Smooth muscle cell–extrinsic vascular spasm arises from cardiomyocyte degeneration in sarcoglycan-deficient cardiomyopathy

Vascular spasm is a poorly understood but critical biomedical process because it can acutely reduce blood supply and tissue oxygenation. Cardiomyopathy in mice lacking γ-sarcoglycan or δ-sarcoglycan is characterized by focal damage. In the heart, sarcoglycan gene mutations produce regional defects in membrane permeability and focal degeneration, and it was hypothesized that vascular spasm was responsible for this focal necrosis. Supporting this notion, vascular spasm was noted in coronary arteries, and disruption of the sarcoglycan complex was observed in vascular smooth muscle providing a molecular mechanism for spasm. Using a transgene rescue strategy in the background of sarcoglycan-null mice, we replaced cardiomyocyte sarcoglycan expression. Cardiomyocyte-specific sarcoglycan expression was sufficient to correct cardiac focal degeneration. Intriguingly, successful restoration of the cardiomyocyte sarcoglycan complex also eliminated coronary artery vascular spasm, while restoration of smooth muscle sarcoglycan in the background of sarcoglycan-null alleles did not. This mechanism, whereby tissue damage leads to vascular spasm, can be partially corrected by NO synthase inhibitors. Therefore, we propose that cytokine release from damaged cardiomyocytes can feed back to produce vascular spasm. Moreover, vascular spasm feeds forward to produce additional cardiac damage.

Sarcoglycan is a multimember transmembrane complex found in all muscle types and is a component of the dystrophin glycoprotein complex (DGC). Sarcoglycan has a complex mechanosignaling role for the maintenance of striated muscle cells (1). In striated muscle, sarcoglycan interacts with dystrophin and dystroglycan connecting the intracellular cytoskeleton to the ECM and contributing to the structural integrity of muscle cells (2–4). Dystrophin, taken together with sarcoglycan, dystroglycan, syntrophins, and dystrobrevins, plays an important role in anchoring diverse signaling proteins to the plasma membrane (5). Sarcoglycan is thought to stabilize the linkages between dystroglycan and dystrophin on the intracellular surface and between dystroglycan and laminin-2 on the extracellular surface.

Mice with null mutations in γ-sarcoglycan, δ-sarcoglycan, or β-sarcoglycan develop cardiomyopathy that is characterized by focal degeneration. δ-Sarcoglycan– and β-sarcoglycan–null mice display disruption of the vascular smooth muscle (VSM) sarcoglycan complex (6–8). In contrast, mice lacking α-sarcoglycan develop muscular dystrophy but not cardiomyopathy (9). In α-sarcoglycan mutant mice, the VSM sarcoglycan complex remains intact. Therefore, it was reasoned that VSM sarcoglycan complex disruption promotes cardiomyopathy (7). Consistent with this, microvascular filling defects were found in β- or δ-sarcoglycan mutant mice, but not α-sarcoglycan mutant mice (6, 7). Moreover, long-term treatment with calcium channel antagonists reduced vasospasm and slowed cardiomyopathy progression (10).

The sarcoglycan complex varies in composition in different muscle tissues. In mice, the major sarcoglycan complex type found in skeletal and cardiac muscle consists of α-, β-, γ-, and δ-sarcoglycans (11). In addition, ζ- and ε-sarcoglycan are expressed in a subset of cardiac and skeletal muscle sarcoglycan complexes performing an as yet unclear function, which may include substituting for other subunits or acting in discreet locations of cells, such as at the neuromuscular junction (11, 12). In contrast, the arterial VSM sarcoglycan complex consists of β-, δ-, ε-, and ζ-sarcoglycan (12, 13).

We examined the role of the VSM sarcoglycan complex as a direct mediator of vascular spasm in sarcoglycan-mediated cardiomyopathy by generating a series of tissue-specific transgenes to express δ-sarcoglycan or γ-sarcoglycan in the background of mice lacking δ-sarcoglycan (dsg^–/–) (8) or γ-sarcoglycan (gsg^–/–) (14), respectively. Using the α-myosin heavy chain (α-MHC) gene promoter (15) to drive expression exclusively in cardiomyocytes, we showed that cardiomyocyte sarcoglycan restoration is sufficient to correct VSM vasospasm. Additionally, rescue of cardiomyocyte sarcoglycan expression corrected the focal degenerative process that leads to cardiomyopathy. Alternatively, using the SM22α promoter (16) to drive expression of sarcoglycan exclusively in arterial VSM, we showed that restoration of the VSM sarcoglycan complex was not sufficient to ameliorate vascular spasm, nor does it correct cardiac or skeletal muscle pathology. We postulate that the mechanism underlying VSM cell-extrinsic spasm involves cytokines released from damaged cardiomyocytes. We previously noted that eNOS was upregulated in regions of cardiomyocyte damage (17), and we now postulate that gradients of NO may create substrate for vascular spasm. We treated mice with a broad spectrum inhibitor of NOS and showed a reduction in vascular spasm. As cardiomyocyte damage occurs in response to a variety of stimuli including infarction, toxic, metabolic, and genetic defects, damage-responsive vascular spasm may be a broad mediator of cardiovascular pathology and a target for therapeutic intervention.

Cardiomyocyte-specific replacement of γ-sarcoglycan or δ-sarcoglycan. To determine the mechanism of coronary artery vascular spasm in sarcoglycan-mediated cardiomyopathy, we generated mice that expressed sarcoglycan solely in the cardiomyocytes but not in skeletal or VSM. We used the α-MHC promoter because this promoter results in sustained ventricular expression from late embryonic stages through adulthood (15). Full-length δ-sarcoglycan was placed under the control of the α-MHC promoter to generate the MHD transgene (Figure 1A). Mice harboring the MHD transgene were bred to mice null for δ-sarcoglycan to create mice with (dsg^–/–/MHD) and without the transgene (dsg^–/–). Southern blot analysis demonstrated a copy number of eight for MHD (data not shown). Similarly, full-length γ-sarcoglycan was placed under the control of α-MHC to generate the MHG transgene (Figure 1A). MHG transgenic mice were bred to γ-sarcoglycan–null (dsg^–/–) mice to generate mice with cardiomyocyte-specific γ-sarcoglycan expression (gsg^–/–/MHG).

Figure 1

Construction and expression of cardiomyocyte-specific sarcoglycan transgenes. (A) Full-length murine γ-sarcoglycan (γ-sg) or δ-sarcoglycan (δ-sg) cDNA was ligated into a vector containing the 5.6-kb murine α-MHC promoter (15) to create MHG or MHD, respectively. Transgenic mice with the MHG transgene were crossed with γ-sarcoglycan–null (gsg^–/–) mice to generate gsg^–/–/MHG mice. Transgenic mice with the MHD transgene were crossed with δ-sarcoglycan–null (dsg^–/–) mice generating dsg^–/–/MHD mice. pA, polyadenylation signals. (B) Immunoblot of whole heart extracts from normal, gsg^–/–, dsg^–/–, gsg^–/–/MHG, and dsg^–/–/MHD animals at 12 weeks of age using γ-sarcoglycan Ab showed γ-sarcoglycan expression was absent in the hearts of gsg^–/– and dsg^–/– animals, but was restored in transgenic animals. Quantitative Western blot analysis determined the level of γ-sarcoglycan expression in gsg^–/–/MHG animals to be sevenfold above normal. (C) δ-Sarcoglycan expression was restored to normal levels by expression of the MHD transgene in dsg^–/–/MHD hearts. Expression of δ-sarcoglycan from the MHD transgene also resulted in recovery of γ-sarcoglycan to normal levels (last lane). (D) Immunoblots for the remaining sarcoglycan subunits showed that α-sarcoglycan (ASG) expression is recovered in hearts with either γ- or δ-sarcoglycan transgene expression. β-Sarcoglycan protein (BSG) is increased to normal levels, and ζ-sarcoglycan protein (ZSG) levels are not significantly different in transgenic hearts. Loading control is shown for B, C, and D. Coom., Coomassie blue.

Immunoblotting with sarcoglycan-specific Ab’s against whole heart extracts showed expression in the hearts of transgenic animals (Figure 1, B and C). Quantitative immunoblotting of γ-sarcoglycan expression in gsg^–/–/MHG determined expression to be between fivefold and sevenfold higher than normal (data not shown). We previously examined the effect of overexpression of γ-sarcoglycan on striated muscle and found that levels between 30-fold and 50-fold over normal are required for striated muscle toxicity (18). Thus, the level of γ-sarcoglycan expression produced from MHG is not expected to produce pathology. Quantitative immunoblotting to determine the level of expression produced from dsg^–/–/MHD indicated that δ-sarcoglycan was approximately twofold to threefold normal level. Expression of γ-sarcoglycan or δ-sarcoglycan in cardiomyocytes restored the sarcoglycan complex in gsg^–/–/MHG and dsg^–/–/MHD hearts (Supplemental Figure 1; supplemental material available at http://www.jci.org/cgi/content/full/113/5/668/DC1). Expression levels of the remaining sarcoglycans (α, β, and ζ) were increased in transgene-rescued mice compared with gsg^–/– and dsg^–/– mice (Figure 1D).

Expression of sarcoglycan subunits at the cardiomyocyte membrane of MHG and MHD hearts. Transgenic expression of δ- and γ-sarcoglycan produced sarcoglycans correctly targeted to the plasma membrane. Figure 2 shows sections from hearts stained with Ab’s against smooth muscle actin (green) and Ab’s specific to either γ- or δ-sarcoglycan (blue). In normal hearts, δ-sarcoglycan is expressed in both cardiomyocytes and VSM (Figure 2A, top row, merged image). Figure 2A shows that the expression of δ-sarcoglycan is restored to the cardiomyocyte membrane in dsg^–/–/MHD transgenic hearts, similar to what is seen in normal hearts. γ-Sarcoglycan is normally only expressed in striated muscle (Figure 2A). In gsg^–/–/MHG hearts (Figure 2B) expression of γ-sarcoglycan (blue) is restored.

Figure 2

Cardiomyocyte-specific transgenes rescue sarcoglycan at the membrane and restore membrane-permeability defects. Immunofluorescence microscopy was performed on sections of hearts containing coronary vessels from normal, gsg^–/–, dsg^–/–, gsg^–/–/MHG, and dsg^–/–/MHD mice at 26 weeks of age. In the left column, smooth muscle α-actin (sm actin) staining coronary VSM is shown in green; in the middle column, EBD staining is shown in red and sarcoglycan staining is shown in blue. The third column shows a merged view. (A) δ-Sarcoglycan Ab is present at the membrane of cardiomyocytes of normal and dsg^–/–/MHD hearts but not dsg^–/– hearts. EBD uptake (red) is found in the cardiomyocytes of dsg^–/– animals but is never found in normal or dsg^–/–/MHD cardiomyocytes. δ-Sarcoglycan is normally expressed in coronary arteries. Transgene expression in dsg^–/–/MHD does not induce δ-sarcoglycan expression in the coronary artery tree (bottom row). (B) γ-Sarcoglycan Ab localizes γ-sarcoglycan in the membrane of normal and gsg^–/–/MHG cardiomyocytes. EBD uptake (red) is seen in gsg^–/– cardiomyocytes but is never seen in gsg^–/–/MHG or normal cardiomyocytes. Scale bars: dsg^–/– and gsg^–/– are 100 μm; others are 20 μm.

Mice were injected with EBD, a small molecular tracer dye to which normal cardiac and skeletal myocytes are impermeable. Mutations that disrupt the sarcoglycan complex alter membrane permeability so that EBD uptake can be seen scattered throughout heart and skeletal muscle. These membrane-permeability defects indicate impending or ongoing cell degeneration. Microscopically, EBD uptake is seen as red fluorescence in cardiomyocytes of gsg^–/– and dsg^–/– hearts (Figure 2). In Figure 2, EBD uptake is seen in immediate proximity to a smooth muscle actin containing coronary vessels. EBD uptake was never seen in dsg^–/–/MHD or gsg^–/–/MHG transgenic hearts, indicating restoration of membrane permeability and correction of the underlying defect. In contrast, skeletal muscle of dsg^–/–/MHD and gsg^–/–/MHG mice showed complete absence of δ-sarcoglycan or γ-sarcoglycan. These data indicate that the transgenic constructs successfully rescued the acute membrane damage seen in the hearts of δ- and γ-sarcoglycan–deficient animals.

Cardiomyocyte sarcoglycan expression corrects focal cardiac degeneration. Masson trichrome staining was performed on both dsg^–/–/MHD and gsg^–/–/MHG paraffin-embedded hearts. The histopathologic process in both dsg^–/– and gsg^–/– mutant hearts is one of focal degeneration accompanied by fibrosis. In Figure 3A, fibrosis (arrow) seen in the dsg^–/– heart is corrected by the presence of the MHD transgene. Phenotypically, dsg^–/–/MHD hearts were indistinguishable from normal hearts. Similarly, the patchy fibrosis seen in gsg^–/– mutant hearts (Figure 3B) was eliminated in gsg^–/–/MHG transgenic hearts. Cardiac pathology in sarcoglycan mutant mice can be fully corrected with cardiomyocyte-transgene rescue of sarcoglycan expression.

Figure 3

Focal areas of fibrosis and necrosis are eliminated with cardiomyocyte-specific sarcoglycan expression. Shown is Masson’s trichrome staining of midventricular cross sections of hearts from 26-week-old animals. (A) Areas of fibrosis and necrosis are seen throughout dsg^–/– hearts; arrow in high-power view shows an area in right ventricle. Evidence of ventricular wall thickening is seen in the low-power view. Expression of δ-sarcoglycan specifically in cardiomyocytes (dsg^–/–/MHD) eliminates foci of fibrosis and necrosis. Transgenic hearts are indistinguishable from normal hearts. (B) Foci of fibrosis, necrosis, and inflammation are also seen in gsg^–/– hearts, shown here in the intraventricular septum. In contrast, exclusive expression of γ-sarcoglycan in cardiomyocytes eliminates areas of degeneration. Scale bars: left column, 1 mm; right column, 100 μm.

Cardiomyocyte expression corrects vascular spasm. The focal nature of fibrosis in sarcoglycan mutant hearts is consistent with the presence of microinfarction. It was suggested that vascular spasm was responsible for cardiomyopathy in sarcoglycan mutations (7, 10, 23). Supporting this suggestion, disruption of the VSM sarcoglycan complex and microvascular filling defects were noted in mice in δ-sarcoglycan–null hearts (7). We analyzed dsg^–/–/MHD and gsg^–/–/MHG using microvascular filling to evaluate whether vasospasm arises as a VSM cell-extrinsic process. Figure 4 (top) shows examples of vasospasm with focal narrowings and microvascular filling defects in both dsg^–/– and gsg^–/– mutant hearts. Notably, gsg^–/– coronary vessels had an equivalent frequency of vascular spasm to coronary vessels in strain-matched dsg^–/– hearts (Table 1). Microvascular filling of transgene-rescued hearts showed no evidence of vasospasm in either dsg^–/–/MHD or gsg^–/–/MHG transgenic hearts. Representative photomicrographs of microvascular filling experiments in dsg^–/–/MHD and gsg^–/–/MHG hearts show smoothly tapered, well-filled coronary arterial vessels and filling of capillaries (Figure 4, bottom). Restoration of γ-sarcoglycan or δ-sarcoglycan specifically in cardiomyocytes was sufficient to eliminate vasospasm in these transgenic hearts.

Figure 4

Vascular spasm is present in dsg^–/– and gsg^–/– but not in dsg^–/–/MHD or gsg^–/–/MHG animals. Microvascular filling defects were seen in dsg^–/– and gsg^–/– animals at similar frequencies. Examples of focal narrowings are shown (arrows). In contrast, no focal narrowings were ever seen in dsg^–/–/MHD and gsg^–/–/MHG animals. Well-filled vascular trees with smoothly tapered vessels are shown at lower power (bottom row).

Table 1

Quantitation of microvascular filling experiments

Cardiomyopathy occurs despite expression of γ-sarcoglycan or δ-sarcoglycan in VSM. To evaluate the role of the sarcoglycan complex in VSM, we specifically restored VSM sarcoglycan expression. We used the SM22α gene (16) to generate transgenes expressing δ-sarcoglycan (SMD) or γ-sarcoglycan (SMG) in arterial smooth muscle (Figure 5A). In the case of γ-sarcoglycan, we expressed γ-sarcoglycan in smooth muscle to determine whether γ-sarcoglycan can substitute for δ-sarcoglycan in VSM. Founder mice carrying the SMD or SMG transgenes were bred to mice lacking δ-sarcoglycan or γ-sarcoglycan, respectively. The copy number for the gsg^–/–/SMG transgene was approximately 19, while for dsg^–/–/SMD, it was approximately 2 (data not shown). Figure 5B shows that δ-sarcoglycan or γ-sarcoglycan expression was restricted to VSM in dsg^–/–/SMD and gsg^–/–/SMG (shown in blue). Smooth muscle actin Ab staining is shown in green and colocalized with δ- and γ-sarcoglycan expression. No evidence of cardiomyocyte membrane δ-sarcoglycan or γ-sarcoglycan expression was seen in hearts of dsg^–/–/SMD and gsg^–/–/SMG animals, respectively. In both dsg^–/–/SMD and gsg^–/–/SMG hearts, EBD uptake was evident (Figure 5C). Therefore, restoration of VSM sarcoglycan expression was insufficient to prevent cardiomyocyte damage. Supporting this, both dsg^–/–/SMD and gsg^–/–/SMG hearts develop cardiac degeneration (Supplemental Figure 2). The cardiac pathology is identical to that which is seen in dsg^–/– and gsg^–/– hearts; that is, it is relatively mild at 8 to 12 weeks and progresses to more extensive and widespread lesions at 6 months to 1 year.

Figure 5

VSM sarcoglycan expression does not prevent vascular spasm. (A) Full-length δ-sarcoglycan was placed under the control of the smooth muscle–specific SM22α 441-bp promoter (16). Mice harboring the transgene were bred with dsg^–/– mice, creating dsg^–/–/SMD mice. Similarly, full-length γ-sarcoglycan was placed behind the SM22α promoter, and gsg^–/–/SMG mice were generated. (B) Expression of δ-sarcoglycan, left column, is seen in the VSM of dsg^–/–/SMD coronary arteries. δ-Sarcoglycan, in blue, colocalizes with smooth muscle α-actin (green) in coronary vessels, as seen in the merged image. Expression of γ-sarcoglycan (blue), right column, is found in coronary VSM of gsg^–/–/SMG mice and colocalizes with smooth muscle α-actin (green). Cardiomyocytes of gsg^–/–/SMG mice do not express γ-sarcoglycan. Although γ-sarcoglycan is not normally found in VSM, we generated gsg^–/–/SMG to determine if γ-sarcoglycan could substitute for δ-sarcoglycan in VSM. (C) EBD uptake (red) is found in cardiomyocytes of dsg^–/–/SMD and gsg^–/–/SMG mice, similar to that which is seen in dsg^–/– and gsg^–/– mice, indicating that cardiomyocyte membrane defects persist despite correction of VSM sarcoglycan expression. (D) Restoration of coronary VSM sarcoglycan does not prevent evidence of coronary vasospasm. Microvascular filling experiments (Microfil) demonstrate areas of focal narrowing (arrows) in dsg^–/–/SMD (left) and gsg^–/–/SMG (right) coronary arteries. An ectatic area of a gsg^–/–/SMG coronary vessel is also seen (arrowhead), as is often identified in gsg^–/– and dsg^–/– hearts (not shown). Scale bars: B and C, gsg^–/–/SMG, 20 μm; dsg^–/–/SMD, 50 μm.

Microvascular-filling experiments in dsg^–/–/SMD and gsg^–/–/SMG transgenic hearts revealed ready evidence of vascular spasm (arrows, Figure 5D). Quantitation of these microvascular filling experiments showed that microvascular filling defects were equally present in VSM sarcoglycan-rescued mice as they were in sarcoglycan-null mice, as shown in Table 1. Therefore, it is the absence of δ- or γ-sarcoglycan in cardiomyocytes, and not in VSM, that promotes vascular spasm.

To evaluate VSM, we measured blood pressures from dsg^–/–, gsg^-–/–, dsg^–/–/MHD, gsg^–/–/MHG, dsg^–/–/SMD, gsg^–/–/SMG, and strain-matched normal control mice at 1 year of age. These mice have fully developed histopathology and would be most likely to show systemic hemodynamic changes. Blood pressure was not altered in sarcoglycan mutant or transgene-rescued mice (Supplemental Figure 3).

Regional NO release influences vascular tone. The mechanism by which primary degeneration in cardiomyocytes can lead to vascular spasm may relate, in part, to cytokine release elicited from degenerating cardiomyocytes or infiltrating inflammatory cells. In sarcoglycan-mediated cardiomyopathy, regional or focal degeneration occurs. The earliest cellular pathologic signs relate to abnormal membrane permeability evidenced by uptake of EBD. The subsequent cytokine release that accompanies cellular degeneration may affect neighboring vessels in a paracrine fashion. Supporting this, we imaged EBD uptake and vasospasm simultaneously. An example of this is shown in Figure 6, where a large focus of EBD uptake can be seen upon gross examination as blue staining in juxtaposition to a vascular stenotic lesion.

Figure 6

Vasospasm occurs as a consequence of focal cardiomyocyte degeneration in sarcoglycan deficiency. (A) Evidence for focal cardiomyocyte damage leading to coronary vasospasm. EBD uptake appears pale blue at gross examination and is seen in a heart from a γ-sarcoglycan–deficient animal (left, demarcated by arrowheads). Microvascular filling in the same heart (left, orange, and right) shows evidence of vasospasm with focal narrowings (arrows) and poor perfusion of the area that has taken up EBD. (B) Diagram of cardiomyocyte degeneration and vasospasm. Disruption of the sarcoglycan complex through gene mutation causes loss of the sarcoglycan complex at the membrane in cardiomyocytes and impairs the function of the dystrophin glycoprotein mechanosignaling complex. The early marker of degeneration in cardiomyocytes, EBD uptake (blue cardiomyocytes in schematic), indicates the active disruption and degenerative changes in the cardiomyocyte due to loss of sarcoglycan in cardiomyocyte sarcolemma. Coronary vessels, in proximity to the areas of local cardiomyocyte degeneration, undergo vasospasm (red arrows, right) due to local changes in the surrounding myocardium (solid arrow). Transient ischemia downstream of these events may lead to further degeneration within the myocardium (dashed arrow).

We noted previously that sarcoglycan-null cardiomyocytes with EBD uptake exhibit marked upregulation of eNOS and NO (17). Because regions of cardiomyocyte degeneration in sarcoglycan-null animals typically constitute less than 5% of the myocardium, an increase in eNOS is not detected in whole cell lysates. We hypothesized the focal upregulation of eNOS may paradoxically contribute to vascular pathology. To evaluate this possibility, we pretreated six mice (n = 4, gsg^–/–; n = 2 dsg^–/–) with L-NAME, an inhibitor of NOS. Blinded analysis found five focal arterial narrowings in 24 filled coronary artery branches (20.8%) in the saline-treated group (n = 4 each of gsg^–/– and dsg^–/–) compared with two focal arterial narrowings of 21 filled branches (9.5%) in the L-NAME–treated group. Thus, NOS inhibition can reduce the frequency of coronary artery vasospasm in sarcoglycan mutant mice supporting the concept that cytokine release from damaged cardiomyocytes predisposes to vascular spasm.

Vascular spasm is an important contributor to cardiac pathology. In myocardial infarction, there is acute coronary artery occlusion and oxygen deprivation to the tissue normally supplied by the occluded vessel. In Prinzmetal variant angina or primary coronary artery vascular spasm there is damage to cardiac tissue in the presence of minimal to no atherosclerotic disease. In Prinzmetal vasospasm, VSM is unusually hyperreactive, leading to spasm and tissue infarction (24, 25). Using a model of focal degeneration that leads to cardiomyopathy, we now demonstrate that vascular spasm arises from a cardiomyocyte-intrinsic process. Mice lacking either γ-sarcoglycan or δ-sarcoglycan display progressive focal cardiomyocyte degeneration that ultimately leads to reduced cardiac function, irregular heart rhythms, and death. This model of cardiomyopathy closely parallels what is seen in humans with sarcoglycan and dystrophin gene mutations (26–29).

We previously described vascular spasm in mice lacking sulfonylurea receptor 2 (SUR2), a model for Prinzmetal variant angina and primary vascular spasm (22). We now compared vasospasm in SUR2 mutant mice with vasospasm in mice lacking sarcoglycan. Cell-intrinsic vasospasm, as it occurs in SUR2 mutant mice, is characterized by approximately double the frequency of focal arterial narrowings than that seen in hearts that lack sarcoglycan. Moreover, vasospasm as it occurs in SUR2 mutant mice is such that transient cardiomyocyte injury/infarction can be detected on a surface ECG. Similar ECG changes were not seen in sarcoglycan mutant mice (data not shown) and may reflect a slower time course of vascular spasm. While every SUR2 heart examined had evidence for vascular spasm as found by the microvascular filling technique, only a fraction of sarcoglycan mutant hearts showed filling defects. Thus, VSM cell-intrinsic spasm differs qualitatively and quantitatively from VSM cell-extrinsic spasm.

Cytokine release accompanies cardiomyocyte degeneration. Liberated cytokines may derive from damaged cardiomyocytes or may derive from the inflammatory infiltrate that accompanies striated muscle damage. Invading macrophages and lymphocytes are commonly noted in sarcoglycan mutant muscle (8, 14) and may serve as a source of NO. In sarcoglycan mutant hearts, eNOS is focally upregulated coincident with EBD uptake into cardiomyocytes (17), suggesting that the degenerating myocyte is the major source of NO. Concomitant with eNOS upregulation, NO is focally increased as well (17). We find that gradients of NO dispersed throughout the myocardium likely have a pathologic effect on neighboring VSM because we now show that inhibition of NO reduced vascular spasm. It should be noted that NO inhibition produced only a partial reduction of vascular spasm suggesting that additional factors contribute to cell-extrinsic vasospasm. There is precedent for the involvement of NO in vascular spasm because a paradoxical response to NO has been described in human patients with Prinzmetal or atheromatous vascular disease (30, 31). In sarcoglycan mutant hearts, the paradoxical response to NO arises as a cardiomyocyte-driven, VSM cell-extrinsic mechanism.

Inhibition of vascular spasm is a therapeutic target to slow cardiomyopathy progression. Vascular spasm, as it occurs in sarcoglycan-deficient cardiomyopathy, likely contributes to the pathogenic process. Supporting this, the calcium channel antagonist verapamil was used to inhibit vascular spasm in the BIO 14.6 cardiomyopathic hamster and in mice mutant for δ-sarcoglycan (10, 23, 32). The BIO14.6 hamster model harbors a large deletion in the δ-sarcoglycan gene and displays a similar phenotype to humans and mice with sarcoglycan gene mutations (32). Verapamil is effective in reducing vascular spasm since it acts directly on VSM calcium channels. Calcium channel antagonists that lack negative inotropic action, however, are likely to be better tolerated in cardiomyopathic hearts. In the sulfonylurea mutant model of Prinzmetal vasospasm, the dihydropyridine calcium channel antagonist nifedipine was effective in suppressing vascular spasm (22). Therefore, in animal models inhibition of vasospasm has been achieved at many levels and may prove useful clinically in human disease.

Finally, cardiomyocyte degeneration is common to many forms of cardiac pathology including myocardial infarction, myocarditis, and idiopathic dilated cardiomyopathy. Some forms of hypertrophic cardiomyopathy have been associated with microvascular disease and focal degeneration occurring within the myocytes (33, 34). The mechanism of VSM cell-extrinsic vascular spasm may occur in any of these pathologic states if the paracrine pathway for vascular spasm is simply one that derives from cardiac degeneration itself. Studies on human subjects with myocardial infarction are consistent with vascular spasm, but further investigation is warranted to identify additional cytokines that mediate this effect and to establish whether this process can be exploited therapeutically.

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This work was supported by NIH grant HL-61322, the Muscular Dystrophy Association, and the Burroughs Wellcome Fund (to E.M. McNally). E.M. McNally is an Established Investigator of the American Heart Association. M.T. Wheeler is supported by an American Heart Association Predoctoral Award (Midwest Affiliate) and the Medical Scientist Training Program. M.J. Allikian is supported by NIH grant HL-68472. A. Heydemann is supported by NIH grant HL-10432.

Matthew T. Wheeler and Michael J. Allikian contributed equally to this work.

Nonstandard abbreviations used: α-myosin heavy chain (α-MHC); dystrophin glycoprotein complex (DGC); Evans Blue Dye (EBD); N_G-nitro-l-arginine methyl ester hydrochloride (l-NAME); NO synthase (NOS); sulfonylurea receptor (SUR); vascular smooth muscle (VSM).

Conflict of interest: The authors have declared that no conflict of interest exists.

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