Local delivery of the KCa3.1 blocker, TRAM-34, prevents acute angioplasty-induced coronary smooth muscle phenotypic modulation and limits stenosis

OBJECTIVE

We previously demonstrated that upregulation of intermediate-conductance Ca(2+)-activated K(+) channels (K(Ca)3.1) is necessary for mitogen-induced phenotypic modulation in isolated porcine coronary smooth muscle cells (SMCs). The objective of the present study was to determine the role of K(Ca)3.1 in the regulation of coronary SMC phenotypic modulation in vivo using a swine model of postangioplasty restenosis.

METHODS AND RESULTS

Balloon angioplasty was performed on coronary arteries of swine using either noncoated or balloons coated with the specific K(Ca)3.1 blocker TRAM-34. Expression of K(Ca)3.1, c-jun, c-fos, repressor element-1 silencing transcription factor (REST), smooth muscle myosin heavy chain (SMMHC), and myocardin was measured using qRT-PCR in isolated medial cells 2 hours and 2 days postangioplasty. K(Ca)3.1, c-jun, and c-fos mRNA levels were increased 2 hours postangioplasty, whereas REST expression decreased. SMMHC expression was unchanged at 2 hours, but decreased 2 days postangioplasty. Use of TRAM-34 coated balloons prevented K(Ca)3.1 upregulation and REST downregulation at 2 hours, SMMHC and myocardin downregulation at 2 days, and attenuated subsequent restenosis 14 and 28 days postangioplasty. Immunohistochemical analysis demonstrated corresponding changes at the protein level.

CONCLUSIONS

Blockade of K(Ca)3.1 by delivery of TRAM-34 via balloon catheter prevented smooth muscle phenotypic modulation and limited subsequent restenosis.

PKCδ mediates anti-proliferative, pro-apoptic effects of testosterone on coronary smooth muscle

Sex hormone status has emerged as an important modulator of coronary physiology and cardiovascular disease risk in both males and females. Our previous studies have demonstrated that testosterone increases protein kinase C (PKC) δ expression and activity in coronary smooth muscle (CSMC). Because PKCδ has been implicated in regulation of proliferation and apoptosis in other cell types, we sought to determine if testosterone modulates CSMC proliferation and/or apoptosis through PKCδ. Porcine CSMC cultures (passages 2–6) from castrated males were treated with testosterone for 24 h. Testosterone (20 and 100 nM) decreased [3H]thymidine incorporation in proliferating CSMC to 59 ± 5.3 and 33.1 ± 4.5% of control. Flow cytometric analysis demonstrated that testosterone induced G1arrest in CSMC with a concomitant reduction in the S phase cells. Testosterone reduced protein levels of cyclins D1and E and phosphorylation of retinoblastoma protein while elevating levels of p21cip1and p27kip1. There were no significant differences in the levels of cyclins D3, CDK2, CDK4, or CDK6. Testosterone significantly reduced kinase activity of CDK2 and -6, but not CDK4, -7, or -1. PKCδ small interfering RNA (siRNA) prevented testosterone-mediated G1arrest, p21cip1upregulation, and cyclin D1and E downregulation. Furthermore, testosterone increased CSMC apoptosis in a dose-dependent manner, which was blocked by either PKCδ siRNA or caspase 3 inhibition. These findings demonstrate that the anti-proliferative, pro-apoptotic effects of testosterone on CSMCs are substantially mediated by PKCδ.

Endothelium-dependent and -independent relaxation in the forelimb and hindlimb vasculatures of swine

Limb differences in endothelial function exist between arm and leg vasculatures of humans. The current investigation tested the hypothesis that forelimb and hindlimb vasorelaxation are similar in the absence of limb differences in blood pressure. Conduit arteries (brachials/femorals) and second order arterioles were harvested from 22 miniature Yucatan swine. In vitro assessment of vasorelaxation was determined by administering increasing doses of bradykinin (BK), acetylcholine (ACh), and sodium nitroprusside (SNP). The role of the nitric oxide synthase (NOS) and cyclooxygenase (COX) pathways was assessed in conduit arteries but not resistance arterioles through L-NAME (300 microM) and INDO (5 microM) incubation, respectively. The relaxation responses to BK and ACh were similar in brachial and femoral arteries. SNP relaxation response was greater in the brachial compared to femoral arteries. There were also no significant differences in the relaxation responses of second order arterioles of the forelimb and hindlimb to BK, ACh, and SNP. Incubation of conduit arterial rings in L-NAME produced a greater reduction in BK and ACh relaxation in the brachial (approximately 25%) compared to femoral (approximately 13%) arterial rings. The current results of this investigation suggest that the forelimb and hindlimb vasculatures of swine have relatively similar vasorelaxation responses to both endothelium-dependent and -independent vasodilators.

Upregulation of intermediate-conductance Ca2+-activated K+ channel (IKCa1) mediates phenotypic modulation of coronary smooth muscle

A hallmark of smooth muscle cell (SMC) phenotypic modulation in atherosclerosis and restenosis is suppression of SMC differentiation marker genes, proliferation, and migration. Blockade of intermediate-conductance Ca(2+)-activated K(+) channels (IKCa1) has been shown to inhibit restenosis after carotid balloon injury in the rat; however, whether IKCa1 plays a role in SMC phenotypic modulation is unknown. Our objective was to determine the role of IKCa1 channels in regulating coronary SMC phenotypic modulation and migration. In cultured porcine coronary SMCs, platelet-derived growth factor-BB (PDGF-BB) increased TRAM-34 (a specific IKCa1 inhibitor)-sensitive K(+) current 20-fold; increased IKCa1 promoter histone acetylation and c-jun binding; increased IKCa1 mRNA approximately 4-fold; and potently decreased expression of the smooth muscle differentiation marker genes smooth muscle myosin heavy chain (SMMHC), smooth muscle alpha-actin (SMalphaA), and smoothelin-B, as well as myocardin. Importantly, TRAM-34 completely blocked PDGF-BB-induced suppression of SMMHC, SMalphaA, smoothelin-B, and myocardin and inhibited PDGF-BB-stimulated migration by approximately 50%. Similar to TRAM-34, knockdown of endogenous IKCa1 with siRNA also prevented the PDGF-BB-induced increase in IKCa1 and decrease in SMMHC mRNA. In coronary arteries from high fat/high cholesterol-fed swine demonstrating signs of early atherosclerosis, IKCa1 expression was 22-fold higher and SMMHC, smoothelin-B, and myocardin expression significantly reduced in proliferating vs. nonproliferating medial cells. Our findings demonstrate that functional upregulation of IKCa1 is required for PDGF-BB-induced coronary SMC phenotypic modulation and migration and support a similar role for IKCa1 in coronary SMC during early coronary atherosclerosis.

Excitation-transcription coupling in arterial smooth muscle

The primary function of the vascular smooth muscle cell (SMC) is contraction for which SMCs express a selective repertoire of genes (eg, SM alpha-actin, SM myosin heavy chain [SMMHC], myocardin) that ultimately define the SMC from other muscle cell types. Moreover, the SMC exhibits extensive phenotypic diversity and plasticity, which play an important role during normal development, repair of vascular injury, and in vascular disease states. Diverse signals modulate ion channel activity in the sarcolemma of SMCs, resulting in altered intracellular calcium (Ca) signaling, activation of multiple intracellular signaling cascades, and SMC contraction or relaxation, a process known as "excitation-contraction coupling" (EC-coupling). Over the past 5 years, exciting new studies have shown that the same signals that regulate EC-coupling in SMCs are also capable of regulating SMC-selective gene expression programs, a new paradigm coined "excitation-transcription coupling" (ET-coupling). This article reviews recent progress in our understanding of the mechanisms by which ET-coupling selectively coordinates the expression of distinct gene subsets in SMCs by disparate transcription factors, including CREB, NFAT, and myocardin, via selective kinases. For example, L-type voltage-gated Ca2+ channels modulate SMC differentiation marker gene expression, eg, SM alpha-actin and SMMHC, via Rho kinase and myocardin and also regulate c-fos gene expression independently via CaMK. In addition, we discuss the potential role of IK channels and TRPC in ET-coupling as potential mediators of SMC phenotypic modulation, ie, negatively regulate SMC differentiation marker genes, in vascular disease.

Assessment of contractility of purified smooth muscle cells derived from embryonic stem cells

The aims of this study were to develop a method for deriving purified populations of contractile smooth muscle cells (SMCs) from embryonic stem cells (ESCs) and to characterize their function. Transgenic ESC lines were generated that stably expressed a puromycin-resistance gene under the control of either a smooth muscle alpha-actin (SMalphaAlpha) or smooth muscle-myosin heavy chain (SM-MHC) promoter. Negative selection, either overnight or for 3 days, was then used to purify SMCs from embryoid bodies. Purified SMCs expressed multiple SMC markers by immunofluorescence, immunoblotting, quantitative reverse transcription-polymerase chain reaction, and flow cytometry and were designated APSCs (SMalphaAlpha-puromycin-selected cells) or MPSCs (SM-MHC-puromycin-selected cells), respectively. Both SMC lines displayed agonist-induced Ca(2+) transients, expressed functional Ca(2+) channels, and generated contractile force when aggregated within collagen gels and stimulated with vasoactive agonists, such as endothelin-1, or in response to depolarization with KCl. Importantly, subcutaneous injection of APSCs or MPSCs subjected to 18 hours of puromycin selection led to the formation of teratomas, presumably due to residual contamination by pluripotent stem cells. In contrast, APSCs or MPSCs subjected to prolonged puromycin selection for 3 days did not form teratomas in vivo. These studies describe for the first time a method for generating relatively pure populations of SMCs from ESCs which display appropriate excitation and contractile responses to vasoactive agonists. However, studies also indicate the potential for teratoma development in ESC-derived cell lines, even after prolonged differentiation, highlighting the critical requirement for efficient methods of separating differentiated cells from residual pluripotent precursors in future studies that use ESC derivatives, whether SMC or other cell types, in tissue engineering applications.

PKCδ Mediates Testosterone-induced Increases in Coronary Smooth Muscle Cav1.2

Sex hormones have emerged as important modulators of cardiovascular physiology and pathophysiology. Our previous studies demonstrated that testosterone increases expression and activity of L-type, voltage-gated calcium channels (Cav1.2) in coronary arteries of males. The purpose of the present study was to determine whether testosterone (T) alters coronary protein kinase C delta (PKCdelta) expression and whether PKCdelta plays a role in coronary Cav1.2 expression. For in vitro studies, porcine right coronary arteries (RCA) and post-confluent (passages 3-6) 5-day, serum-restricted coronary smooth muscle cell cultures (CSMC) were incubated in the presence and absence of T or dihydrotestosterone (10 and 100 nm) for 18 h at 37 degrees C in a humidified chamber. For sex and endogenous testosterone-dependent effects, RCA were obtained from intact males, castrated males, castrated males with T replacement, and intact females. In vitro T and dihydrotestosterone caused an approximately 2-3-fold increase in PKCdelta protein levels, approximately 1.5-2-fold increase in PKCdelta kinase activity, and localization of PKCdelta toward the plasma membrane and nuclear envelope. PKCdelta protein levels were higher in coronary arteries of intact males compared with intact females. Elimination of endogenous testosterone by castration reduced RCA PKCdelta protein levels, an effect partially (approximately 45%) reversed by exogenous T (castrated males with T replacement). In CSMC, PKC inhibition with either the general PKC inhibitor, cheylerythrine, or the putative PKCdelta inhibitor, rottlerin, completely inhibited the T-mediated increase in coronary Cav1.2 protein levels. Conversely, Go6976, a conventional PKC isoform inhibitor, failed to inhibit T-induced increases in coronary Cav1.2 protein levels. PKCdelta short interference RNA completely blocked T-induced increases in Cav1.2 protein levels in CSMC. These results demonstrate for the first time that 1) endogenous T is a primary modulator of coronary PKCdelta protein and activity in males and 2) T increases Cav1.2 protein expression in a PKCdelta-dependent manner.

Exercise and hypercholesterolemia produce disparate shifts in coronary PKC expression

INTRODUCTION

Sedentary lifestyle and high-fat, high-cholesterol diets are each associated with elevated risk for coronary heart disease (CHD); however, the mechanisms by which they increase risk are unclear. Specific PKC isoforms have been implicated in the development of CHD, regulation of coronary vasoreactivity, as well as exercise-induced cardioprotection. Thus, diet and physical inactivity may increase CHD risk by altering coronary protein kinase C (PKC) isoform profiles.

PURPOSE

To determine whether coronary PKC isoform profiles are altered in a model of early CHD and whether exercise can prevent these changes.

METHODS

Male and female Yucatan miniature swine were either fed a normal (NF) or high-fat (HF) diet (8 vs 46% kilocalories from fat) and remained sedentary (Sed) or were treadmill-trained (Ex) at 75% of; VO2max (6 mph, 60 min) for 16 wk. Groups were as follows: NFSed (N=8/N=7), NFEx (N=8/N=7), HFSed (N=8/N=7), and HFEx (N=8/N=7). Western blotting was performed on right coronary conduit artery (CCA) segments (>1 mm I.D.) to measure total protein levels of PKC-alpha, -betaI, -betaII, -delta, -epsilon, and -zeta.

RESULTS

HF diet increased total cholesterol by more than sixfold with no increase in triglycerides. Hypercholesterolemia increased PKC-betaII and -epsilon protein levels in CCA of both male and female pig; Ex had no effect on this response. Ex-induced increases in PKC-betaI, PKC-delta, and PKC-zeta were observed in HF male pigs. Female pigs had higher baseline amounts of PKC-alpha (25%), PKC-betaI (33%), PKC-betaII (39%), and PKC-epsilon (29%), whereas male pigs had higher amounts of PKC-delta (308%). Further analyses revealed a direct relationship between androgens and PKC-delta levels.

CONCLUSION

Hypercholesterolemia and exercise exert disparate effects on coronary PKC expression. Observed sex differences in PKC protein profiles may also contribute to altered cardiovascular risk patterns in males versus females.

Exaggerated coronary vasoreactivity to endothelin-1 in aged rats: Role of protein kinase C

OBJECTIVE

The interaction between advanced age and increased susceptibility to ischemic insult is well documented. Age-related increases in coronary vascular resistance, in part due to impaired dilator responses, have been reported. Our aim was to determine the role of endothelin-1 (ET-1) on enhanced constrictor responses in aged coronary arteries (CAs) and whether protein kinase C (PKC) signaling mechanisms impact ET-1 responses.

METHODS

Vasoreactivity was assessed in CAs isolated from aged (24 months; n=16) and adult (4 months; n=21) male F344 rats following ET-1 (10(-10)-10(-8)) with and without specific ETA/ETB receptor antagonists (BQ-123, 1 microM; BQ-788, 30 nM) or the PKC inhibitor bisindolylmaleimide (Bis; 10(-6) M). Constrictor responses to KCl (80 mM) were also measured and voltage-gated Ca2+ channel (VGCC) determined in isolated coronary smooth muscle cells. Dilator responses to acetylcholine (ACH) and sodium nitroprusside (SNP) were assessed.

RESULTS

Passive diameter was greater (357+/-19 vs. 309+/-9; p<0.02) while spontaneous tone was similar in 24 months vs. 4 months. ET-1 resulted in greater constriction in 24 months vs. 4 months (79% vs. 67%; p<0.01). Group differences persisted following selective ETB inhibition with BQ-788 (p<0.02), while BQ-123 abolished contractile responses to ET-1. Importantly, inhibition of ET-1 constriction by Bis occurred in 24 months but not 4 months (p<0.01). Constrictor responses to KCl and peak VGCC current density were similar in 24 months vs. 4 months (48% vs. 50%). No age-related differences were observed in ACH- or SNP-mediated dilation. Western blotting revealed increases in Ca2+-sensitive PKCalpha, -betaI, and -betaII levels with age, while eNOS and ETA receptor protein levels were unchanged.

CONCLUSION

Aberrant ETA constrictor responses and directional changes in PKC are likely to contribute to coronary vascular pathology with advanced age.

Isoform-specific modulation of coronary artery PKC by glucocorticoids

Glucocorticoids (GC) exert diverse cellular effects in response to both acute and chronic stress, the functional consequences of which have been implicated in the development of cardiovascular pathology such as hypertension and atherosclerosis. However, the mechanisms by which GCs activate divergent signaling pathways are poorly understood. The present study examined the direct effects of natural (cortisol) and synthetic (dexamethasone) GCs on protein kinase C (PKC) isoform expression in coronary arteries. Porcine right coronary arteries were treated in vitro for 18 h in the presence and absence of either dexamethasone (10, 100, or 500 nM) or cortisol (50, 125, 250, or 500 nM). PKC isoform levels and subcellular distribution were determined by immmunoblotting of whole cell homogenates and immunocytofluorescence using PKC-alpha, -betaII, -epsilon, -delta, and -zeta specific antibodies. Dexamethasone caused a approximately 4-fold increase in PKC-alpha, a approximately 2.5-fold increase in PKC-betaII, and a 2-fold increase in PKC-epsilon (p<0.05). In contrast, dexamethasone had no effect on PKC-delta or PKC- zeta levels. Dexamethasone also caused an increase in the activity of PKC-alpha (285%), -betaII (170%), and -epsilon (210%). Cortisol produced similar effects on PKC isoform expression. Confocal microscopy revealed that while dexamethasone altered localization patterns for PKC-alpha, -betaII and -epsilon, no such effect was observed for PKC-delta or PKC-zeta. The stimulatory effects of dexamethasone and cortisol on coronary PKC levels and translocation were prevented by the GC receptor (GR) blocker, RU486. These results demonstrate, for the first time, that GCs modulate coronary PKC expression and subcellular distribution in an isoform-specific manner through a GR-dependent mechanism.

Hypercholesterolemia abolishes voltage-dependent K+ channel contribution to adenosine-mediated relaxation in porcine coronary arterioles

Hypercholesterolemic patients display reduced coronary flow reserve in response to adenosine infusion. We previously reported that voltage-dependent K+ (Kv) channels contribute to adenosine-mediated relaxation of coronary arterioles isolated from male miniature swine. For this study, we hypothesized that hypercholesterolemia attenuates Kv channel contribution to adenosine-induced vasodilatation. Pigs were randomly assigned to a control or high fat/high cholesterol diet for 20–24 wk, and then killed. After completion of the experimental treatment, arterioles (∼150 μm luminal diameter) were isolated from the left-ventricular free wall near the apical region of the heart, cannulated, and pressurized at 40 mmHg. Adenosine-mediated relaxation was significantly attenuated in both endothelium-intact and -denuded arterioles from hypercholesterolemic compared with control animals. The classic Kv channel blocker, 4-aminopyridine (1 mM), significantly attenuated adenosine-mediated relaxation in arterioles isolated from control but not hypercholesterolemic animals. Furthermore, the nonselective K+ channel blocker, tetraethylammonium (TEA; 1 mM) significantly attenuated adenosine-mediated relaxation in arterioles from control but not hypercholesterolemic animals. In additional experiments, coronary arteriolar smooth muscle cells were isolated, and whole cell K v currents were measured. Kv currents were significantly reduced (∼15%) in smooth muscle cells from hypercholesterolemic compared with control animals. Furthermore, Kv current sensitive to low concentrations of TEA was reduced (∼45%) in smooth muscle cells from hypercholesterolemic compared with control animals. Our data indicate that hypercholesterolemia abolishes Kv channel contribution to adenosine-mediated relaxation in coronary arterioles, which may be attributable to a reduced contribution of TEA-sensitive K v channels in smooth muscle of hypercholesterolemic animals.

L-type Voltage-Gated Ca 2+ Channels Modulate Expression of Smooth Muscle Differentiation Marker Genes via a Rho Kinase/Myocardin/SRF–Dependent Mechanism

Vascular smooth muscle cell (SMC) contraction is mediated in part by calcium influx through L-type voltage-gated Ca 2+ channels (VGCC) and activation of the RhoA/Rho kinase (ROK) signaling cascade. We tested the hypothesis that Ca 2+ influx through VGCCs regulates SMC differentiation marker expression and that these effects are dependent on RhoA/ROK signaling. Depolarization-induced activation of VGCCs resulted in a nifedipine-sensitive increase in endogenous smooth muscle myosin heavy chain (SMMHC) and SM α-actin expression and CArG-dependent promoter activity, as well as c-fos promoter activity. The ROK inhibitor, Y-27632, prevented depolarization-induced increase in SMMHC/SM α-actin but had no effect on c-fos expression. Conversely, the Ca 2+ /calmodulin-dependent kinase inhibitor, KN93, prevented depolarization-induced increases in c-fos expression with no effect on SMMHC/SM α-actin. Depolarization increased expression of myocardin, a coactivator of SRF that mediates CArG-dependent transcription of SMC marker gene promoters containing paired CArG cis regulatory elements (SMMHC/SM α-actin). Both nifedipine and Y-27632 prevented the depolarization-induced increase in myocardin expression. Moreover, short interfering RNA (siRNA) specific for myocardin attenuated depolarization-induced SMMHC/SM α-actin transcription. Chromatin immunoprecipitation (ChIP) assays revealed that depolarization increased SRF enrichment of the CArG regions in the SMMHC, SM α-actin, and c-fos promoters in intact chromatin. Whereas Y-27632 decreased basal and depolarization-induced SRF enrichment in the SMMHC/SM α-actin promoter regions, it had no effect of SRF enrichment of c-fos. Taken together, these results provide evidence for a novel mechanism whereby Ca 2+ influx via VGCCs stimulates expression of SMC differentiation marker genes through mechanisms that are dependent on ROK, myocardin, and increased binding of SRF to CArG cis regulatory elements.

Endogenous testosterone increases L-type Ca2+ channel expression in porcine coronary smooth muscle

Evidence indicates that gender and sex hormonal status influence cardiovascular physiology and pathophysiology. We recently demonstrated increased L-type voltage-gated Ca2+ current (ICa,L) in coronary arterial smooth muscle (CASM) of male compared with female swine. The promoter region of the L-type voltage-gated Ca2+ channel (VGCC) (Cav1.2) gene contains a hormone response element that is activated by testosterone. Thus the purpose of the present study was to determine whether endogenous testosterone regulates CASM ICa,L through regulation of VGCC expression and activity. Sexually mature male and female Yucatan swine (7-8 mo; 35-45 kg) were obtained from the breeder. Males were left intact (IM, n=8), castrated (CM, n=8), or castrated with testosterone replacement (CMT, n=8; 10 mg/day Androgel). Females remained gonad intact (n=8). In right coronary arteries, both Cav1.2 mRNA and protein were greater in IM compared with intact females. Cav1.2 mRNA and protein were reduced in CM compared with IM and restored in CMT. In isolated CASM, both peak and steady-state ICa were reduced in CM compared with IM and restored in CMT. In males, a linear relationship was found between serum testosterone levels and ICa. In vitro, both testosterone and the nonaromatizable androgen, dihydrotestosterone, increased Cav1.2 expression. Furthermore, this effect was blocked by the androgen receptor antagonist cyproterone. We conclude that endogenous testosterone is a primary regulator of Cav1.2 expression and activity in coronary arteries of males.

Hypercholesterolemia inhibits L-type calcium current in coronary macro-, not microcirculation

Hypercholesterolemia (HC) is a mary risk factor for the development of coronary heart disease. Coronary ion regulation, especially calcium, is thought to be important in coronary heart disease development; however, the influence of high dietary fat and cholesterol on coronary arterial smooth muscle (CASM) ion channels is unknown. The purpose of this study was to determine the effect of diet-induced HC on CASM voltage-gated calcium current ( ICa). Male miniature swine were fed a high-fat, high-cholesterol diet (40% kcal fat, 2% wt cholesterol) for 20–24 wk, resulting in elevated serum total and low-density lipoprotein cholesterol. Histochemistry indicated early atherosclerosis in large coronary arteries. CASM were isolated from the right coronary artery (>1.0 mm ID), small arteries (∼200 μm), and large arterioles (∼100 μm). ICawas determined by whole cell voltage clamp. L-type ICawas reduced ∼30% by HC compared with controls in the right coronary artery (-5.29 ± 0.42 vs. -7.59 ± 0.41 pA/pF) but not the microcirculation (small artery, -8.39 ± 0.80 vs. -10.13 ± 0.60; arterioles, -10.78 ± 0.93 vs. -11.31 ± 0.95 pA/pF). Voltage-dependent activation was unaffected by HC in both the macro- and microcirculation. L-type voltage-gated calcium channel (Cav1.2) mRNA and membrane protein levels were unaffected by HC. Inhibition of ICaby HC was reversed in vitro by the cholesterol scavenger methyl-β-cyclodextrin and mimicked in control CASM by incubation with the cholesterol donor cholesterol:methyl-β-cyclodextrin. These data indicate that CASM L-type ICais decreased in large coronary arteries in early stages of atherosclerosis, whereas ICain the microcirculation is unaffected. The inhibition of calcium channel activity in CASM of large coronary arteries is likely due to increases in membrane free cholesterol.

Alterations in PKC signaling underlie enhanced myogenic tone in exercise-trained porcine coronary resistance arteries

The intracellular mechanisms underlying enhanced myogenic contraction (MC) in coronary resistance arteries (CRAs) from exercise-trained (EX) pigs have not been established. The purpose of this study was to test the hypothesis that exercise-induced alterations in protein kinase C (PKC) signaling underlie enhanced MC. Furthermore, we sought to determine whether modulation of intracellular Ca2+signaling by PKC underlies enhanced MC in EX animals. Male Yucatan miniature swine were treadmill trained ( n = 7) at ∼75% of maximal O2uptake for 16 wk (6 miles/h, 60 min) or remained sedentary (SED, n = 6). Diameter measurements in response to intraluminal pressure (60, 75, and 90 cmH2O) or 60 mM KCl were determined in single, cannulated CRAs (∼100 μm ID) with and without the PKC inhibitor chelerythrine (CE, 1 μM). Confocal imaging of Ca2+signaling [myogenic Ca2+(Cam)] was also performed in CRAs of similar internal diameter after abluminal loading of the Ca2+indicator dye fluo 4 (1 μM, 37°C, 30 min). We observed significantly greater MC in CRAs isolated from EX than from SED animals at 90 cmH2O, as well as greater reductions in MC after CE at all pressures studied. At intraluminal pressures of 75 and 90 cmH2O, CE produced greater decreases in Camin CRAs from EX than from SED animals (64% vs. 25%, P < 0.05). Inhibition of KCl constriction and Camby CE was also greater in EX animals ( P < 0.05). Western blotting revealed significant increases in Ca2+-dependent PKC-α (∼50%) but not Ca2+-independent PKC-ϵ levels in CRAs isolated from EX animals ( P < 0.05). We also observed significant group differences in phosphorylated PKC-α levels. Finally, voltage-gated Ca2+current (VGCC) was effectively blocked by CE, bisindolylmaleimide, and staurosporine in isolated smooth muscle cells from CRAs, providing evidence for a mechanistic link between VGCCs and PKC in our experimental paradigm. These results suggest that enhanced MC in CRAs from EX animals involves PKC-dependent modulation of intracellular Ca2+, including regulation of VGCCs.

Insulin-like growth factor-induced transcriptional activity of the skeletal alpha-actin gene is regulated by signaling mechanisms linked to voltage-gated calcium channels during myoblast differentiation

IGF-I activates signaling pathways that increase the expression of muscle-specific genes in differentiating myoblasts. Induction of skeletal alpha-actin expression occurs during differentiation through unknown mechanisms. The purpose of this investigation was to examine the mechanisms that IGF-I uses to induce skeletal alpha-actin gene expression in C2C12 myoblasts. IGF-I increased skeletal alpha-actin promoter activity by 107% compared with the control condition. Ni(+) [T-type voltage-gated Ca(2+) channel (VGCC) inhibitor] reduced basal-induced activation of the skeletal alpha-actin promoter by approximately 84%, and nifedipine (L-type VGCC inhibitor) inhibited IGF-I-induced activation of the skeletal alpha-actin promoter by 29-48%. IGF-I failed to increase skeletal alpha-actin promoter activity in differentiating dysgenic (lack functional L-type VGCC) myoblasts; 30 mm K(+) and 30 mm K(+)+IGF-I increased skeletal alpha-actin promoter activity by 162% and 76% compared with non-IGF-I or IGF-I-only conditions, respectively. IGF-I increased calcineurin activity, which was inhibited by cyclosporine A. Further, cyclosporine A inhibited K(+)+IGF-I-induced activation of the skeletal alpha-actin promoter. Constitutively active calcineurin increased skeletal alpha-actin promoter activity by 154% and rescued the nifedipine-induced inhibition of L-type VGCC but failed to rescue the Ni(+)-inhibition of T-type VGCC. IGF-I-induced nuclear factor of activated T-cells transcriptional activity was not inhibited by nifedipine or Ni(+). IGF-I failed to increase serum response factor transcriptional activity; however, serum response factor activity was reduced in the presence of Ni(+). These data suggest that IGF-I-induced activation of the skeletal alpha-actin promoter is regulated by the L-type VGCC and calcineurin but independent of nuclear factor of activated T-cell transcriptional activity as C2C12 myoblasts differentiate into myotubes.

Altered calcium sensitivity contributes to enhanced contractility of collateral-dependent coronary arteries

Coronary arteries distal to chronic occlusion exhibit enhanced vasoconstriction and impaired relaxation compared with nonoccluded arteries. In this study, we tested the hypotheses that an increase in peak Ca(2+) channel current density and/or increased Ca(2+) sensitivity contributes to altered contractility in collateral-dependent coronary arteries. Ameroid occluders were surgically placed around the proximal left circumflex coronary artery (LCX) of female miniature swine. Segments of epicardial arteries ( approximately 1 mm luminal diameter) were isolated from the LCX and nonoccluded left anterior descending (LAD) arteries 24 wk after Ameroid placement. Contractile responses to depolarization (10-100 mM KCl) were significantly enhanced in LCX compared with size-matched LAD arterial rings [concentration of KCl causing 50% of the maximal contractile response (EC(50)); LAD = 41.7 +/- 2.3, LCX = 34.3 +/- 2.7 mM]. However, peak Ca(2+) channel current was not altered in isolated smooth muscle cells from LCX compared with LAD (-5.29 +/- 0.42 vs. -5.68 +/- 0.55 pA/pF, respectively). Furthermore, whereas half-maximal activation of Ca(2+) channel current occurred at nearly the same membrane potential in LAD and LCX, half-maximal inactivation was shifted to a more positive membrane potential in LCX cells. Simultaneous measures of contractile tension and intracellular free Ca(2+) (fura 2) levels in arterial rings revealed that significantly more tension was produced per unit change in fura 2 ratio in LCX compared with LAD in response to KCl but not during receptor-agonist stimulation with endothelin-1. Taken together, our data indicate that coronary arteries distal to chronic occlusion display increased Ca(2+) sensitivity in response to high KCl-induced depolarization, independent of changes in whole cell peak Ca(2+) channel current. Unaltered Ca(2+) sensitivity in endothelin-stimulated arteries suggests more than one mechanism regulating Ca(2+) sensitization in coronary smooth muscle.

C-reactive protein correlates with macrophage accumulation in coronary arteries of hypercholesterolemic pigs

A growing body of evidence supports the hypothesis that C-reactive protein (CRP) is a marker of inflammation in coronary artery disease. The purpose of the present study was to test the hypothesis that CRP correlates with macrophage accumulation during the initial stages of coronary vascular disease. Adult male pigs were fed a normal chow (NF) or a high-fat high-cholesterol (HF) diet for 20 wk. After 20 wk, blood was collected for analyses of interleukin-6 (IL-6), CRP, and lipids. After blood collection, the pigs were euthanized and the right coronary arteries (RCA) were harvested and fixed in neutral buffered formalin. Paraffin-embedded sections of RCA were stained immunohistochemically for CRP, scavenger receptor A (SRA), and monocyte chemoattractant protein-1 (MCP-1). All cholesterol fractions were elevated in the HF vs. the NF group (P < 0.05). There was little or no positive staining for CRP, SRA, or MCP-1 in the RCA of NF pigs, but there was extensive staining in lipidladen macrophage foam cells in the HF pigs. Double staining revealed colocalization of CRP with SRA and CRP with MCP-1 in foam cells. Serum IL-6 was below the assay detection limit in all pigs. Serum CRP correlated directly with plasma total cholesterol (R = 0.727, P = 0.041) and accumulation of SRA-positive macrophages (R = 0.938, P < 0.001) in RCA of HF pigs. We conclude that serum CRP correlates with macrophage accumulation and coronary artery disease in hypercholesterolemic pigs.

Coronary smooth muscle adaptation to exercise: does it play a role in cardioprotection?

Substantial evidence exists supporting the role of chronic exercise in reducing the incidence and severity of coronary vascular disease. Physical inactivity is an independent risk factor for coronary heart disease suggesting that the cardioprotective effect of exercise is due, in part, to an intrinsic adaptation within the coronary vasculature. Surprisingly, a paucity of information exists regarding the intrinsic cellular changes within the coronary vasculature associated with exercise training and even less is known regarding the effect of physical activity on long-term phenotypic modulation of coronary smooth muscle (CSM). The purpose of this symposium is to provide a concise update on the current knowledge regarding CSM adaptation to exercise training and the potential for these adaptations to contribute to exercise-induced cardioprotection. The potential role of CSM in exercise-induced cardioprotection will be approached from two perspectives. First, endurance exercise training effects on the regulation of coronary vasomotor tone via changes in CSM calcium regulation will be reviewed, i.e. short-term functional adaptation. Secondly, we will discuss potential long-term consequences of this altered calcium regulation, i.e. exercise-induced phenotypic modulation of CSM. We propose that exercise training alters CSM intracellular calcium regulation to reduce Ca2+-dependent activation of the contractile apparatus and Ca2+-dependent gene transcription and increase activation of sarcolemmal potassium channels. The overall effect is to increase the gain of the vasomotor system and maintain a stable, contractile CSM phenotype.

Exercise training attenuates coronary smooth muscle phenotypic modulation and nuclear Ca2+ signaling

Physical inactivity is an independent risk factor for coronary heart disease, yet the mechanism(s) of exercise-related cardioprotection remains unknown. We tested the hypothesis that coronary smooth muscle after exercise training would have decreased mitogen-induced phenotypic modulation and enhanced regulation of nuclear Ca(2+). Yucatan swine were endurance exercise trained (EX) on a treadmill for 16-20 wk. EX reduced endothelin-1-induced DNA content by 40% compared with sedentary (SED) swine (P < 0.01). EX decreased single cell peak endothelin-1-induced cytosolic Ca(2+) responses compared with SED by 16% and peak nuclear Ca(2+) responses by 33% (P < 0.05), as determined by confocal microscopy. On the basis of these results, we hypothesized that sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) and intracellular Ca(2+) stores in native smooth muscle are spatially localized to dissociate cytosolic Ca(2+) and nuclear Ca(2+). Subcellular localization of SERCA in living and fixed cells revealed a distribution of SERCA near the sarcolemma and on the nuclear envelope. These results show that EX enhances nuclear Ca(2+) regulation, possibly via SERCA, which may be one mechanism by which coronary smooth muscle cells from EX are less responsive to mitogen-induced phenotypic modulation.

Nonuniform changes in arteriolar myogenic tone within skeletal muscle following hindlimb unweighting

Hindlimb unweighting (HLU) has been shown to alter myogenic tone distinctly in arterioles isolated from skeletal muscles composed predominantly of fast-twitch (white gastrocnemius) compared with slow-twitch (soleus) fibers. Based on these findings, we hypothesized that HLU would alter myogenic tone differently in arterioles isolated from distinct fiber-type regions within a single skeletal muscle. We further hypothesized that alterations in myogenic tone would be associated with alterations in voltage-gated Ca(2+) channel current (VGCC) density of arteriolar smooth muscle. After 14 days of HLU or weight bearing (control), first-order arterioles were isolated from both fast-twitch and mixed fiber-type regions of the gastrocnemius muscle, cannulated, and pressurized at 90 cmH(2)O. Mixed gastrocnemius arterioles of HLU rats demonstrated increased spontaneous tone [43 +/- 5% (HLU) vs. 27 +/- 4% (control) of possible constriction] and an approximately twofold enhanced myogenic response when exposed to step changes in intraluminal pressure (10-130 cmH(2)O) compared with control rats. In contrast, fast-twitch gastrocnemius arterioles of HLU rats demonstrated similar levels of spontaneous tone [6 +/- 2% (HLU) vs. 6 +/- 2% (control)] and myogenic reactivity to control rats. Neither KCl-induced contractile responses (10-50 mM KCl) nor VGCC density was significantly different between mixed gastrocnemius arterioles of HLU and control rats. These results suggest that HLU produces diverse adaptations in myogenic reactivity of arterioles isolated from different fiber-type regions of a single skeletal muscle. Furthermore, alterations in myogenic responses were not attributable to altered VGCC density.

Gender-specific K(+)-channel contribution to adenosine-induced relaxation in coronary arterioles

We examined the contribution of K(+)-channel activity on basal tone and adenosine-mediated relaxation of coronary arterioles isolated from sexually mature male and female miniature swine. Arterioles (approximately 100-200 microm ID) isolated from the apical region of the heart were cannulated and studied using videodimensional analysis under constant intraluminal pressure. Coronary arterioles from male and female pigs demonstrated similar levels of basal tone and reductions in basal diameter in response to the K(+)-channel blockers 4-aminopyridine (4-AP; 1 mM), tetraethylammonium (1 mM), and glibenclamide (Glib; 10 microM), with 4-AP producing significantly greater constriction than tetraethylammonium or Glib. After endothelin-induced preconstriction, relaxation responses to adenosine were not significantly different between coronary arterioles of male and female pigs. Inhibition of 4-AP-sensitive channels significantly impaired adenosine-mediated relaxation in arterioles from male but not female pigs. However, inhibition of K(+) channels with iberiotoxin (100 nM) or Glib had no effect on adenosine-induced relaxation in either sex. Results obtained in the presence of nitric oxide synthase inhibition suggest a potential interaction of 4-AP-sensitive channels and nitric oxide at low adenosine concentrations. In conclusion, our data indicate that 4-AP-sensitive channels 1) contribute significantly to basal tone in coronary arterioles of both male and female pigs, 2) contribute to adenosine-mediated relaxation in male but not female pigs, and 3) can contribute to adenosine-induced relaxation independent of nitric oxide production in male pigs. These data are consistent with a significant role for voltage-dependent K(+) channels in adenosine-mediated relaxation of coronary arterioles from males.

Gender influences coronary L-type Ca(2+) current and adaptation to exercise training in miniature swine

Endurance exercise training increases smooth muscle L-type Ca(2+) current density in both resistance and proximal coronary arteries of female miniature swine. The purpose of the present study was to determine 1) whether gender differences exist in coronary smooth muscle (CSM) L-type Ca(2+) current density and 2) whether endurance training in males would demonstrate a similar adaptive response as females. Proximal, conduit (approximately 1.0 mm), and resistance [~200 microm (internal diameter)] coronary arteries were obtained from sedentary and treadmill-trained swine of both sexes. CSM were isolated by enzymatic digestion (collagenase plus elastase), and voltage-gated Ca(2+)-channel current (I(Ca)) was determined by using whole cell voltage clamp during superfusion with 75 mM tetraethylammonium chloride and 10 mM BaCl(2). Current-voltage relationships were obtained at test potentials from -60 to 70 mV from a holding potential of -80 mV, and I(Ca) was normalized to cell capacitance (pA/pF). Endurance treadmill training resulted in similar increases in heart weight-to-body weight ratio, endurance time, and skeletal muscle citrate synthase activity in male and female swine. I(Ca) density was significantly greater in males compared with females in both conduit (-7.57 +/- 0.58 vs. -4.14 +/- 0.47 pA/pF) and resistance arteries (-11.25 +/- 0.74 vs. -6.49 +/- 0.87 pA/pF, respectively). In addition, voltage-dependent activation of I(Ca) in resistance arteries was shifted to more negative membrane potentials in males. Exercise training significantly increased I(Ca) density in both conduit and resistance arteries in females (-7.01 +/- 0.47 and -9.73 +/- 1.13 pA/pF, respectively) but had no effect in males (-8.61 +/- 0.50 and -12.04 +/- 1.07 pA/pF, respectively). Thus gender plays a significant role in determining both the magnitude and voltage dependence of I(Ca) in CSM and the adaptive response of I(Ca) to endurance training.

Hydrogen peroxide activates Na(+)-dependent Ca(2+) influx in coronary endothelial cells

The purpose of the present study was to examine the effect of short duration H(2)O(2) exposure on coronary artery endothelial cell [Ca(2+)](i) regulation. Freshly dispersed cells from porcine coronary artery were exposed to H(2)O(2) (300 micromol/L) for 3 min while monitoring [Ca(2+)](i) using fura-2 microfluorometry. H(2)O(2) increased [Ca(2+)](i) from 0.86 +/- 0.03 to 2.19 +/- 0.41 ratio units at 3 min of H(2)O(2) (P < 0.05). Intracellular Ca(2+) remained elevated 3 min following removal of H(2)O(2), yet H(2)O(2) had no effect on the subsequent [Ca(2+)](i) response to bradykinin (0.1 micromol/L). The H(2)O(2)-induced [Ca(2+)](i) increase was completely abolished either by removal of extracellular Ca(2+) or lowering extracellular Na(+). Cells exposed to the Na(+) ionophore, monensin, showed an increase in [Ca(2+)](i) with a time course similar to that seen with H(2)O(2). Furthermore, H(2)O(2)-induced Ca(2+) influx was not attenuated by either Ni(2+) (300 micromol/L) or econazole (10 micromol/L), excluding Ca(2+) influx via the agonist-sensitive pathway. Thus, in coronary arterial endothelial cells, H(2)O(2) increases Ca(2+) influx in an extracellular Na(+)-dependent manner via an agonist-insensitive pathway.

Enhanced L-type Ca2+ channel current density in coronary smooth muscle of exercise-trained pigs is compensated to limit myoplasmic free Ca2+ accumulation

We hypothesized that enhanced voltage-gated Ca2+ channel current (VGCC) density in coronary smooth muscle cells of exercise-trained miniature Yucatan pigs is compensated by other cellular Ca2+ regulatory mechanisms to limit net myoplasmic free Ca2+ accumulation. Whole-cell voltage clamp experiments demonstrated enhanced VGCC density in smooth muscle cells freshly dispersed from coronary arteries of exercise-trained vs. sedentary animals. In separate experiments using fura-2 microfluorometry, we measured depolarization-induced (80 mM KCl) accumulation of myoplasmic free Ba2+ and free Ca2+. Both maximal rate and net accumulation of free Ba2+ in response to membrane depolarization were increased in smooth muscle cells isolated from exercise-trained pigs, consistent with an increased VGCC density. Depolarization also produced an enhanced maximal rate of free Ca2+ accumulation in cells of exercise-trained pigs; however, net accumulation of free Ca2+ was not significantly increased suggesting enhanced Ca2+ influx was compensated to limit net free Ca2+ accumulation. Inhibition of sarco-endoplasmic reticulum Ca2+-transporting ATPase (SERCA; 10 microM cyclopiazonic acid) and/or sarcolemmal Na+-Ca2+ exchange (low extracellular Na+) suggested neither mechanism compensated the enhanced VGCC in cells of exercise-trained animals. Local Ca2+-dependent inactivation of VGCC, assessed by buffering myoplasmic Ca2+ with EGTA in the pipette and using Ca2+ and Ba2+ as charge carriers, was not different between cells of sedentary and exercise-trained animals. Our findings indicate that increased VGCC density is compensated by other cellular Ca2+ regulatory mechanisms to limit net myoplasmic free Ca2+ accumulation in smooth muscle cells of exercise-trained animals. Further, SERCA, Na+-Ca2+ exchange and local Ca2+-dependent inactivation of VGCC do not appear to function as compensatory mechanisms. Additional potential compensatory mechanisms include Ca2+ extrusion via plasma membrane Ca2+-ATPase, mitochondrial uptake, myoplasmic Ca2+-binding proteins and other sources of VGCC inactivation.

Coronary smooth muscle and endothelial adaptations to exercise training

Exercise training produces complex changes in intrinsic control of coronary vascular resistance. In smooth muscle, adaptations that alter Ca2+ regulation seem central, including changes in the function of sarcolemmal K+ and L-type Ca2+ channels and the sarcoplasmic reticulum. Exercise training also increases the ability of the endothelium to release vasoactive factors, with increased expression and activity of endothelial cell nitric oxide synthase playing a key role.

Exercise training increases L-type calcium current density in coronary smooth muscle

Exercise training produces numerous adaptations in the coronary circulation, including an increase in coronary tone, both in conduit and resistance arteries. On the basis of the importance of voltage-gated Ca2+ channels (VGCC) in regulation of vascular tone, we hypothesized that exercise training would increase VGCC current density in coronary smooth muscle. To test this hypothesis, VGCC current was compared in smooth muscle from conduit arteries (>1.0 mm), small arteries (200-250 micrometer), and large arterioles (75-150 micrometer) from endurance-trained (Ex) or sedentary miniature swine (Sed). After 16-20 wk of treadmill training, VGCC current was determined using whole cell voltage-clamp techniques. In both Ex and Sed, VGCC current density was inversely related to arterial diameter, i.e., large arterioles > small arteries > conduit arteries. Exercise training increased peak inward currents approximately twofold in smooth muscle from all arterial sizes compared with those from Sed (large arteriole, -12.52 +/- 2.05 vs. -5.74 +/- 0.99 pA/pF; small artery, -6.20 +/- 0.97 vs. -3.18 +/- 0.44 pA/pF; and conduit arteries, -4.22 +/- 0.30 vs. -2.41 +/- 0.55 pA/pF; 10 mM Ba2+ external). Dihydropyridine sensitivity, voltage dependence, and inactivation kinetics identified this Ca2+ current to be L-type current in all arterial sizes from both Sed and Ex. Furthermore, peak VGCC current density was correlated with treadmill endurance in all arterial sizes. We conclude that smooth muscle L-type Ca2+ current density is increased within the coronary arterial bed by endurance exercise training. This increased VGCC density may provide an important mechanistic link between functional and cellular adaptations in the coronary circulation to exercise training.

Exercise training increases K+-channel contribution to regulation of coronary arterial tone

The present study examined whether regulation of coronary tone in conduit arteries (>1.0 mm ID) is altered by exercise training. Yucatan miniature swine were treadmill trained for 16-20 wk (Ex) and compared with sedentary counterparts (Sed). Endothelium-denuded arterial rings were stretched to optimal length and allowed to equilibrate for 60 min. Inhibition of either Ca2+-activated channels [1 mM tetraethylammonium (TEA) or 10 nM iberiotoxin (IBTX)] or voltage-dependent K+ channels [1 mM 4-aminopyridine (4-AP)] significantly increased resting tension in both groups; however, the effect of all K+-channel blockers was greater in Ex. Addition of 1 mM sodium nitroprusside reduced resting tension in both groups, confirming the presence of active basal tone; however, sodium nitroprusside-sensitive tone was increased approximately twofold in Ex compared with Sed group. Perforated patch-clamp experiments on isolated smooth muscle cells demonstrated no effect of exercise training on whole cell TEA-sensitive, 4-AP-sensitive, or basal K+ current. Similarly, whereas TEA, 4-AP, and IBTX all decreased resting membrane potential, there was no difference in depolarization between groups. The greater effect of TEA on resting tension in Ex could be mimicked in Sed by addition of the Ca2+-channel agonist BAY K 8644. In conclusion, the greater response to K+-channel blockers after exercise training is consistent with an increased contribution of K+ channels to regulation of basal tone in conduit coronary arteries. The lack of an effect of training on K+ current characteristics or membrane potential responses in isolated cells suggests that a requisite factor for enhanced K+-channel activation in arteries from Ex, possibly stretch, is absent in isolated cells.

Exercise training-induced adaptations in the coronary circulation

Aerobic exercise training induces an increase in coronary blood flow capacity that is associated with altered control of coronary vascular resistance and, therefore, coronary blood flow. The relative importance of metabolic, myogenic, endothelium-mediated, and neurohumoral control systems varies throughout the coronary arterial tree, and these control systems contribute in parallel to regulating coronary vascular resistance to differing degrees at each level in the coronary arterial tree. In addition to this nonuniformity of the relative importance of vascular control systems in the coronary arterial tree, it appears that exercise training-induced adaptations are also distributed spatially, in a nonuniform manner throughout the coronary tree. As a result, it is necessary to examine training-induced adaptations throughout the coronary arterial tree. Adaptations in endothelium-mediated control play a role in training-induced changes in control of coronary vascular resistance, and there is evidence that the effects of training may be different in large coronary arteries than in the microcirculation. Also, there is evidence that the mode, frequency, and intensity of exercise training bouts and duration of training may influence the adaptive changes in endothelial function. Exercise training has also been shown to induce changes in responses of coronary vascular smooth muscle to vasoactive agents and alterations in the cellular-molecular control of intracellular Ca2+ in coronary vascular smooth muscle of conduit coronary arteries and to enhance myogenic reactivity of coronary resistance arteries. Exercise training also appears to have different effects on vascular smooth muscle in large coronary arteries than in the microcirculation. For example, adenosine sensitivity is increased in conduit coronary arteries and large resistance arteries after training but is not altered in small coronary resistance arteries of trained animals. Although much remains to be studied, evidence clearly indicates that chronic exercise alters the phenotype of coronary endothelial and vascular smooth muscle cells and that plasticity of these cells plays a role in adaptation of the cardiovascular system in exercise training.

Heterogeneity of L-type calcium current density in coronary smooth muscle

Heterogeneity of vascular responses to physiological and pharmacological stimuli has been demonstrated throughout the coronary circulation. Typically, this heterogeneity is based on vessel size. Although the cellular mechanisms for this heterogeneity are unknown, one plausible factor may be heterogeneous distribution of ion channels important in regulation of vascular tone. Because of the importance of voltage-gated Ca2+ channels in regulation of vascular tone, we hypothesized that these channels would be unequally distributed throughout the coronary arterial bed. To test this hypothesis, voltage-gated Ca2+ current was measured in smooth muscle from conduit arteries (>1.0 mm), small arteries (200-250 microm), and large arterioles (75-125 microm) of miniature swine using whole cell voltage-clamp techniques. With 2 mM Ca2+ or 10 mM Ba2+ as charge carrier, voltage-gated Ca2+ current density was inversely related to arterial diameter, i.e., large arterioles > small arteries > conduit. Peak inward currents (10 mM Ba2+) were increased approximately 2.5- and approximately 1.5-fold in large arterioles and small arteries, respectively, compared with conduit arteries (-5.58 +/- 0.53, -3.54 +/- 0.34, and -2.26 +/- 0.31 pA/pF, respectively). In physiological Ca2+ (2 mM), small arteries demonstrated increased inward current at membrane potentials within the physiological range for vascular smooth muscle (as negative as -40 mV) compared with conduit arteries. In addition, cells from large arterioles showed a negative shift in the membrane potential for half-maximal activation compared with small and conduit arteries (-13.23 +/- 0.88, -6.22 +/- 1.35, and -8.62 +/- 0.81 mV, respectively; P < 0.05). Voltage characteristics and dihydropyridine sensitivity identified this Ca2+ current as predominantly L-type current in all arterial sizes. We conclude that L-type Ca2+ current density is inversely related to arterial diameter within the coronary arterial vasculature. This heterogeneity of Ca2+ current density may provide, in part, the basis for functional heterogeneity within the coronary circulation.

Myocardial injury after hypoxia in immature, adult and aged rats

We evaluated the abilities of isolated perfused hearts from immature (IM) (2.5-3 months), ADULT (11-13 months) and OLD (24-26 months) Fischer 344 rats to tolerate and recover from oxygen deprivation. Hearts were perfused at 60 mmHg for a 30-minute prehypoxic period with oxygenated buffer supplemented with 10 mM glucose (+insulin) and 2 mM acetate, then 30 minutes with substrate-free, hypoxic buffer gassed with 95% N2:5% CO2, and finally reoxygenated for an additional 45 minutes with the same buffer used during the prehypoxic period. During prehypoxia, all groups were similar in ventricular mechanical function, glycogen content, high-energy phosphates (HEP), reduced glutathione (GSH), Ca+2 content, and mitochondrial state 3 rates. At the end of the hypoxic period, glycogen levels were similar and almost completely depleted in all groups, HEP were lower (p < 0.05) in ADULT vs other groups, mitochondrial state 3 rates were decreased (24%, p < 0.05) only in ADULT, and GSH was depleted by 34% in IM vs only 13% in OLD (p < 0.05). After 45 minutes of reoxygenation, IM and OLD had recovered 48% and 45% of their respective prehypoxic function which was two-fold greater than the 23% recovery by ADULT. Loss of cytosolic enzymes, an indicator of sarcolemmal damage, was estimated by measuring lactate dehydrogenase (LDH) release. LDH release and Ca+2 content during reoxygenation in IM were only about half of that observed in ADULT or OLD. We conclude that immature and aged hearts tolerate and recover from hypoxia better than hearts from adults, and that the sarcolemmal membranes of immature rat hearts are less susceptible to damage from hypoxic stress than those of either older group.

Exercise training alters the Ca2+ and contractile responses of coronary arteries to endothelin

We tested the hypothesis that alterations in myoplasmic free Ca2+ (Ca(m)) regulation in coronary smooth muscle after exercise training (Ex) underlie changes in vasomotor function. Yucatan miniature pigs were endurance trained by treadmill running for 16-20 wk. Simultaneous determination of Ca(m) (fura-2 microfluorometry) and contraction during endothelin exposure in coronary arteries were then performed. Endothelin (10(-9) to 10(-7) M) was administered either cumulatively or as a single concentration. Ex significantly attenuated the Ca(m) response to 10(-9) and 10(-8) M endothelin. Developed tension was significantly diminished at 10(-8) M endothelin in Ex pigs, producing a rightward shift in the concentration-developed tension response. Attenuated Ca(m) and contractile response to 10(-8) M endothelin were present after Ex whether endothelin was applied cumulatively or as a single concentration. The developed tension-Ca(m) relationship showed an increased Ca(m) sensitivity of contraction with Ex. Endothelin (10(-8) M)-induced Ca2+ influx, estimated by Ba2+ influx in low-Na+ solution, was increased threefold in coronary arteries from Ex pigs. The decreased Ca(m) in the presence of increased divalent cation (i.e., Ca2+) influx during 10(-8) M endothelin suggests a greatly enhanced sarcolemmal Ca2+ cycling in coronary arteries from Ex pigs.

Heterogeneity of caffeine- and bradykinin-sensitive Ca2+ stores in vascular endothelial cells

The filling state of Ca2+ stores in endothelial cells regulates Ca2+ entry. The functional relationship between the major Ca2+ stores [i.e. Ins(1,4,5)P3-sensitive (= bradykinin-sensitive stores, 'BsS') and caffeine-sensitive stores] is unknown. In pig right-coronary-artery endothelial cells, caffeine failed to release Ca2+ in 68% of the cells (quiet-responders), but increased bradykinin (Bk)-induced Ca2+ release 2.5-fold. In Bk-pre-stimulated cells, caffeine increased Ca2+ release upon a second stimulation with Bk 3.2-fold. In quiet-responders caffeine alone did not affect net Ca2+ storage, whereas Bk or caffeine followed by Bk decreased the intracellular Ca2+ pool to 45% and 15%, respectively. Blockade of the endoplasmic-reticulum Ca2+ pump by thapsigargin unmasked the effect of caffeine in quiet-responders, resulting in a transient increase in intracellular free Ca2+ concentration ([Ca2+]i). In 37% of the cells caffeine alone transiently increased [Ca2+]i and depleted BsS. This study suggests a heterogeneity in functional organization of endothelial Ca2+ stores. In quiet-responders, caffeine translocates Ca2+ towards the BsS, whereas in overt-responders caffeine empties the BsS.

Exercise training improves metabolic response after ischemia in isolated working rat heart

Hearts from treadmill-trained and sedentary rats were perfused in the working heart mode. Mechanical and metabolite status was evaluated before ischemia, after 25 min of global ischemia, and after 30 min of retrograde reperfusion. After reperfusion, hearts from trained rats were found to have better recovery of contractile function, lower diastolic stiffness, greater efficiency of work, and greater extracellular calcium responsiveness than hearts from sedentary rats. Training had no significant impact on bioenergetic status before or at the end of ischemia. However, after reperfusion, both phosphocreatine and ATP were significantly higher in hearts from trained rats than from sedentary control rats. Mitochondrial function in both subsarcolemmal and intermyofibrillar subpopulations was unaffected by ischemia-reperfusion. 45Ca2+ uptake during reperfusion was significantly higher in hearts from sedentary rats than from exercise-trained rats. No differences were found in free radical production or tolerance due to training. Therefore, hearts from exercise-trained rats demonstrated an increased metabolic tolerance to ischemic-reperfusion damage, which may contribute to the improved postischemic functional recovery.

Exercise training improves cardiac function after ischemia in the isolated, working rat heart

The aim of this study was to determine whether exercise training produces a myocardium intrinsically more tolerant to ischemic-reperfusion injury. Male Fischer 344 rats were treadmill trained for 11-16 wk at one of the following intensities: LOW (20 m/min, 0% grade, 60 min/day), moderate (MOD; 30 m/min, 5% grade, 60 min/day) or intensive (INT; 10 bouts of alternating 2-min runs at 16 and 60 m/min, 5% grade). Cardiac function was evaluated both before and after 25 min of global, zero-flow ischemia in the isolated, working heart model. Compared to hearts from sedentary (SED) rats, postischemic cardiac output (CO) and work were significantly higher in all trained groups. Percent recovery of CO (relative to preischemia) was 36.0 +/- 7.1 in SED and 61.2 +/- 6.5, 68.1 +/- 9.3, and 73.2 +/- 5.0 in LOW, MOD, and INT, respectively. Postischemic increases in stroke volume with increased preload and cardiac work at high work load were significantly higher in INT compared with SED. Coronary flow during initial retrograde reperfusion was significantly enhanced with training and correlated with subsequent recovery of CO (R2 = 0.613). Furthermore, trained hearts had higher phosphocreatine (P less than 0.05) and ATP (P less than 0.01) contents after 45 min reperfusion. It is concluded that exercise training results in an intrinsic myocardial adaptation, allowing greater recovery of cardiac pump function after global ischemia in the isolated rat heart.

Effect of downhill running on motoneuron pool excitability

The purpose of this study was to compare alterations in motoneuron pool excitability after eccentric-biased (ECC-B) downhill running exercise with non-biased (NO-B) level running exercise. Six male subjects (25-34 yr) participated in the study, which included ECC-B exercise (-10% grade) and NO-B exercise (0% grade) at 50% of maximal O2 uptake for 20 min. The control trial consisted of 20 min of quiet rest with all subjects participating in all conditions (repeated measures). Motoneuron pool excitability was measured by the Hoffman reflex (H-wave), which was expressed as a ratio (H/M ratio) of the maximal electrically stimulated muscle action potential (M-wave). NO-B exercise resulted in a 9.3 +/- 2.7% (SE) reduction in the H/M ratio. ECC-B exercise resulted in a 24.6 +/- 5.7% reduction in the ratio (P less than 0.05 for both). The two exercise treatment conditions were also significantly different from one another (P less than 0.05). Twenty-four-hour postexercise H/M ratios were similar to baseline (P greater than 0.05). Postexercise subjective muscle soreness assessment (DOMS) produced significant increases in DOMS of 36 and 166% immediately and 24 h after exercise, respectively, for the ECC-B trial only (P less than 0.001). The data show that ECC-B exercise results in greater postexercise H/M ratio reductions than NO-B exercise and that H/M ratio changes post-ECC-B exercise are not solely associated with DOMS.

Fructose-1,6-bisphosphate improves efficiency of work in isolated perfused rat hearts

The purpose of this study was to determine whether exogenous fructose-1,6-bisphosphate (F-1,6-P2) directly affects myocardial hemodynamics and certain metabolic parameters. Isolated working rat hearts were perfused for 30 min with 10 mM glucose (+insulin) as the exclusive exogenous substrate followed by 15 min with glucose plus one of the following concentrations (in mM) of F-1,6-P2: 1.25, 2.5, 5, or 10, and finally returned to the glucose only buffer. Additions of 2.5 and 5 mM F-1,6-P2 decreased (P less than 0.01) oxygen consumption (VO2) by 10.8 and 17.0% and coronary flow by 8.3 and 10.3%, respectively. No changes were observed in lactate release, cardiac output (CO), peak systolic pressure, heart rate, or pressure work (PW). Efficiency, expressed as PW divided by VO2, increased with F-1,6-P2 by 8.6% with 1.25 mM (P less than 0.05), 13.2% with 2.5 mM (P less than 0.01), and 16.9% with 5 mM (P less than 0.01). F-1,6-P2 at 10 mM produced no further improvements in VO2 or efficiency but was associated with declines (P less than 0.05) in CO and PW. Glucose plus 10 mM fructose had no effects on any of the above parameters, indicating that the F-1,6-P2-induced changes were not due to changes in osmolarity or to end products of F-1,6-P2 hydrolysis. Some chelation of buffer calcium by F-1,6-P2 occurred, but when free calcium was equalized in glucose and glucose plus 5 mM F-1,6-P2 buffers, the decline in VO2 (11.5%) was still far greater than could be explained by exogenous F-1,6-P2 metabolism in the glycolytic pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of acute, submaximal exercise on skeletal muscle vitamin E

Vitamin E is the major lipid soluble anti-oxidant and may play an important protective role against free radicals produced during exercise. The purpose of this study was to determine the effect of a submaximal exercise bout on vitamin E levels in selected tissues. Five week- old lean, female Zucker rats were randomly divided into sedentary and run groups. At least 4 days following a maximal VO2 test, the run group (n = 7) ran on a treadmill at 70.3 +/- 1.5% VO2 max for 34-42 minutes. Duration was varied according to body weight to keep total work constant. Immediately post-exercise, animals were decapitated, exsanguinated and the quadriceps (red and white vastus lateralis), liver and heart quickly excised and stored under liquid nitrogen until analyzed. Lipids were extracted in heptane and alpha-tocopherol levels determined by reverse-phase HPLC with electrochemical detection. Quadriceps vitamin-E levels declined post-exercise p less than 0.01), and in the white quadriceps from 22 +/- 2 to 16 +/- 2 (p less than 0.05) nmol/g wet weight. No change in vitamin E content was noted for either heart (113 +/- 6 vs. 110 +/- 7, p less than 0.05) or liver (68 +/- 6 vs. 78 +/- 5, p greater than 0.05). It is concluded that a single bout of submaximal treadmill running can result in a significant depletion of vitamin E in skeletal muscle.

Protection against hypoxic injury in isolated-perfused rat heart by ruthenium red

Changes in intracellular calcium content and energy production during the period of hypoxia appear to be necessary for the development of cellular injury. Ruthenium red, a hexavalent dye which inhibits the active uptake of calcium by mitochondria, might improve a cell's energy status thereby minimizing hypoxic injury. Rat heart tissue was perfused retrogradely with Krebs-Henseleit medium containing 2.5 mM calcium and 10 mM glucose. The infusion of 0.1, 1.0 or 1.24, but not 0.01 microM, ruthenium red throughout 60 min of hypoxia and 30 min of reoxygenation decreased, in a dose-dependent manner, the release of lactate dehydrogenase normally seen at reoxygenation. When the infusion of 1.24 microM ruthenium red was begun after 45 min of hypoxia, lactate dehydrogenase release at reoxygenation after 60 min of hypoxia was decreased, but to a lesser extent than when this agent was present throughout hypoxia. Ruthenium red, 1.24 microM, had no significant effects on coronary flow or function in oxygenated heart tissue. When present throughout hypoxia and reoxygenation, 1.24 microM ruthenium red prevented the decrease in coronary flow normally seen and allowed recovery of heart rate, +dP/dT, -dP/dT and work (defined as the product of developed pressure and heart rate) to normal levels. Significant functional protection was not evident at reoxygenation when ruthenium red was infused after 45 min of hypoxia or in the absence of glucose. Cardiac ATP, creatine phosphate and energy charge were decreased after 60 min of hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)