in SMCs from control and SMMHC-Ryr2^−/- mice. F/F0, fluorescence/background fluorescence. Bar, 2 µm. *P < 0.05. Modified from [6].

Table 1. Voltage-dependent Ca²⁺ channels (VDCCs) expressed in vascular smooth muscle.

α1 Subunit	Localization	Reference
L-type Cav1.2	(vascular) smooth muscle, cardiac muscle, neurons, endocrine cells	[7,83–88]
L-type Cav1.3	sensory cells, endocrine cells, neurons, low density in heart and vascular smooth muscle	[85,89]
T-type Cav3.1	(vascular) smooth muscle, cardiac muscle, neurons	[83–86]
T-type Cav3.2	(vascular) smooth muscle, cardiac muscle, neurons, kidney, liver	[83–86]

Figure 4. Proposed model of the role of Ca_v1.2 and Ca_v3.2 channels in arterial smooth muscle Ca²⁺ spark generation. Ca²⁺ sparks are produced by opening of clustered ryanodine receptors (RyR2) in the SR, which produces a negative-feedback effect on vasoconstriction. This vasodilatory effect is mediated by activation of large-conductance Ca²⁺-activated K⁺ (BKCa) channels, which results in hyperpolarization of VSMCs and reduced global cytosolic [Ca²⁺]. The majority (~70-80%) of Ca²⁺ sparks are triggered by Ca_v1.2 channels contributing to global cytosolic [Ca²⁺], which in turn influences luminal SR calcium via SERCA [3,104]. Ca_v3.2 T-type channels contribute to a minor extent [5]. SERCA, calcium pump; SR, sarcoplasmic reticulum; VSMC, mesenteric artery vascular smooth muscle cell.

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