Myogenic activation and calcium sensitivity of cannulated rat mesenteric small arteries

Characterization of Contractile Machinery of Vascular Smooth Muscles in Hypertension

Hypertension is a key risk factor for cardiovascular disease and it is a growing public health problem worldwide. The pathophysiological mechanisms of vascular smooth muscle (VSM) contraction contribute to the development of hypertension. Calcium (Ca²⁺)-dependent and -independent signaling mechanisms regulate the balance of the myosin light chain kinase and myosin light chain phosphatase to induce myosin phosphorylation, which activates VSM contraction to control blood pressure (BP). Here, we discuss the mechanism of the contractile machinery in VSM, especially RhoA/Rho kinase and PKC/CPI-17 of Ca²⁺ sensitization pathway in hypertension. The two signaling pathways affect BP in physiological and pathophysiological conditions and are highlighted in pulmonary, pregnancy, and salt-sensitive hypertension.

Inositol 1,4,5-Trisphosphate Receptors in Hypertension

Chronic hypertension remains a major cause of global mortality and morbidity. It is a complex disease that is the clinical manifestation of multiple genetic, environmental, nutritional, hormonal, and aging-related disorders. Evidence supports a role for vascular aging in the development of hypertension involving an impairment in endothelial function together with an alteration in vascular smooth muscle cells (VSMCs) calcium homeostasis leading to increased myogenic tone. Changes in free intracellular calcium levels ([Ca²⁺] _i ) are mediated either by the influx of Ca²⁺ from the extracellular space or release of Ca²⁺ from intracellular stores, mainly the sarcoplasmic reticulum (SR). The influx of extracellular Ca²⁺ occurs primarily through voltage-gated Ca²⁺ channels (VGCCs), store-operated Ca²⁺ channels (SOC), and Ca²⁺ release-activated channels (CRAC), whereas SR-Ca²⁺ release occurs through inositol trisphosphate receptor (IP₃R) and ryanodine receptors (RyRs). IP₃R-mediated SR-Ca²⁺ release, in the form of Ca²⁺ waves, not only contributes to VSMC contraction and regulates VGCC function but is also intimately involved in structural remodeling of resistance arteries in hypertension. This involves a phenotypic switch of VSMCs as well as an alteration of cytoplasmic Ca²⁺ signaling machinery, a phenomena tightly related to the aging process. Several lines of evidence implicate changes in expression/function levels of IP₃R isoforms in the development of hypertension, VSMC phenotypic switch, and vascular aging. The present review discusses the current knowledge of these mechanisms in an integrative approach and further suggests potential new targets for hypertension management and treatment.

Vasoconstrictor stimulus determines the functional contribution of myoendothelial feedback to mesenteric arterial tone

KEY POINTS

In isolated resistance arteries, endothelial modulation of vasoconstrictor responses to α₁ -adrenoceptor agonists occurs via a process termed myoendothelial feedback: localized inositol trisphosphate (InsP₃ )-dependent Ca²⁺ transients activate intermediate conductance Ca²⁺ -activated K⁺ (IK_Ca ) channels, hyperpolarizing the endothelial membrane potential to limit further reductions in vessel diameter. We demonstrate that IK_Ca channel-mediated myoendothelial feedback limits responses of isolated mesenteric arteries to noradrenaline and nerve stimulation, but not to the thromboxane A₂ mimetic U46619 or to increases in intravascular pressure. In contrast, in the intact mesenteric bed, although responses to exogenous noradrenaline were limited by IK_Ca channel-mediated myoendothelial feedback, release of NO and activation of endothelial small conductance Ca²⁺ -activated K⁺ (SK_Ca ) channels in response to increases in shear stress appeared to be the primary mediators of endothelial modulation of vasoconstriction. We propose that (1) the functional contribution of myoendothelial feedback to arterial tone is determined by the nature of the vasoconstrictor stimulus, and (2) although IK_Ca channel-mediated myoendothelial feedback may contribute to local control of arterial diameter, in the intact vascular bed, increases in shear stress may be the major stimulus for engagement of the endothelium during vasoconstriction.

ABSTRACT

Constriction of isolated resistance arteries in response to α₁ -adrenoceptor agonists is limited by reciprocal engagement of inhibitory endothelial mechanisms via myoendothelial feedback. In the current model of feedback, agonist stimulation of smooth muscle cells results in localized InsP₃ -dependent Ca²⁺ transients that activate endothelial IK_Ca channels. The subsequent hyperpolarization of the endothelial membrane potential then feeds back to the smooth muscle to limit further reductions in vessel diameter. We hypothesized that the functional contribution of InsP₃ -IK_Ca channel-mediated myoendothelial feedback to limiting arterial diameter may be influenced by the nature of the vasoconstrictor stimulus. To test this hypothesis, we investigated the functional role of myoendothelial feedback in modulating responses of rat mesenteric resistance arteries to the adrenoceptor agonist noradrenaline, the thromboxane A₂ mimetic U46619, increases in intravascular pressure and stimulation of perivascular sympathetic nerves. In isolated arteries, responses to noradrenaline and stimulation of sympathetic nerves, but not to U46619 and increases in intravascular pressure, were modulated by IK_Ca channel-dependent myoendothelial feedback. In the intact mesenteric bed perfused under conditions of constant flow, responses to exogenous noradrenaline were modulated by myoendothelial feedback, but shear stress-induced release of NO and activation of endothelial SK_Ca channels appeared to be the primary mediators of endothelial modulation of vasoconstriction to agonists and nerve stimulation. Thus, we propose that myoendothelial feedback may contribute to local control of diameter within arterial segments, but at the level of the intact vascular bed, increases in shear stress may be the major stimulus for engagement of the endothelium during vasoconstriction.

Involvement of Epithelial Na+ Channel in the Elevated Myogenic Response in Posterior Cerebral Arteries from Spontaneously Hypertensive Rats

Hypertension is characterized by increased peripheral vascular resistance which is related with elevated myogenic response. Recent findings have indicated that epithelial sodium channel (ENaC) is involved in mechanotransduction of the myogenic response. The purpose of this study was to investigate the involvement of ENaC in the elevated myogenic response of posterior cerebral arteries (PCAs) from spontaneously hypertensive rats (SHRs). Sixteen to eighteen weeks old male wistar kyoto rats (WKYs) and SHRs were used in this study. We found that wall to lumen (W/L) ratio was increased in the PCAs from SHRs compared with WKYs at the resting state. Interestingly, amiloride significantly inhibited myogenic response in the PCAs from SHRs and WKYs, however, the magnitude of the blockade was greater in SHRs. The transfection of γENaC-siRNA significantly reduced the expression of γENaC protein and inhibited myogenic response in the PCAs from SHRs. Furthermore, these data were supported by the findings that serum/glucocorticoid-induced kinase (Sgk1) and neural precursor cell-expressed developmentally downregulated gene 4-2 (Nedd4-2) were increased in SHRs compared with WKYs. Our results suggest that γENaC may play an important role in the elevated myogenic response in PCAs from SHRs.

ROK and Arteriolar Myogenic Tone Generation: Molecular Evidence in Health and Disease

The myogenic response is an inherent property of resistance arteries that warrants a relatively constant blood flow in response to changes in perfusion pressure and protect delicate organs from vascular insufficiencies and excessive blood flow. This fundamental phenomenon has been extensively studied aiming to elucidate the underlying mechanisms triggering smooth muscle contraction in response to intraluminal pressure elevation, particularly, Rho-associated kinase (ROK)-mediated Ca²⁺-independent mechanisms. The size of the resistance arteries limits the capacity to examine changes in protein phosphorylation/expression levels associated with ROK signaling. A highly sensitive biochemical detection approach was beneficial in examining the role of ROK in different force generation mechanisms along the course of myogenic constriction. In this mini review, we summarize recent results showing direct evidence for the contribution of ROK in development of myogenic response at the level of mechanotransduction, myosin light chain phosphatase inhibition and dynamic actin cytoskeleton reorganization. We will also present evidence that alterations in ROK signaling could underlie the progressive loss in myogenic response in a rat model of type 2 diabetes.

T-type Ca2+ channels and autoregulation of local blood flow

L-type voltage gated Ca²⁺ channels are considered to be the primary source of calcium influx during the myogenic response. However, many vascular beds also express T-type voltage gated Ca²⁺ channels. Recent studies suggest that these channels may also play a role in autoregulation. At low pressures (40-80 mmHg) T-type channels affect myogenic responses in cerebral and mesenteric vascular beds. T-type channels also seem to be involved in skeletal muscle autoregulation. This review discusses the expression and role of T-type voltage gated Ca²⁺ channels in the autoregulation of several different vascular beds. Lack of specific pharmacological inhibitors has been a huge challenge in the field. Now the research has been strengthened by genetically modified models such as mice lacking expression of T-type voltage gated Ca²⁺ channels (Ca_V3.1 and Ca_V3.2). Hopefully, these new tools will help further elucidate the role of voltage gated T-type Ca²⁺ channels in autoregulation and vascular function.

Mechanics of Vascular Smooth Muscle

Vascular smooth muscle (VSM; see Table 1 for a list of abbreviations) is a heterogeneous biomaterial comprised of cells and extracellular matrix. By surrounding tubes of endothelial cells, VSM forms a regulated network, the vasculature, through which oxygenated blood supplies specialized organs, permitting the development of large multicellular organisms. VSM cells, the engine of the vasculature, house a set of regulated nanomotors that permit rapid stress-development, sustained stress-maintenance and vessel constriction. Viscoelastic materials within, surrounding and attached to VSM cells, comprised largely of polymeric proteins with complex mechanical characteristics, assist the engine with countering loads imposed by the heart pump, and with control of relengthening after constriction. The complexity of this smart material can be reduced by classical mechanical studies combined with circuit modeling using spring and dashpot elements. Evaluation of the mechanical characteristics of VSM requires a more complete understanding of the mechanics and regulation of its biochemical parts, and ultimately, an understanding of how these parts work together to form the machinery of the vascular tree. Current molecular studies provide detailed mechanical data about single polymeric molecules, revealing viscoelasticity and plasticity at the protein domain level, the unique biological slip-catch bond, and a regulated two-step actomyosin power stroke. At the tissue level, new insight into acutely dynamic stress-strain behavior reveals smooth muscle to exhibit adaptive plasticity. At its core, physiology aims to describe the complex interactions of molecular systems, clarifying structure-function relationships and regulation of biological machines. The intent of this review is to provide a comprehensive presentation of one biomachine, VSM.

A PLCγ1-dependent, force-sensitive signaling network in the myogenic constriction of cerebral arteries

Maintaining constant blood flow in the face of fluctuations in blood pressure is a critical autoregulatory feature of cerebral arteries. An increase in pressure within the artery lumen causes the vessel to constrict through depolarization and contraction of the encircling smooth muscle cells. This pressure-sensing mechanism involves activation of two types of transient receptor potential (TRP) channels: TRPC6 and TRPM4. We provide evidence that the activation of the γ1 isoform of phospholipase C (PLCγ1) is critical for pressure sensing in cerebral arteries. Inositol 1,4,5-trisphosphate (IP3), generated by PLCγ1 in response to pressure, sensitized IP3 receptors (IP3Rs) to Ca(2+) influx mediated by the mechanosensitive TRPC6 channel, synergistically increasing IP3R-mediated Ca(2+) release to activate TRPM4 currents, leading to smooth muscle depolarization and constriction of isolated cerebral arteries. Proximity ligation assays demonstrated colocalization of PLCγ1 and TRPC6 with TRPM4, suggesting the presence of a force-sensitive, local signaling network comprising PLCγ1, TRPC6, TRPM4, and IP3Rs. Src tyrosine kinase activity was necessary for stretch-induced TRPM4 activation and myogenic constriction, consistent with the ability of Src to activate PLCγ isoforms. We conclude that contraction of cerebral artery smooth muscle cells requires the integration of pressure-sensing signaling pathways and their convergence on IP3Rs, which mediate localized Ca(2+)-dependent depolarization through the activation of TRPM4.

Integrative modeling of small artery structure and function uncovers critical parameters for diameter regulation

Organ perfusion is regulated by vasoactivity and structural adaptation of small arteries and arterioles. These resistance vessels are sensitive to pressure, flow and a range of vasoactive stimuli. Several strongly interacting control loops exist. As an example, the myogenic response to a change of pressure influences the endothelial shear stress, thereby altering the contribution of shear-dependent dilation to the vascular tone. In addition, acute responses change the stimulus for structural adaptation and vice versa. Such control loops are able to maintain resistance vessels in a functional and stable state, characterized by regulated wall stress, shear stress, matched active and passive biomechanics and presence of vascular reserve. In this modeling study, four adaptation processes are identified that together with biomechanical properties effectuate such integrated regulation: control of tone, smooth muscle cell length adaptation, eutrophic matrix rearrangement and trophic responses. Their combined action maintains arteries in their optimal state, ready to cope with new challenges, allowing continuous long-term vasoregulation. The exclusion of any of these processes results in a poorly regulated state and in some cases instability of vascular structure.

Role of endogenous ENaC and TRP channels in the myogenic response of rat posterior cerebral arteries

AIMS

Mechanogated ion channels are predicted to mediate pressure-induced myogenic vasoconstriction in small resistance arteries. Recent findings have indicated that transient receptor potential (TRP) channels and epithelial sodium channels (ENaC) are involved in mechanotransduction. The purpose of this study was to investigate the role of TRP channels and ENaC in the myogenic response. Our previous study suggested that ENaC could be a component of the mechanosensitive ion channels in rat posterior cerebral arteries (PCA). However, the specific ion channel proteins mediating myogenic constriction are unknown. Here we found, for the first time, that ENaC interacted with TRPM4 but not with TRPC6 using immunoprecipitation and confocal microscopy.

METHODS AND RESULTS

Treatment with a specific βENaC inhibitor, amiloride, a specific TRPM4 inhibitor, 9-phenanthrol, and a TRPC6 inhibitor, SKF96365, resulted in inhibition of the pressure-induced myogenic response. Moreover, the myogenic response was inhibited in rat PCA transfected with small interfering RNA of βENaC, TRPM4, and TRPC6. Co-treatment with amiloride and 9-phenanthrol showed a similar inhibitory effect on myogenic contraction compared to single treatment with amiloride or 9-phenanthrol. The myogenic response was not affected by 9-phenanthrol or amiloride treatment in PCA transfected with βENaC or TRPM4 siRNA, respectively. However, pressure-induced myogenic response was fully inhibited by co-treatment with amiloride, 9-phenanthrol, and SKF96365, and by treatment with SKF96365 in PCA transfected with βENaC siRNA.

CONCLUSION

Our results suggest that ENaC, TRPM4, and TRPC6 play important roles in the pressure-induced myogenic response, and that ENaC and TRPM4 interact in rat PCA.

An experimental system for the study of ultrasound exposure of isolated blood vessels

An experimental system designed for the study of the effects of diagnostic or therapeutic ultrasound exposure on isolated blood vessels in the presence or absence of intraluminal contrast agent is described. The system comprised several components. A microscope was used to monitor vessel size (and thus vessel functionality), and potential leakage of intraluminal 70 kDa FITC-dextran fluorescence marker. A vessel chamber allowed the mounting of an isolated vessel whilst maintaining its viability, with pressure regulation for the control of intraluminal pressure and induction of flow for the infusion of contrast microbubbles. A fibre-optic hydrophone sensor mounted on the vessel chamber using a micromanipulator allowed pre-exposure targeting of the vessel to within 150 µm, and monitoring of acoustic cavitation emissions during exposures. Acoustic cavitation was also detected using changes in the ultrasound drive voltage and by detection of audible emissions using a submerged microphone. The suitability of this system for studying effects in the isolated vessel model has been demonstrated using a pilot study of 6 sham exposed and 18 high intensity focused ultrasound exposed vessels, with or without intraluminal contrast agent (SonoVue) within the vessels.

Ca2+ sensitization due to myosin light chain phosphatase inhibition and cytoskeletal reorganization in the myogenic response of skeletal muscle resistance arteries

Abstract  The myogenic response of resistance arteries to intravascular pressure elevation is a fundamental physiological mechanism of crucial importance for blood pressure regulation and organ-specific control of blood flow. The importance of Ca(2+) entry via voltage-gated Ca(2+) channels leading to phosphorylation of the 20 kDa myosin regulatory light chains (LC20) in the myogenic response is well established. Recent studies, however, have suggested a role for Ca(2+) sensitization via activation of the RhoA/Rho-associated kinase (ROK) pathway in the myogenic response. The possibility that enhanced actin polymerization is also involved in myogenic vasoconstriction has been suggested. Here, we have used pressurized resistance arteries from rat gracilis and cremaster skeletal muscles to assess the contribution to myogenic constriction of Ca(2+) sensitization due to: (1) phosphorylation of the myosin targeting subunit of myosin light chain phosphatase (MYPT1) by ROK; (2) phosphorylation of the 17 kDa protein kinase C (PKC)-potentiated protein phosphatase 1 inhibitor protein (CPI-17) by PKC; and (3) dynamic reorganization of the actin cytoskeleton evoked by ROK and PKC. Arterial diameter, MYPT1, CPI-17 and LC20 phosphorylation, and G-actin content were determined at varied intraluminal pressures ± H1152, GF109203X or latrunculin B to suppress ROK, PKC and actin polymerization, respectively. The myogenic response was associated with an increase in MYPT1 and LC20 phosphorylation that was blocked by H1152. No change in phospho-CPI-17 content was detected although the PKC inhibitor, GF109203X, suppressed myogenic constriction. Basal LC20 phosphorylation at 10 mmHg was high at ∼40%, increased to a maximal level of ∼55% at 80 mmHg, and exhibited no additional change on further pressurization to 120 and 140 mmHg. Myogenic constriction at 80 mmHg was associated with a decline in G-actin content by ∼65% that was blocked by inhibition of ROK or PKC. Taken together, our findings indicate that two mechanisms of Ca(2+) sensitization (ROK-mediated phosphorylation of MYPT1-T855 with augmentation of LC20 phosphorylation, and a ROK- and PKC-evoked increase in actin polymerization) contribute to force generation in the myogenic response of skeletal muscle arterioles.

Mechanotransduction through fibronectin-integrin focal adhesion in microvascular smooth muscle cells: is calcium essential?

It is believed that increased transmural pressure exerts force on vascular smooth muscle cells (VSMCs) and triggers Ca(2+) signaling as an initiating event responsible for the arteriolar myogenic response. However, the mechanisms linking the pressure increase to Ca(2+) signaling are unclear. We have shown previously using atomic force microscopy (AFM) that mechanical force induces a VSMC contractile response when applied to single fibronectin (FN; Sun Z, Martinez-Lemus LA, Hill MA, Meininger GA. Am J Physiol Cell Physiol 295; C268-C278, 2008) focal adhesion sites. This current study seeks to determine whether application of force to single focal adhesions can cause a change in VSMC Ca(2+). Experiments were performed in low passage (p3∼10) as well as in freshly isolated skeletal muscle arteriole VSMCs. AFM-attached microbeads (5 μm) were coated with FN or collagen type I (CN-I) or type IV (CN-IV) and placed on a VSMC for 20 min, resulting in formation of a focal adhesion between the cell and the microbead. In low passage VSMCs, mechanically pulling on the FN-coated beads (800∼3000 pN) did not induce a Ca(2+) increase but did cause a contractile response. In freshly isolated VSMCs, application of an FN or CN-I-coated bead onto the cell surface induced global Ca(2+) increases. However, these Ca(2+) increases were not correlated with the application of AFM pulling force to the bead or with the VSMC contractile responses to FN-coupled pulling. Chelating cytosolic Ca(2+) using BAPTA loading had no negative effect on the focal adhesion-related contractile response in both freshly isolated and low passage VSMCs, while the Rho-kinase inhibitor Y27632 abolished the micromyogenic response in both cases. These observations suggest that, in freshly isolated and cultured VSMCs, application of mechanical force to a focal adhesion does not invoke an acute global Ca(2+) increase. On the other hand, our data support a role for Rho-linked signaling mechanism involved in mechanotransduction leading to focal contraction that is independent of the need for a global increase in VSMC Ca(2+).

Calcium homeostasis in vascular smooth muscle cells is altered in type 2 diabetes by Bcl-2 protein modulation of InsP3R calcium release channels

This study examines the extent to which the antiapoptotic Bcl-2 proteins Bcl-2 and Bcl-x(L) contribute to diabetic Ca(2+) dysregulation and vessel contractility in vascular smooth muscle cells (VSMCs) through their interaction with inositol 1,4,5-trisphosphate receptor (InsP(3)R) intracellular Ca(2+) release channels. Measurements of intracellular ([Ca(2+)](i)) and sarcoplasmic reticulum ([Ca(2+)](SR)) calcium concentrations were made in primary cells isolated from diabetic (db/db) and nondiabetic (db/m) mice. In addition, [Ca(2+)](i) and constriction were recorded simultaneously in isolated intact arteries. Protein expression levels of Bcl-x(L) but not Bcl-2 were elevated in VSMCs isolated from db/db compared with db/m age-matched controls. In single cells, InsP(3)-evoked [Ca(2+)](i) signaling was enhanced in VSMCs from db/db mice compared with db/m. This was attributed to alterations in the intrinsic properties of the InsP(3)R itself because there were no differences between db/db and db/m in the steady-state [Ca(2+)](SR) or InsP(3)R expression levels. Moreover, in permeabilized cells the rate of InsP(3)R-dependent SR Ca(2+) release was increased in db/db compared with db/m VSMCs. The enhanced InsP(3)-dependent SR Ca(2+) release was attenuated by the Bcl-2 protein inhibitor ABT-737 only in diabetic cells. Application of ABT-737 similarly attenuated enhanced agonist-induced [Ca(2+)](i) signaling only in intact aortic and mesenteric db/db vessels. In contrast, ABT-737 had no effect on agonist-evoked contractility in either db/db or db/m vessels. Taken together, the data suggest that in type 2 diabetes the mechanism for [Ca(2+)](i) dysregulation in VSMCs involves Bcl-2 protein-dependent increases in InsP(3)R excitability and that dysregulated [Ca(2+)](i) signaling does not appear to contribute to increased vessel reactivity.

Differential recruitment of mechanisms for myogenic responses according to luminal pressure and arterial types

Mechanosensitive nonselective cation channels (NSC(ms)), protein kinase C (PKC), and Rho kinase (ROCK) are suggested as underlying mechanisms for the myogenic contractile response (MR) to luminal pressure (P(lum)). Here we compared relative contributions from these mechanisms using pharmacological inhibitors in rabbit middle cerebral (RbCA), rat middle cerebral (RtCA), rat femoral (RtFA), and rat mesenteric (RtMA) small arteries. Inner diameters of pressurized arteries under various P(lum) were video-analyzed. 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS, 10 microM) was used as a blocker of NSC(ms). In general, RbCA and RtCA showed higher P(lum) sensitivity of MR than RtFA and RtMA. Ten micromolars of DIDS commonly decreased MRs more effectively at low P(lum) (40-60 mmHg) in all tested arteries except RtCA. In RbCA, PKC inhibitors (100 nM of Go6976 or Go6983) decreased the MR at relatively high P(lum) (80-100 mmHg) whereas ROCK inhibitor (Y-27632, 1 microM) showed a P(lum)-independent inhibition. In RtMA and RtCA, PKC inhibitors (Go6976 and Go6983) had no significant effect whereas Y-27632 generally inhibited the MR. In RtFA, neither PKC inhibitor nor Y-27632 alone affected MRs. Interestingly, in the presence of 10 microM DIDS, Go6983 and Y-27632 decreased the MR of RtFA. In RtMA, it was notable that the MR decreased spontaneously on repeated protocol of P(lum) increase, and the 'run-down' could be effective reversed by maxi-K(+) channel blocker (tetraethylammonium or iberiotoxin). In summary, our study shows the variability of MRs according to the arterial types in terms of their pressure sensitivity and underlying mechanisms that are recruited according to P(lum).

Altered whole kidney blood flow autoregulation in a mouse model of reduced beta-ENaC

Renal blood flow (RBF) autoregulation is mediated by at least two mechanisms, the fast acting myogenic response (approximately 5 s) and slow acting tubuloglomerular feedback (TGF; approximately 25 s). Previous studies suggest epithelial Na(+) channel (ENaC) family proteins, beta-ENaC in particular, mediate myogenic constriction in isolated renal interlobar arteries. However, it is unknown whether beta-ENaC-mediated myogenic constriction contributes to RBF autoregulation in vivo. Therefore, the goal of this investigation was to determine whether the myogenic mediated RBF autoregulation is inhibited in a mouse model of reduced beta-ENaC (m/m). To address this goal, we evaluated the temporal response of RBF and renal vascular resistance (RVR) to a 2-min step increase in mean arterial pressure (MAP). Pressure-induced changes in RBF and RVR at 0-5, 6-25, and 110-120 s after step increase in MAP were used to assess the contribution of myogenic and TGF mechanisms and steady-state autoregulation, respectively. The rate of the initial increase in RVR, attributed to the myogenic mechanism, was reduced by approximately 50% in m/m mice, indicating the speed of the myogenic response was inhibited. Steady-state autoregulation was similar between beta-ENaC +/+ and m/m mice. Although the rate of the secondary increase in RVR, attributed to TGF, was similar in beta-ENaC +/+ and m/m mice, however, it occurred over a longer period (+10 s), which may have allowed TGF to compensate for a loss in myogenic autoregulation. Our findings suggest beta-ENaC is an important mediator of renal myogenic constriction-mediated RBF autoregulation in vivo.

Rho-kinase inhibition reduces pressure-mediated autoregulatory adjustments in afferent arteriolar diameter

Preglomerular resistance is regulated by calcium influx- and mobilization-dependent mechanisms; however, the role of Rho-kinase in calcium sensitization in the intact kidney has not been carefully examined. Experiments were performed to test the hypothesis that Rho-kinase inhibition blunts pressure-mediated afferent arteriolar autoregulatory behavior and vasoconstrictor responses evoked by angiotensin II and P2X1 receptor activation. Rat kidneys were studied in vitro using the blood-perfused juxtamedullary nephron technique. Autoregulatory behavior was assessed before and during Rho-kinase inhibition with Y-27632 (1.0 microM; n = 5). Control diameter averaged 14.3 +/- 0.8 microm and increased to 18.1 +/- 0.9 microm (P < 0.05) during Y-27632 treatment. In the continued presence of Y-27632, reducing perfusion pressure to 65 mmHg slightly increased diameter to 18.7 +/- 1.0 microm. Subsequent pressure increases to 130 and 160 mmHg yielded afferent arteriolar diameters of 17.5 +/- 0.8 and 16.6 +/- 0.6 microm (P < 0.05). This 11% decline in diameter is significantly smaller than the 40% decrease obtained in untreated kidneys. The inhibitory effects of Y-27632 on autoregulatory behavior were concentration dependent. Angiotensin II responses were blunted by Y-27632. Angiotensin II (1.0 nM) reduced afferent diameter by 17 +/- 1% in untreated arterioles and by 6 +/- 2% during exposure to Y-27632. The P2X1 receptor agonist, alpha, beta-methylene ATP, reduced afferent arteriolar diameter by 8 +/- 1% but this response was eliminated during exposure to Y-27632. Western blot analysis confirms expression of the Rho-kinase signaling pathway. Thus, Rho-kinase may be important in pressure-mediated autoregulatory adjustments in preglomerular resistance and responsiveness to angiotensin II and autoregulatory P2X1 receptor agonists.

Vasomotion dynamics following calcium spiking depend on both cell signalling and limited constriction velocity in rat mesenteric small arteries

Vascular smooth muscle cell contraction depends on intracellular calcium. However, calcium-contraction coupling involves a complex array of intracellular processes. Quantitating the dynamical relation between calcium perturbations and resulting changes in tone may help identifying these processes. We hypothesized that in small arteries accurate quantitation can be achieved during rhythmic vasomotion, and questioned whether these dynamics depend on intracellular signalling or physical vasoconstriction. We studied calcium-constriction dynamics in cannulated and pressurized rat mesenteric small arteries (∼300 μm in diameter). Combined application of tetra-ethyl ammonium (TEA) and BayK8644 induced rhythmicity, consisting of regular and irregular calcium spiking and superposition of spikes. Calcium spikes induced delayed vasomotion cycles. Their dynamic relation could be fitted by a linear second-order model. The dirac impulse response of this model had an amplitude that was strongly reduced with increasing perfusion pressure between 17 and 98 mmHg, while time to peak and relaxation time were the largest at an intermediate pressure (57 mmHg: respectively 0.9 and 2.3 sec). To address to what extent these dynamics reside in intracellular signalling or vasoconstriction, we applied rhythmic increases in pressure counteracting the vasoconstriction. This revealed that calcium-activation coupling became faster when vasoconstriction was counteracted. During such compensation, a calcium impulse response remained that lasted 0.5 sec to peak activation, followed by a 1.0 sec relaxation time, attributable to signalling dynamics. In conclusion, this study demonstrates the feasibility of quantitating calcium-activation dynamics in vasomoting small arteries. These dynamics relate to both intracellular sig-nalling and actual vasoconstriction. Performing such analyses during pharmacological intervention and in genetic models provides a tool for unravelling calcium-contraction coupling in small arteries.

Chronic hypoxia induces Rho kinase-dependent myogenic tone in small pulmonary arteries

Myogenic tone in the pulmonary vasculature of normoxic adult animals is minimal or nonexistent. Whereas chronic hypoxia (CH) increases basal tone in pulmonary arteries, it is unclear if a portion of this elevated tone is due to development of myogenicity. Since basal arterial RhoA activity and Rho kinase (ROK) expression are augmented by CH, we hypothesized that CH elicits myogenic reactivity in pulmonary arteries through ROK-dependent vascular smooth muscle (VSM) Ca(2+) sensitization. To test this hypothesis, we assessed the contribution of ROK to basal tone and pressure-induced vasoconstriction in endothelium-disrupted pulmonary arteries [50-300 microm inner diameter (ID)] from control and CH [4 wk at 0.5 atmosphere (atm)] rats. Arteries were loaded with fura-2 AM to continuously monitor VSM intracellular Ca(2+) concentration ([Ca(2+)](i)). Basal VSM [Ca(2+)](i) was not different between groups. The ROK inhibitor, HA-1077 (100 nM to 30 microM), caused a concentration-dependent reduction of basal tone in CH arteries but had no effect in control vessels. In contrast, PKC inhibition with GF109203X (1 microM) did not alter basal tone. Furthermore, significant vasoconstriction in response to stepwise increases in intraluminal pressure (5-45 mmHg) was observed at 12, 15, 25, and 35 mmHg in arteries (50-200 microm ID) from CH rats. This myogenic reactivity was abolished by HA-1077 (10 microM) but not by GF109203X. VSM [Ca(2+)](i) was unaltered by HA-1077, GF109203X, or increases in pressure in either group. Myogenicity was not observed in larger vessels (200-300 microm ID). We conclude that CH induces myogenic tone in small pulmonary arteries through ROK-dependent myofilament Ca(2+) sensitization.

Characteristics of myogenic tone in the rat ophthalmic artery

The pressure-induced constriction in the rat ophthalmic artery was characterized. Ophthalmic arteries were isolated, cannulated in an arteriograph, and pressurized. Arteries developed 25% constriction at 70 mmHg of intraluminal pressure. Arteries maintained almost similar diameter over the range of pressures 50–210 mmHg, and forced dilatation was observed at pressures >210 mmHg. Denudation of endothelium increased the sensitivity of arteries to pressure-induced constriction, and significantly higher myogenic tone was observed in the pressure range of 10–100 mmHg. Indomethacin and cyclooxygenase-2 inhibition by SC-236 decreased myogenic tone, whereas cyclooxygenase-1 inhibition by SC-560 potentiated myogenic tone in a lower concentration range and decreased at a higher concentration. Pressure-induced constriction was completely blocked by 1 μM nifedipine. Phospholipase C inhibition by 6 μM U-73122 decreased myogenic tone by 39%, whereas PKC inhibitor GF-109203X (3 μM) had no effect. Constriction to phenylephrine was significantly decreased by U-73122 (1 μM) and GF-109203X (3 μM) at an intraluminal pressure of 10 mmHg. Rho-kinase inhibition by Y-27632 (30 μM) and HA-1077 (30 μM) decreased myogenic tone by 75% and 73%, respectively, and 1 μM Y-27632 significantly decreased myogenic tone developed in response to graded increases in pressure. These results suggest that rat ophthalmic artery has an efficient pressure-dependent autoregulatory function that is modulated by endothelium. Contribution of phospholipase C-activation to myogenic tone is minimal, whereas Rho-kinase activation plays a predominant role in the myogenic reactivity in this artery.

Myogenic vasoconstriction in mouse renal interlobar arteries: role of endogenous β and γENaC

Mechanosensitive ion channels are thought to initiate pressure-induced vasoconstriction, however, the molecular identity of these channels is unknown. Recent work from our laboratory suggests that members of the Degenerin/Epithelial Na+ Channel (DEG/ENaC) family may be components of the mechanosensitive ion channel complex in vascular smooth muscle (VSM); however, the specific DEG/ENaC proteins mediating myogenic constriction are unknown. The goal of this study is to determine if specific knockdown of β or γENaC, using dominant-negative (DN) or small-interference RNA (siRNA) molecules, inhibits pressure-induced vasoconstriction in mouse renal interlobar arteries. To address this goal, isolated arteries were transiently transfected with β or γENaC DN-cDNA or siRNA molecules. After 24 h, vessels were either 1) cannulated and pressurized for pressure-diameter response curves or 2) dissociated and immunolabeled to determine VSM cell endogenous ENaC protein expression. We found that transfection of βENaC DN-cDNA or siRNA suppresses β-, but not γENaC protein expression. Similarly, γENaC DN-cDNA or siRNA suppresses γ-, but not βENaC protein expression. In addition, transfection of β- or γENaC DN-cDNA or siRNA molecules inhibits pressure-induced vasoconstriction, but does not block agonist-induced vasoconstriction. Our results provide the first direct evidence that β and γENaC proteins are essential in mediating myogenic vasoconstriction in mouse renal interlobar arteries.

The effects of pH on beta-endorphin and morphine inhibition of calcium transients in dorsal root ganglion neurons

During inflammation, immune cells migrate into inflamed tissue and release opioid peptides that activate opioid receptors on peripheral sensory neurons to reduce pain. A characteristic of the inflamed environment in which these opioids act is acidic pH. Activation of opioid receptors leads to a decrease in the calcium component of neuronal action potentials. We investigated the hypothesis that inhibitory effects of opioids on intracellular calcium transients in dorsal root ganglion neuronal cultures are potentiated at acidic extracellular pH. Intracellular calcium responses to stimulation with capsaicin were measured in untreated neurons or after preincubation with beta-endorphin or morphine. beta-Endorphin significantly inhibited calcium responses to 300 nmol/L capsaicin at the lowest experimental extracellular pH (6.1, 6.5, and 7.2), whereas morphine inhibited capsaicin (300 nmol/L) responses significantly at pH 6.1 with a trend of inhibition at pH 6.5. The effect of pH on morphine inhibition of K+ -evoked calcium responses was also assessed. Morphine inhibition of calcium responses was significantly enhanced at pH 6.8 compared with pH 7.2 and pH 7.6. The inhibitory effects were reversed by naloxone, an opioid receptor antagonist. In conclusion, low extracellular pH potentiated beta-endorphin and morphine inhibition of calcium transients and might contribute to improved opioid efficacy during inflammation.

PERSPECTIVE

The results of the current study suggest that acidic pH might contribute to increased opioid efficacy in inflamed tissue. This highlights the therapeutic potential of endogenous opioid analgesia, whereby opioid peptides are delivered locally in inflamed tissues, as well as the use of locally applied opioids in inflammatory conditions.

Mechanisms of agonist-induced constriction in isolated human mesenteric arteries

We determined the calcium signalling pathways involved in the mechanisms of contraction of the vasoconstrictive agonists KCl, U46619 and PDBu in isolated human mesenteric arteries. The influence of gender, vessel diameter and age of the patients was also investigated. Human mesenteric arteries (n = 76) were loaded in a wire myograph and maintained at a tension equivalent to the in vivo pressure of 100 mm Hg, bubbled with 95%O2/5%CO2 to maintain pH 7.4 in physiological saline solution (PSS). Cumulative concentration-response curves were obtained to KCl (100 microM-100 mM), U46619 (1 nM-1 microM) or PDBu (1 nM-1 microM), before or after a 30 min incubation with either the voltage-gated calcium channel (VGCC) blocker nifedipine (10 microM), the store-operated calcium channel (SOCC) blocker SK&F96365 (50 microM) or in calcium-free PSS (-Ca2+ PSS). The KCl response was abolished in -Ca2+ PSS and with nifedipine. The U46619 response was partially blocked in -Ca2+ PSS and with nifedipine and predominantly blocked by SK&F96365. Incubation in -Ca2+ PSS had no effect on the response to PDBu. Arteries from male patients responded significantly higher to KCl than arteries from female patients. This study demonstrates that KCl induces mesenteric vasoconstriction via activation of VGCCs, U46619 induces mesenteric vasoconstriction via activation of SOCCs, but also VGCCs and PDBu induce mesenteric vasoconstriction via a calcium-independent pathway.

Pregnancy attenuates uterine artery pressure-dependent vascular tone: role of PKC/ERK pathway

The mechanisms of adaptation of uterine artery vascular tone to pregnancy are not fully understood. The present study tested the hypothesis that pregnancy decreases the PKC-mediated Ca(2+) sensitivity of the contractile process and attenuates myogenic tone in resistance-sized uterine arteries. In pressurized uterine arteries from nonpregnant (NPUA) and near-term pregnant (PUA) sheep, we measured, simultaneously in the same tissue, vascular diameter and vessel wall intracellular Ca(2+) concentration ([Ca(2+)](i)) as a function of intraluminal pressure. In both NPUA and PUA, membrane depolarization with KCl caused a rapid increase in [Ca(2+)](i) and a decrease in diameter. A pressure increase from 20 to 100 mmHg resulted in a transient increase in diameter that was associated with an increase in [Ca(2+)](i), followed by myogenic contractions in the absence of further changes in [Ca(2+)](i). In addition, activation of PKC by phorbol 12,13-dibutyrate induced a decrease in diameter in the absence of changes in [Ca(2+)](i). Pressure-dependent myogenic responses were significantly decreased in PUA compared with NPUA. However, pressure-induced increases in [Ca(2+)](i) were not significantly different between PUA and NPUA. The ratio of changes in diameter to changes in [Ca(2+)](i) was significantly greater for pressure-induced contraction of NPUA than that of PUA. Inhibition of PKC by calphostin C significantly attenuated the pressure-induced vascular tone and eliminated the difference of myogenic responses between NPUA and PUA. In contrast, the MAPKK (MEK) inhibitor PD-098059 had no effect on NPUA but significantly enhanced myogenic responses of PUA. In the presence of PD-098059, there was no difference in pressure-induced myogenic responses between NPUA and PUA. The results suggest that pregnancy downregulates pressure-dependent myogenic tone of the uterine artery, which is partly due to increased MEK/ERK activity and decreased PKC signal pathway leading to a decrease in Ca(2+) sensitivity of myogenic mechanism in the uterine artery during pregnancy.

Modulation of the myogenic response by neurogenic influences in rat small arteries

The hypothesis that the amplitude of the myogenic response is modulated by factors released from nerve endings was tested in rat tail small arteries. A pressure myograph in conjunction with direct stimulation of nerve endings by electrical field stimulation (EFS) was used to determine rat small artery contractile reactions. Vessel pretreatment with 10(- 5) M phentolamine abolished EFS-induced reactions completely indicating that they are mediated mainly by an adrenoceptor agonist, probably noradrenaline. In the absence and presence of 10(- 5) M phentolamine, vessel diameter changes in the pressure range from 10 to 120 mmHg were not different. Vessel stimulation by (i) EFS, (ii) noradrenaline, (iii) selective stimulation of alpha1- and alpha2-receptors, (iv) serotonin, or (v) vasopressin significantly reduced the diameter change induced by stepping pressure from 10 to 40 mmHg compared to unstimulated, control vessels. Vessel diameter changes induced by stepping pressure from 40 to 80 and from 80 to 120 mmHg, however, were not different in vessels stimulated with EFS and noradrenaline compared to controls. In conclusion, these data show that factors released from unstimulated adrenergic nerve endings (i.e., not stimulated by EFS) are not involved in the myogenic response. In contrast, factors released upon stimulation of nerve endings can modulate the amplitude of the myogenic response, but only at low pressures. Thus, the pressure range for myogenic blood flow autoregulation is extended to lower pressures. Myogenic autoregulation of blood flow at physiological pressures is unaltered.

Acute and chronic effects of relaxin on vasoactivity, myogenic reactivity and compliance of the rat mesenteric arterial and venous vasculature

We investigated the effect of relaxin on vasodilation, myogenic reactivity, and compliance of small mesenteric arteries and veins. In acute experiments, small (second order) mesenteric arteries and veins from female rats were mounted in a pressure myograph, perfused intraluminally with relaxin, and exposed to incremental increases in intraluminal pressure (20-120 mm Hg for arteries, 2-12 mm Hg for veins). We expressed myogenic reactivity as the ratio of active to passive diameter at each pressure step. In chronic experiments, relaxin was administered to rats (4 microg/h) for 3 days prior to isolating the vessels and measuring myogenic reactivity. Arteries were more sensitive than were veins to the acute vasodilatory activity of relaxin (EC50: arteries=1.32+/-0.18x10(-8) M; veins=3.19+/-0.88x10(-8) M, P<0.05). Acute relaxin reduced myogenic reactivity of mesenteric arteries, but not veins. Chronic pretreatment with relaxin did alter the pressure/diameter relationship in Ca(2+)-containing medium, but this was due to increased passive compliance (control: 2.96+/-0.14 microm mm Hg(-1), n=5; relaxin: 3.72+/-0.16 microm mm Hg(-1), n=5) rather than to reduced myogenic reactivity. Chronic relaxin did not alter myogenic reactivity or compliance (control: 43.8+/-1.4 microm mm Hg(-1), n=5; relaxin: 46.1+/-2.3 microm mm Hg(-1), n=5) of veins. Thus, although relaxin reduces total peripheral resistance, it does not affect splanchnic venous capacitance or tone. In the face of elevated plasma relaxin levels, such as during pregnancy, cardiac preload may thus be maintained, concurrent with a reduction in cardiac afterload and blood pressure. We caution that, if an experimental treatment alters compliance, myogenic reactivity must be expressed as the ratio of active:passive diameter.

Relative contribution of Rho kinase and protein kinase C to myogenic tone in rat cerebral arteries in hypertension

Arterial smooth muscle constriction in response to pressure, i.e., myogenic tone, may involve calcium-dependent and calcium-sensitization mechanisms. Calcium sensitization in vascular smooth muscle is regulated by kinases such as PKC and Rho kinase, and activity of these kinases is known to be altered in cardiovascular disorders. In the present study, we evaluated the relative contribution of PKC and Rho kinase to myogenic tone in cerebral arteries in hypertension. Myogenic tone and arterial wall calcium in Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) were measured simultaneously, and the effect of PKC and Rho kinase inhibitors on myogenic tone was evaluated. SHR arteries showed significantly greater myogenic tone than WKY arteries. Pressure/wall tension-arterial wall calcium curves showed a hyperbolic relation in WKY rats, but the curves for SHR arteries were parabolic. Myogenic tone was decreased by the Rho kinase inhibitors Y-27632 and HA-1077, with a significantly greater effect in SHR than in WKY arteries. Reduction in myogenic tone produced by the PKC inhibitor bisindolylmaleimide I in WKY and SHR arteries was significantly less than that produced by Rho kinase inhibition. The pressure-dependent increase in myogenic tone was significantly decreased by Y-27632, and the decrease was markedly greater than that produced by bisindolylmaleimide I in SHR arteries. In WKY arteries, the pressure-dependent increase in myogenic tone was decreased to a similar extent by Y-27632 and bisindolylmaleimide I. These results suggest greater myogenic tone with increased calcium sensitization in SHR arteries, largely because of Rho kinase activation, with a minor contribution of PKC activation.

Disruption of smooth muscle gap junctions attenuates myogenic vasoconstriction of mesenteric resistance arteries

Communication between vascular smooth muscle (VSM) cells via low-resistance gap junctions may facilitate vascular function by synchronizing the contractile state of individual cells within the vessel wall. We hypothesized that inhibition of gap junctional communication would impair constrictor responses of mesenteric resistance arteries. Immunohistochemical experiments revealed positive staining for connexin 37 (Cx37) in both endothelium and smooth muscle of rat mesenteric arterioles, whereas connexin 43 (Cx43) immunoreactivity was not detected in the mesenteric vasculature. Administration of the gap junction inhibitory peptide Gap27, which targets Cx37 and Cx43, significantly diminished myogenic vasoconstriction (8.6 +/- 3.8% of passive diameter at 100 Torr) and changes in vessel wall intracellular [Ca2+] of mesenteric resistance arteries compared with vessels treated with either vehicle (physiological saline solution) (33.5 +/- 6.1%) or a control peptide (32.1 +/- 6.5%). Administration of 18alpha-glycyrrhetinic acid, structurally distinct from Gap27, also significantly attenuated myogenic constriction compared with its vehicle control (DMSO) (9.6 +/- 3.2% vs. 23.8 +/- 4.6%). In contrast, phenylephrine-induced vasoconstriction was not altered by gap junction blockers. Attenuated myogenic vasoconstriction resulting from inhibition of gap junctions persisted after disruption of the endothelium. In additional experiments, VSM cell membrane potential was recorded in mesenteric resistance arteries pressurized to 20 or 100 Torr. VSM membrane potential was depolarized at 100 Torr compared with 20 Torr. However, VSM cells in arteries treated with Gap27 were significantly hyperpolarized (-48.6 +/- 1.4 mV) at the higher pressure compared with vehicle (-41.4 +/- 1.5 mV) and Gap20-treated (-38.4 +/- 0.7 mV) vessels. Our findings suggest that inhibition of smooth muscle gap junctions attenuates pressure-induced VSM cell depolarization and myogenic vasoconstriction.

Ionic mechanisms mediating the myogenic response in newborn porcine cerebral arteries

Mechanisms that underlie autoregulation in the newborn vasculature are unclear. Here we tested the hypothesis that in newborn porcine cerebral arteries intravascular pressure elevates wall tension, leading to an increase in intracellular calcium concentration ([Ca2+]i) and a constriction that is opposed by pressure-induced K+channel activation. Incremental step (20 mmHg) elevations in intravascular pressure between 10 and 90 mmHg induced an immediate transient elevation in arterial wall [Ca2+]iand a short-lived constriction that was followed by a smaller steady-state [Ca2+]ielevation and sustained constriction. Pressures between 10 and 90 mmHg increased steady-state arterial wall [Ca2+]ibetween ∼142 and 299 nM and myogenic (defined as passive-active) tension between 25 and 437 dyn/cm. The relationship between pressure and myogenic tension was strongly Ca2+dependent until forced dilation. At low pressure, 60 mM K+induced a steady-state elevation in arterial wall [Ca2+]iand a constriction. Nimodipine, a voltage-dependent Ca2+channel blocker, and removal of extracellular Ca2+similarly dilated arteries at low or high pressures. 4-Aminopyridine, a voltage-dependent K+(Kv) channel blocker, induced significantly larger constrictions at high pressure, when compared with those at low pressure. Although selective Ca2+-activated K+(KCa) channel blockers and intracellular Ca2+release inhibitors induced only small constrictions at low and high pressures, a low concentration of caffeine (1 μM), a ryanodine-sensitive Ca2+release (RyR) channel activator, increased KCachannel activity and induced dilation. These data suggest that in newborn cerebral arteries, intravascular pressure elevates wall tension, leading to voltage-dependent Ca2+channel activation, an increase in wall [Ca2+]iand Ca2+-dependent constriction. In addition, pressure strongly activates Kvchannels that opposes constriction but only weakly activates KCachannels.

Different roles of ryanodine receptors and inositol (1,4,5)-trisphosphate receptors in adrenergically stimulated contractions of small arteries

The functions of ryanodine receptors (RyRs) and inositol (1,4,5)-trisphosphate receptors [Ins(1,4,5)P(3)Rs] in adrenergically activated contractions of pressurized rat mesenteric small arteries were investigated. Caffeine (20 mM) but not phenylephrine (PE; 10 microM) facilitated the depletion of smooth muscle sarcoplasmic reticulum (SR) Ca(2+) stores by ryanodine (40 microM). In ryanodine-treated SR-depleted arteries, 1) Ca(2+) sparks were absent, 2) low concentrations of PE failed to elicit either vasoconstriction or normal asynchronous propagating Ca(2+) waves, and 3) high [PE] induced abnormally slow oscillatory contractions (vasomotion) and synchronous Ca(2+) oscillations. In ryanodine-treated SR-depleted arteries denuded of endothelium, high [PE] induced steady contraction and steady elevation of intracellular [Ca(2+)]. In contrast, 2-aminoethyl diphenylborate (2-APB), a putative blocker of Ins(1,4,5)P(3)Rs, produced opposite effects to ryanodine: 1) Ca(2+) sparks were present; 2) Ca(2+) waves were absent; 3) caffeine-releasable Ca(2+) stores were intact; and 4) PE, even at high concentrations on endothelial-denuded arteries, failed to elicit contraction, asynchronous Ca(2+) waves, or synchronous Ca(2+) oscillations or maintained elevated [Ca(2+)]. We conclude that 1) Ins(1,4,5)P(3)Rs are essential for adrenergically induced asynchronous Ca(2+) waves and the associated steady vasoconstriction, 2) RyRs are not appreciably opened during adrenergic activation (because PE did not facilitate the development of the effects of ryanodine), and 3) Ins(1,4,5)P(3)Rs are not essential for Ca(2+) sparks. This provides an explanation of the fact that adrenergic stimulation decreases the frequency of Ca(2+) sparks (previously reported) while simultaneously increasing the frequency of asynchronous propagating Ca(2+) waves; different SR Ca(2+)-release channels are involved.

Remodeling of resistance arteries in organoid culture is modulated by pressure and pressure pulsation and depends on vasomotion

The hypothesis was tested that pressure and pressure pulsation modulate vascular remodeling. Arterioles (∼200 μm lumen diameter) were dissected from rat cremaster muscle and studied in organoid culture. In the first series, arterioles were kept at a stable pressure level of either 50 or 100 mmHg for 3 days. Both groups showed a progressive increase in myogenic tone during the experiment. Arterioles kept at 50 mmHg showed larger endothelium-dependent dilation, compared with vessels kept at 100 mmHg on day 3. Remodeling, as indicated by the reduction in maximally dilated diameter at 100 mmHg, was larger in arterioles kept at 50 mmHg compared with 100 mmHg: 34 ± 4.5 versus 10 ± 4.8 μm ( P < 0.05). In the second series, arterioles were subjected to a stable pressure of 60 mmHg or oscillating pressure of 60 ± 10 mmHg (1.5 Hz) for 4 days. Pressure pulsation induced partial dilation and was associated with less remodeling: 34 ± 4.0 versus 19 ± 4.5 μm ( P < 0.01) for stable pressure versus oscillating pressure. Vasomotion was frequently observed in all groups, and inward remodeling was larger in vessels with vasomotion: 30 ± 2.5 μm compared with vessels that did not exhibit vasomotion: 8.0 ± 5.0 μm ( P < 0.01). In conclusion, these results indicate that remodeling is not enhanced by high pressure. Pressure pulsation causes partial dilation and reduces inward remodeling. The appearance of vasomotion is associated with enhanced inward remodeling.

Alpha1-adrenergic signaling mechanisms in contraction of resistance arteries

Our goal in this review is to provide a comprehensive, integrated view of the numerous signaling pathways that are activated by alpha(1)-adrenoceptors and control actin-myosin interactions (i.e., crossbridge cycling and force generation) in mammalian arterial smooth muscle. These signaling pathways may be categorized broadly as leading either to thick (myosin) filament regulation or to thin (actin) filament regulation. Thick filament regulation encompasses both "Ca(2+) activation" and "Ca(2+)-sensitization" as it involves both activation of myosin light chain kinase (MLCK) by Ca(2+)-calmodulin and regulation of myosin light chain phosphatase (MLCP) activity. With respect to Ca(2+) activation, adrenergically induced Ca(2+) transients in individual smooth muscle cells of intact arteries are now being shown by high resolution imaging to be sarcoplasmic reticulum-dependent asynchronous propagating Ca(2+) waves. These waves differ from the spatially uniform increases in [Ca(2+)] previously assumed. Similarly, imaging during adrenergic activation has revealed the dynamic translocation, to membranes and other subcellular sites, of protein kinases (e.g., Ca(2+)-activated protein kinases, PKCs) that are involved in regulation of MLCP and thus in "Ca(2+) sensitization" of contraction. Thin filament regulation includes the possible disinhibition of actin-myosin interactions by phosphorylation of CaD, possibly by mitogen-activated protein (MAP) kinases that are also translocated during adrenergic activation. An hypothesis for the mechanisms of adrenergic activation of small arteries is advanced. This involves asynchronous Ca(2+) waves in individual SMC, synchronous Ca(2+) oscillations (at high levels of adrenergic activation), Ca(2+) sparks, "Ca(2+)-sensitization" by PKC and Rho-associated kinase (ROK), and thin filament mechanisms.

Nitric Oxide and Prostaglandins Modulate Pressure-Induced Myogenic Responses of Intramural Coronary Arterioles

The myogenic response, an active constriction and dilation of vessels to changes in intravascular pressure, can play an important role in the regulation of coronary blood flow. The characteristics of the myogenic response and its modulation by endothelium-derived factors are organ and location specific and have not been studied extensively in intramural coronary arterioles. Thus, distal intramural branches (approximately 100 and approximately 170 microm active and passive diameter, respectively) of the left anterior descending coronary artery of rats were isolated and cannulated. Step increases in intraluminal pressure from 0 to 40 mm Hg elicited increases in diameter, whereas further increases in pressure from 50 to 150 mm Hg resulted in constrictions. In control, the pressure-induced myogenic tone of coronary arterioles was 67.3 +/- 2.7% of passive diameter (PD, obtained in Ca2+-free solution) at 60 mm Hg. Nomega-nitro-L-arginine (L-NNA, 10(-5) M), an inhibitor of nitric oxide synthase, reduced the initial arteriolar diameter (by 44.8 +/- 5.1 microm at 2 mm Hg, P < 0.05) and significantly mitigated increases in diameter to lower pressures and constrictions to higher pressures (41.1 +/- 5.6% of PD at 60 mm Hg). Administration of adenosine restored the initial diameter in the presence of l-NNA, but the increase in diameter to lower pressures and the decrease in diameter to higher pressures observed under control conditions remained greatly inhibited. Inhibition of prostaglandin synthesis, or PGH2/TxA2 receptors significantly reduced the constrictions to higher pressures as compared with control (indomethacin: from 57.9 +/- 4.8% of PD to 67.0 +/- 4.7% of PD at 150 mm Hg). Thus, because in isolated intramural coronary arterioles of rats a negative slope for the pressure-diameter curve develops only in the presence of nitric oxide and constrictor prostaglandins, they seem to be essential for the normal development of the myogenic response.

Modulation of Intrinsic Cavernous Tone and Nitric Oxide Production by Arginase in Rabbit Corpus Cavernosum

PURPOSE

Arginase shares the common substrate L-arginine with nitric oxide synthase (NOS). We investigated the roles of NOS and arginase for modulating intrinsic and vasoconstrictor tone in rabbit corpus cavernosum (RCC).

MATERIALS AND METHODS

Isolated RCC tissues were used for isometric tension experiments, and NOS and arginase activities. The endothelium lining RCC lacunar spaces was disrupted and/or removed by saponin treatment.

RESULTS

Following stretch of approximately 1gm NG-nitro-L-arginine methyl ester (L-NAME) as a NOS inhibitor caused endothelium dependent contraction, while the potent and specific arginase inhibitor N omega-hydroxy-nor-L-arginine (nor-NOHA) caused endothelium dependent relaxation. Relaxation with nor-NOHA was reversed by L-NAME. In the presence of 10 mM L-arginine 0.1 mM nor-NOHA was ineffective. Pretreatment with 0.1 mM L-NAME and 0.1 mM nor-NOHA did not significantly affect the vasoconstrictor response to phenylephrine. The magnitude of contraction with 0.1 mM L-NAME and relaxation with 0.1 mM nor-NOHA during contraction induced by phenylephrine were not significantly different from changes with L-NAME and nor-NOHA under intrinsic basal tone. In the enzymatic study NOS and arginase were detectable in cavernous homogenates. Nor-NOHA inhibited arginase but not NOS activity.

CONCLUSIONS

Results indicate that basal nitric oxide production from the endothelium regulates intrinsic cavernous tone and endogenous arginase activity in the endothelium modulates tone by inhibiting nitric oxide production, presumably through competition with constitutive NOS for the common substrate L-arginine.

Elasticity of passive blood vessels: a new concept

The mechanical properties of passive blood vessels are generally thought to depend on the parallel arrangement of elastin and collagen with linear elasticity and collagen recruitment depending on vessel strain [hook-on (HO) model]. We evaluated an alternative model [serial element (SE) model] consisting of the series arrangement of an infinite number of elements, each containing elastin with a constant elastic modulus and collagen that switches stepwise from slack (zero stress) to fully rigid (infinite stiffness) on ongoing element strain. Both models were implemented with Weibull distributions for collagen recruitment strain (HO model) and collagen tightening strain (SE model). The models were tested in experiments on rat mesenteric small arteries. Strain-tension relations were obtained before and after two rounds of digestion by collagenase. Both models fitted the data prior to digestion. However, for the HO model, this required unrealistically low estimates for collagen recruitment or elastic modulus and unrealistically high estimates for distension of collagen fibers. Furthermore, the data after digestion were far better predicted by the SE model compared with the HO model. Finally, the SE model required one parameter less (collagen elastic modulus). Therefore, the SE model provides a valuable starting point for the understanding of vascular mechanics and remodeling of vessels.

Recruitment of smooth muscle cells and arterial vasomotion

Investigating the recruitment and synchronization of smooth muscle cells (SMCs) is the key to understanding the physical mechanisms leading to contraction and spontaneous diameter oscillations of arteries, called vasomotion. We improved a method that allows the correlation of calcium oscillations (flashing) of individual SMCs with mean calcium variations and arterial contraction using confocal microscopy. Endothelium-stripped rat mesenteric arteries were cut open, loaded with dual calcium fluorescence probes, and stimulated by increasing concentrations of the vasoconstrictors phenylephrine (PE) and KCl. We found that the number and synchronization of flashing cells depends on vasoconstrictor concentration. At low vasoconstrictor concentration, few cells flash asynchronously and no local contraction is detected. At medium concentration, recruitment of cells is complete and synchronous, leading to strip contraction after KCl stimulation and to vasomotion after PE stimulation. High concentration of PE leads to synchronous calcium oscillations and fully contracted vessels, whereas high concentration of KCl leads to a sustained nonoscillating increase of calcium and to fully contracted vessels. We conclude that the number of simultaneously recruited cells is an important factor in controlling rat mesenteric artery contraction and vasomotion.

Myogenic responses and compliance of mesenteric and splenic vasculature in the rat

In the rat, the spleen is a major site of fluid efflux out of the blood. By contrast, the mesenteric vasculature serves as a blood reservoir. We proposed that the compliance and myogenic responses of these vascular beds would reflect their different functional demands. Mesenteric and splenic arterioles ( approximately 150-200 microm) and venules (<250 microm) from rats anesthetized with pentobarbital sodium were mounted in a pressurized myograph. Mesenteric arterial diameter decreased from 146 +/- 6 to 133 +/- 6 microm on raising intraluminal pressures from 80 to 120 mmHg. This response was enhanced in the presence of N(omega)-nitro-l-arginine methyl ester (l-NAME; 139 +/- 6 to 112 +/- 7 microm). There was no such myogenic response in the splenic arterioles, except in the presence of l-NAME (194 +/- 4 to 164 +/- 4.2 microm). We propose that, whereas mesenteric arterioles exhibit myogenic responses, this is normally masked by NO-mediated dilation in the splenic vessels. The mesenteric venules were highly distensible (active, 184 +/- 15 to 320 +/- 30.9 microm; passive in Ca(2+)-free media, 209 +/- 31 to 344 +/- 27 microm; 4-8 mmHg) compared with the splenic vessels (active, 169 +/- 11 to 184 +/- 16 microm; passive, 187 +/- 12 to 207 +/- 17 microm). We conclude that, in response to an increase in perfusion pressure, mesenteric arterial diameter would decrease to limit the changes in flow and microvascular pressure. In addition, mesenteric venous capacitance would increase. By contrast, splenic arterial diameter would increase, while there would be little change in venous diameter. This would enhance the increase in intrasplenic microvascular pressure and increase fluid extravasation.

Rho kinase inhibition partly weakens myogenic reactivity in rat small arteries by changing calcium sensitivity

The hypothesis that Rho kinase is involved in myogenic reactivity was investigated in pressurized rat tail small arteries using videomicroscopic diameter determination and calcium fluorimetry. The potent Rho kinase inhibitor Y-27632 reversibly increased vessel diameter at 80 mmHg without changing the intracellular calcium concentration ([Ca](i)) shifting the relationship between diameter change and [Ca](i) to higher calcium levels. Neither endothelium removal nor inhibition of neural transmission affected the Y-27632-induced effect. Y-27632 at 3 x 10(-6) mol/l attenuated the myogenic response in the pressure range from 10 to 120 mmHg, shifting the relationship between vessel tone and [Ca](i) to higher calcium levels. In addition, the Y-27632-induced shift of the relationship between vessel tone and [Ca](i) was larger at 80 than at 10 mmHg. These results suggest that smooth muscle cell Rho kinase in rat tail small arteries 1) is in an active state partly determining the level of the myogenic tone, and 2) alters the strength of the myogenic response by changing calcium sensitivity, probably caused by the pressure-induced activation of the kinase.

Pressure-dependent myogenic constriction of cerebral arteries occurs independently of voltage-dependent activation

Pressure-induced decreases in arterial diameter are accompanied by membrane depolarization and Ca(2+) entry via voltage-gated Ca(2+) channels. Recent evidence also suggests the involvement of Ca(2+) sensitization of the contractile proteins. Both PKC and Rho kinase are candidate second messengers for the mediation of the sensitization process. We investigated the signaling pathways of pressure-induced decreases in rat cerebral artery diameter in vessels that were depolarized with a 60 mM potassium-physiological salt solution (KPSS). Arteries were mounted on a pressure myograph, and pressure-induced constrictions were recorded. In some experiments simultaneous changes in intracellular Ca(2+) concentration ([Ca(2+)](i)) were recorded by using fura 2 fluorescence photometry. Pressure increases induced constriction with significant changes in [Ca(2+)](i) at high pressures (60-100 mmHg). The ratio of the change in diameter to change in [Ca(2+)](i) was greater for pressure-induced constriction compared with constriction produced by depolarization with 60 mM KPSS, suggesting that in addition to increases in [Ca(2+)](i), enhanced myofilament Ca(2+) sensitivity occurs during pressure-induced decreases in arterial diameter. Depolarizing the membrane with 60 mM KPSS increased [Ca(2+)](i) via a Ca(2+) influx pathway insensitive to PKC inhibition. Cerebral arteries were able to maintain their diameters in the continued presence of 60 mM KPSS. Pressure-induced constriction under these conditions was not associated with further increases in Ca(2+) but was abolished by selective inhibitors of PLC, PKC, and Rho kinase. We report for the first time that in rat cerebral arteries, pressure-induced decreases in arterial diameter are not only due to increases in voltage-gated Ca(2+) influx but also to accompanying increases in myofilament sensitivity to Ca(2+) mediated by PKC/Rho kinase activation.

Effects of longitudinal stretch on VSM tone and distensibility of muscular conduit arteries

With progressing age, large arteries diminish their longitudinal stretch, which in extreme cases results in tortuosity. Increased age is also associated with loss of vessel distensibility. We measured pressure-diameter curves from muscular porcine carotid arteries ex vivo at different longitudinal stretch ratios (lambda(z) = 1.4 and 1.8) and under different vascular smooth muscle (VSM) conditions (fully relaxed, normal VSM tone, and maximally contracted). Distensibility was found to be halved by decreasing longitudinal stretch from lambda(z) = 1.8 to 1.4 at physiological pressures. This counterintuitive observation is possible because highly nonlinear elastic modulus of the artery and anisotropic properties. Furthermore, a significantly larger basal VSM contraction was observed at lambda(z) = 1.8 than 1.4, although this was clearly not related to a myogenic response during inflation. This dependence of VSM tone to longitudinal stretch may have possible implications on the functional characteristics of the arterial wall.

Role of T-type calcium channels in myogenic tone of skeletal muscle resistance arteries

T-type calcium channels may be involved in the maintenance of myogenic tone. We tested their role in isolated rat cremaster arterioles obtained after CO(2) anesthesia and decapitation. Total RNA was analyzed by RT-PCR and Southern blotting for calcium channel expression. We observed expression of voltage-operated calcium (Ca(V)) channels Ca(V)3.1 (T-type), Ca(V)3.2 (T-type), and Ca(V)1.2 (L-type) in cremaster arterioles (n = 3 rats). Amplification products were observed only in the presence of reverse transcriptase and cDNA. Concentration-response curves of the relatively specific L-type blocker verapamil and the relatively specific T-type blockers mibefradil and nickel were made on cannulated vessels with either myogenic tone (75 mmHg) or a similar level of constriction induced by 30 mM K(+) at 35 mmHg. Mibefradil and nickel were, respectively, 162-fold and 300-fold more potent in inhibiting myogenic tone compared with K(+)-induced constriction [log(IC(50), M): mibefradil, basal -7.3 +/- 0.2 (n = 9) and K(+) -5.1 +/- 0.1 (n = 5); nickel, basal -4.1 +/- 0.2 (n = 5) and K(+) -1.6 +/- 0.5 (n = 5); means +/- SE]. Verapamil had a 17-fold more potent effect [log(IC(50), M): basal -6.6 +/- 0.1 (n = 5); K(+) -5.4 +/- 0.3 (n = 4); all log(IC(50)) P < 0.05, basal vs. K(+)]. These data suggest that T-type calcium channels are expressed and involved in maintenance of myogenic tone in rat cremaster muscle arterioles.

Direct effects of acute hypoxia on the reactivity of peripheral arteries of the chicken embryo

In the chicken embryo, acute hypoxemia results in cardiovascular responses, including an increased peripheral resistance. We investigated whether local direct effects of decreased oxygen tension might participate in the arterial response to hypoxemia in the chicken embryo. Femoral arteries of chicken embryos were isolated at 0.9 of incubation time, and the effects of acute hypoxia on contraction and relaxation were determined in vitro. While hypoxia reduced contraction induced by high K(+) to a small extent (-21.8 +/- 5.7%), contractile responses to exogenous norepinephrine (NE) were markedly reduced (-51.1 +/- 3.2%) in 80% of the arterial segments. This effect of hypoxia was not altered by removal of the endothelium, inhibition of NO synthase or cyclooxygenase, or by depolarization plus Ca(2+) channel blockade. When arteries were simultaneously exposed to NE and ACh, hypoxia resulted in contraction (+49.8 +/- 9.3%). Also, relaxing responses to ACh were abolished during acute hypoxia, while the vessels became more sensitive to the relaxing effect of the NO donor sodium nitroprusside (pD(2): 5.81 +/- 0.21 vs. 5.31 +/- 0.27). Thus, in chicken embryo femoral arteries, acute hypoxia blunts agonist-induced contraction of the smooth muscle and inhibits stimulated endothelium-derived relaxation factor release. The consequences of this for in vivo fetal hemodynamics during acute hypoxemia depend on the balance between vasomotor influences of circulating catecholamines and those of the endothelium.

Myogenic response of rat femoral small arteries in relation to wall structure and [Ca(2+)](i)

The present study investigated the influence of media thickness on myogenic tone and intracellular calcium concentration ([Ca(2+)](i)) in rat skeletal muscle small arteries. A ligature was loosely tied around one external iliac artery of 5-wk-old spontaneously hypertensive rats. At 18 wk of age, femoral artery blood pressure was 102 +/- 11 mmHg (n = 15) on the ligated side and 164 +/- 6 mmHg (n = 15) on the contralateral side. Small arteries feeding the gracilis muscle had a reduced media cross-sectional area and a reduced media-to-lumen ratio on the ligated side, where also the range of myogenic constriction was shifted to lower pressures. However, when expressed as a function of wall stress, diameter responses were nearly identical. [Ca(2+)](i) was higher in vessels from the ligated hindlimb at pressures above 10 mmHg, but vasoconstriction was not accompanied by changes in [Ca(2+)](i). Thus the myogenic constriction here seems due primarily to changes in intracellular calcium sensitivity, which are determined mainly by the force per cross-sectional area of the wall and therefore altered by changes in vascular structure.

Force-velocity relationship of myogenically active arterioles

We compared the shortening velocity of smooth muscle in arterioles that had low or high levels of myogenic tone or norepinephrine (NE)-induced tone. We hypothesized that enhanced myogenic tone of arterioles reflects an enhanced maximum velocity of shortening of arteriolar smooth muscle in a way that is different from that produced by NE. These concepts are untested assumptions of arteriolar mechanics. Second-order arterioles from hamster cheek pouch (passive diameter at 40 mmHg = 42 microm) were isolated and cannulated for in vitro study. In the absence of flow, pressure was controlled by hydraulic pumps so that servo control of wall tension could be achieved from measurement of internal diameter and pressure. Isotonic quick-release protocols were used to measure the initial velocity of shortening following release from control wall tension (afterload) to a series of fractional afterloads. After release, the initial rates of shortening were fit to the Hill equation to obtain coefficients for a hyperbolic fit of the velocity-afterload relationship. The maximal unloaded shortening velocity for partially activated arterioles (V'(max)) was determined from the y-intercept of each plot. Using this procedure, we compared V'(max) from two groups of arterioles equilibrated at low or high pressure, i.e., with low or high myogenic tone. Arterioles with higher myogenic tone had higher values of V'(max) than arterioles with lower myogenic tone. V'(max) for arterioles partially activated with NE at low pressure was comparable to V'(max) for arterioles with high myogenic tone, but NE produced high velocities at low force, whereas enhanced myogenic tone produced roughly parallel shifts in velocity and force. The results suggest that increased myogenic tone does indeed reflect enhanced activation of arteriolar smooth muscle, and this effect is mechanically different from that produced by NE.