INTRODUCTION

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ARTERIOLES TYPICALLY EXHIBIT a state of partial contraction, or myogenic tone, that is dependent on the level of intraluminal pressure. An increase in pressure leads to vasoconstriction, whereas a decrease in pressure leads to vasodilation. The physiological significance of this vasomotor response relates to its participation in local blood flow autoregulation, setting of basal peripheral vascular resistance, and regulation of capillary hydrostatic pressure (16, 44). Importantly, myogenic vasoconstriction provides a level of basal tone that enables arterioles to respond to neurohumoral stimuli with appropriate changes in vessel diameter, particularly vasodilation.

Although the exact signal transduction pathways underlying myogenic tone remain uncertain, it is clear that the phenomenon resides within the vascular smooth muscle cell and that it can be modulated by a variety of endothelial and neurohumoral factors (16). Common with smooth muscle contractile responses in general (for review, see Refs. 38, 96), the underlying mechanism of contraction involves Ca²⁺/calmodulin-myosin light chain kinase-mediated phosphorylation of the regulatory light chains of myosin with subsequent interaction of actin and myosin (124, 125).

The first description of arterial vessels responding to mechanical stimuli is generally attributed to Bayliss in 1902 (5) who, using plethysmography, investigated volume changes in the dog hindlimb as an indicator of changes in blood flow. Importantly, his studies advanced the idea that the level of intravascular pressure, in part, determined vascular tone. The availability of whole organ techniques and later isolated vessel preparations allowed the characteristics of pressure-dependent vascular phenomena to be quantitatively documented. In addition, the increased usage of the isolated microvessel as a tool for study of vascular biology has permitted more in-depth investigation of the underlying mechanisms. More recently, the development of sophisticated imaging techniques, electrophysiological approaches, and refinement of biochemical and molecular methods have enabled studies to progress beyond the cellular level.

Although this review will focus on arteriolar mechanotransduction and Ca²⁺ signaling pathways, the reader is referred to other recent reviews covering other aspects of the arteriolar myogenic response (see Refs. 16, 18, 33, 76, 91).

	IMPORTANCE OF CA2+ IN THE MYOGENIC RESPONSE

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The requirement of an extracellular Ca²⁺ supply for the maintenance of arteriolar tone was first demonstrated by Uchida and Bohr (102). Using small skeletal muscle arteries, these authors demonstrated that their preparations exhibited a level of vascular smooth muscle tone, which was abolished by removing Ca²⁺ from the bathing solutions. This observation was extended to true resistance vessels by Duling and colleagues (24), who first adapted renal tubule perfusion methods to the study of isolated and cannulated arterioles. Subsequently, numerous studies of isolated arteriole preparations exhibiting spontaneous myogenic contraction have demonstrated that removal of Ca²⁺ from superfusate solutions results in both a loss of tone and passive responses to subsequent changes in intraluminal pressure. Combining the isolated arteriole preparation with fluorescent Ca²⁺-sensitive indicators (for example, fura 2) allowed such studies to be extended to demonstrate relationships between the extent of myogenic tone and a given level of intracellular Ca²⁺ (12, 64, 124, 125).

The realization that agonist-induced smooth muscle contractile mechanisms typically involve a release component from the sarcoplasmic reticulum (SR; for review, see Ref. 37) stimulated interest of the role of such Ca²⁺ sources in the arteriolar myogenic response. Furthermore, recent interest in the regulation of intracellular Ca²⁺ has begun to incorporate consideration of localized and temporal components of intracellular Ca²⁺ signaling in the myogenic response. These facets of arteriolar Ca²⁺ signaling will be discussed in subsequent sections.

	COUPLING OF THE MECHANICAL STIMULUS TO CA2+ MOBILIZATION

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Major deficiencies in our knowledge of myogenic signaling pathways include an understanding of exactly how an increase in intraluminal pressure is 1) sensed by the vessel wall and 2) coupled to events that increase smooth muscle intracellular Ca²⁺ and/or other cell signaling pathways, ultimately leading to contraction. Candidate mechanisms for mechanotransduction include the activation of mechanosensitive ion channels, activation of membrane-bound enzymes, and modulation of the cytoskeleton (16). Growing evidence suggests that mechanical forces may be transmitted from the extracellular matrix via cell surface receptors, such as integrins, to the cytoskeleton to regulate ion channels and other intracellular signaling pathways (18).

The possible contribution of integrins, a family of heterodimeric, membrane-spanning, glycoproteins that bind extracellular matrix proteins, to vascular smooth muscle mechanotransduction has recently been reviewed by Davis et al. (18). Consistent with a general role in mechanotransduction processes, integrin activation has been shown to affect second-messenger pathways, alter cytoskeletal characteristics, and influence cell motility (63, 90, 99, 108). Specifically relevant to the topic of this review, recent studies have shown that voltage-gated Ca²⁺ channels (VGCCs), which are critical to myogenic reactivity, are modulated by integrins. Binding of the _v3-integrin inhibits dihydropyridine-sensitive VGCCs, whereas binding of the ₅₁- and ₄₁-integrin increases Ca²⁺ current (117). Furthermore, peptides that specifically bind integrins [for example, analogs of the peptide sequence Arg-Gly-Asp (RGD)] cause a reduction in intracellular Ca²⁺ (12) and a loss of myogenic tone (66) in isolated arterioles. Peptides that contain the Leu-Asp-Val (LDV) sequence enhance tone and L-type Ca²⁺ current (106). Whether these demonstrated links between integrins and regulation of Ca²⁺ entry play a specific role in the mechanotransduction process associated with the myogenic response has yet to be demonstrated.

Interaction between the extracellular environment and intracellular signaling and contractile pathways could also involve specialized membrane and/or attachment regions such as dense bodies and caveolae. Dense bodies are known to provide points of attachment for both the cytoskeleton and actin filaments of the contractile apparatus (94). Caveolae have been shown to be enriched in molecules such as Rho A, protein kinase C, and Ca²⁺ regulatory molecules (13, 98), which are implicated in the regulation of smooth muscle contraction and implicated to be closely opposed to the underlying SR (67). The latter has fueled interest in the role of caveolae in local Ca²⁺ dynamics such as the generation of Ca²⁺ sparks (57). Given the mechanical nature of a pressure stimulus with possible direct effects on the plasma membrane and related structures, the role of dense bodies and caveolae in myogenic responsiveness should continue to be considered.

An additional possibility is that membrane deformation occurring during a mechanical stimulus directly affects the gating of ion channels (see also CA²⁺ sources and related ion channels). Several classes of ion channels present in vascular smooth muscle have been reported to exhibit stretch sensitivity (15, 21, 31, 111). A mechanical effect on the membrane could conceivably change the relationship between signaling molecules to either facilitate or inhibit transduction pathways. In this regard, it is of interest to note that fluid shear stress alters membrane fluidity (29, 30), which in turn is capable of modulating GTPase activity of G proteins (29). Thus one could conceive of a mechanotransduction system focused on direct modulation of membrane function.

A difficulty in understanding the coupling of mechanical stimuli to Ca²⁺ mobilization is that, in addition to a lack of knowledge regarding the specific "mechanosensor," the sensed variable, itself, in the myogenic response is uncertain. Possible candidates are stretch of vascular smooth muscle cells or an increase in wall tension (44). In the case of arteriolar smooth muscle, cell stretch (i.e., a simple length change) would seem an unlikely signal, since, during a steady-state myogenic contraction, overt smooth muscle cell length would be shorter than that observed before the pressure increase. Despite this, it cannot be discounted that a membrane or cytoskeletal element remains physically deformed despite the myogenic contraction or that contraction per se alters the relationship between the cell membrane and extracellular matrix activating additional processes. Furthermore, it is possible that vascular smooth muscle cells respond to multiple mechanical stimuli, including both stretch and changes in tension. In support of this, smooth muscle cells in the walls of cannulated arterioles show differing intracellular Ca²⁺ profiles depending on directionality of a stretch stimulus and the rate of application of a change in intraluminal pressure (36). Consequently, it is likely that there may be several underlying mechanosensitive mechanisms that are operative and contribute to the overall integrated myogenic response.

	CA2+ SOURCES AND RELATED ION CHANNELS

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Extracellular

Intracellular

Sarcoplasmic reticulum. The role of the release of Ca²⁺ from intracellular stores, in particular the SR, in arteriolar smooth muscle mechanotransduction remains relatively poorly defined. This in part relates to the difficulties associated with studying microvessels in the absence of extracellular Ca²⁺. Arterioles rapidly dilate toward their maximal passive diameter when superfused with solutions lacking Ca²⁺, particularly when a Ca²⁺ chelator such as EGTA is present. This changes the mechanical properties of the vessels relative to the state of spontaneous tone. Also, SR Ca²⁺ stores in arterioles tend to deplete with time under zero-Ca²⁺ extracellular conditions (39). In addition to difficulties with studying the functional properties of the SR in arterioles, comparatively little information exists with respect to either its structural properties or spatial arrangement within cells of microvessels. On the basis of functional studies, Low et al. (59) have, however, concluded that the role of the SR (relative to extracellular Ca²⁺ supply) in contraction diminishes as arterial diameter decreases.

Mitochondria. Although little is known regarding the role of other organelles in myogenic responsiveness, it is important to illustrate that Ca²⁺ handling could potentially be affected by sites other than the SR. In the case of mitochondria, this is particularly so when consideration is given to the potential for interaction between myogenic reactivity and the metabolic state of a tissue. In this regard, uptake of Ca²⁺ into mitochondria was first described several decades ago; however, only recently has its physiological relevance been considered. Thus Greenwood et al. (28) demonstrated that uptake of Ca²⁺ into mitochondria is involved in the regulation of Ca²⁺-activated Cl (Cl_Ca) channel current in rabbit portal vein. (see also Cl Channels below) More recently, in studies of isolated rat pulmonary artery smooth muscle cells, Drummond and Tuft (23), by simultaneously monitoring cytosolic and mitochondrial Ca²⁺ with the fluorescent indicators rhod 2 and fura 2, respectively, provided direct evidence that mitochondrial Ca²⁺ uptake is an important determinant of cytosolic Ca²⁺. Release of Ca²⁺ from SR stores by activation of either ryanodine or IP₃ receptors was found to increase both mitochondrial and cytosolic Ca²⁺. The increase in mitochondrial Ca²⁺ was, however, found to be delayed compared with the increase in cytosolic Ca²⁺. In contrast to that observed after acute SR Ca²⁺ release, mitochondrial Ca²⁺ uptake was not considered to be a major determinant for the setting of resting cytosolic Ca²⁺ (23).

Membrane Potential and Voltage-Gated Ca²+ Entry

A number of studies have provided evidence against a principal role of membrane depolarization and hence voltage-gated Ca²⁺ entry, in particular the observation that myogenic vasoconstriction persists in vessels depolarized with high extracellular K⁺ solutions. An analysis of such data (16), however, indicates that the sustained phase of a myogenic constriction as described by the myogenic index (77) is attenuated under depolarizing conditions, despite retention of acute myogenic reactivity. Such studies may also be complicated by indirect effects of the high K⁺ environment (121) and the need to substantially decrease extracellular Na⁺ to maintain isosmotic conditions. Furthermore, permeabilized vessel preparations, which by definition lack a membrane potential difference, do not appear to exhibit myogenic reactivity (62).

Stretch-Activated Channels and Cation Channels

In addition to direct activation of ion channels by stretch, it has been suggested that mechanical perturbation of cell membranes may release factors that modulate the activity of such channels. For example, it has been suggested that fatty acids released by cell stretch modify ion channel gating (75). This concept of mechanical stimulus-induced production of second messengers that secondarily modulate ion channels, for example activation of cation channels, can be extended to the generation of other molecules, including metabolites of arachidonic acid (e.g., cytochrome P-450 monooxygenase products), diacylglycerol, and cyclic nucleotides (33, 58). Activation of cation channels by diacylglycerol has been shown to occur following norepinephrine stimulation of portal vein myocytes (2), and, furthermore, a population of Trp channels has been demonstrated to be activated by this lipid (Ref. 58; see Store-Mediated Ca²⁺ Entry). Although differences in conductances have been reported (2, 101), the existence of any relationship between these cation channels is unknown. Conceivably, such a mechanism could provide a link between many of the messengers that have been implicated in myogenic reactivity and the gating of ions, including various mechanisms of Ca²⁺ entry from the extracellular space. Whether such a sequence of events would be consistent with the time course of myogenic constriction observed in all arterioles, however, is uncertain.

K+ Channels

Synchronized openings of a group of K_Ca channels can generate spontaneous transient outward currents (STOCs) (6). Studies from both visceral (123) and vascular (80) smooth muscle suggest a close association or functional coupling between sparks and STOCs with sparks causing STOCs. Furthermore, stretch-induced depolarization of single smooth muscle cells has been observed to secondarily increase STOC activity (111, 116). This is additional evidence that one or more K⁺ currents act to attenuate stretch-induced changes in membrane potential and myogenic constriction.

Other K⁺ channels [ATP-sensitive K⁺ channel (K_ATP), voltage-gated K⁺ channels, and inward rectifier K⁺ channels], although not necessarily related to intracellular Ca²⁺, are also reported to be involved in the setting of resting arteriolar tone (for review, see Refs. 41, 84). K_ATP channels do not appear to be directly involved in myogenic responses because the inhibitor glibenclamide often has little effect on myogenic tone (51).

Cl Channels

The finding that volume-regulated Cl channels are expressed in vascular smooth muscle (120) suggests a possible role for volume-regulated Cl channels in regulating myogenic tone (71, 120). However, a recent study by Welsh et al. (113) has injected additional controversy into this field. Although they confirmed the presence of a swelling-activated current being sensitive to Cl channel blockers such as DIDS and tamoxifen, subsequent experiments suggested the presence of a cation current as opposed to a Cl current. The reversal potential for the swelling-activated whole cell current did not change when bath Cl concentration was reduced but did shift accordingly when Na⁺ concentration was reduced. Moreover, Gd³⁺, which does not appear to block swelling-activated Cl channel current in vascular smooth muscle cells (120), blocked the current, further suggesting a cation-selective current. Therefore, the role for Cl channels in regulating myogenic tone requires further research for a definitive understanding.

Store-Mediated Ca²+ Entry

Although most studies of capacitative Ca²⁺ entry have been performed in nonexcitable tissues, there is convincing evidence that such a mechanism(s) exists in vascular smooth muscle (100, 101). These studies demonstrate a nonselective cation channel that is activated by Ca²⁺ store depletion and would be expected to favor membrane depolarization with Ca²⁺ entry subsequently occurring via voltage-gated channels. Few studies, however, have specifically examined functional capacitative Ca²⁺ entry in arteriolar smooth muscle (25), and reports examining cannulated arterioles are generally lacking.

The existence of capacitative Ca²⁺ entry channels in arterioles is supported by the demonstration of mRNA encoding for TrpC1 and antibody-based localization of the protein in isolated pial vessels (119). The difficulty in studying cannulated and myogenically active arterioles in part relates to the fact that vessels exhibiting spontaneous tone have a membrane potential favoring activation of L-type VGCCs; furthermore, this level of contraction is abolished by inhibitors of VGCCs. Despite this, the existence of capacitative Ca²⁺ entry in arterioles can be demonstrated by depletion of SR Ca²⁺ with the Ca²⁺-ATPase inhibitor thapsigargin followed by addition of the IP₃ receptor modulator 2-aminoethoxydiphenyl borate (2APB) (81). 2APB has previously been shown to block Ca²⁺ entry in response to agonists in a variety of nonexcitable cells, including embryonic kidney cells (60), endothelial cells (unpublished observations), and platelets (20) through modulation of IP₃ receptor-mediated processes, although this compound may also exert direct effects on the capacitative entry channels (11, 20) . To further demonstrate capacitative entry and ensure that the above did not result from an effect on Ca²⁺ channels, additional studies were performed after blockade of voltage-gated Ca²⁺ entry with nifedipine. Vessels were superfused with nifedipine in a 0 mM Ca²⁺ buffer after which Ca²⁺ was added to the superfusate in the continued presence of VGGC blockade. The resulting Ca²⁺ entry was again inhibited by 2APB, supporting the existence of a capacitative Ca²⁺ entry mechanism possibly linked to the IP₃ receptor and occurring independently of voltage-gated Ca²⁺ entry (81).

Although arterioles show the potential to exhibit capacitative Ca²⁺ entry, little evidence to date has been found to directly link the extent of capacitative Ca²⁺ entry with the level of intraluminal pressure. In preliminary studies with 2APB, Potocnik et al. (81) suggested that capacitative Ca²⁺ entry is involved in myogenic constriction following an acute pressure step but does not play a major role in steady-state tone. In apparent contrast, Welsh and Brayden (112) reported a decreased level of stable myogenic tone in cerebral vessels in which an antisense approach had been used to reduce the expression of Trp6. Interestingly, expression of a truncated form of Trp6 in COS cells decreases a nonselective cation current similar to that implicated in smooth muscle capacitative Ca²⁺ entry (107). Clearly, further work is necessary to establish a role for such Ca²⁺ entry mechanisms in myogenic tone and reactivity.

	SPATIOTEMPORAL ASPECTS OF CA2+ SIGNALING

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It is apparent that Ca²⁺ signaling during a myogenic response exhibits spatiotemporal patterns. This is not surprising given the multiple roles that Ca²⁺ appears to serve as a second messenger and the number of potential Ca²⁺ compartments within a cell. Furthermore, the phases of the mechanical response during a myogenic contraction lend themselves to temporal differences in Ca²⁺ signaling (36, 125). For example, during a step increase in intraluminal pressure, an arteriole undergoes an initial phase of distension followed by active contraction to a diameter often less than its diameter before the pressure increase. Such variation in the mechanical state of an arteriole during a myogenic response, together with the involvement of secondary regulatory mechanisms such as Ca²⁺ sensitization, might be expected to be associated with temporal variation in intracellular Ca²⁺ levels.

Focal Ca²+ Release

Ca²+ Sensitization

Ca²⁺ sensitization has been explained by inhibition of myosin phosphatase through protein kinase C (PKC) and/or Rho/Rho kinase-based mechanisms (47, 97). Early studies using pharmacological inhibitors and activators of PKC implicated a role for this kinase in arteriolar myogenic responsiveness (34, 35, 78). For example, PKC inhibitors were shown to dilate cannulated arterioles, whereas PKC activators caused vasoconstriction in the absence of an overt increase in intracellular Ca²⁺ by a mechanism requiring myosin light-chain phosphorylation. More recently, Dessy et al. (19) in studies of ferret coronary arterioles demonstrated that a pressure increase from 40 to 100 mmHg was associated with Ca²⁺ sensitization and translocation of the Ca²⁺-dependent isoform, PKC-. It was not possible to conclude, however, whether PKC- activation played a causative role in Ca²⁺ sensitization and myogenic contraction or whether it was involved in an unrelated pathway. Furthermore, the involvement of other PKC isoforms known to be present in arteriolar smooth muscle could not be excluded. Additional studies using isoform-specific inhibitors of PKC, site and isoform-specific genetic manipulations, and microbiochemical approaches will be required to definitively establish a role for this family of enzymes in Ca²⁺ sensitization associated with myogenic reactivity.

Although the Rho/Rho kinase pathway has been shown to be involved in G protein-coupled mechanisms of smooth muscle Ca²⁺ sensitization (97) and mechanical activation of growth-related pathways in conduit artery smooth muscle (74) and cardiac muscle (1), there is little direct evidence to specifically link such a mechanism to arteriolar myogenic vasoconstriction. Supporting a role for this pathway in resistance vessels, Bolz et al. (8) recently reported that oxidized lipoproteins enhance vasoconstrictor responses in isolated small skeletal muscle arteries by a mechanism involving both Rho and Rho kinase. Further studies are required, however, to conclusively link this potentially important pathway to arteriolar myogenic responsiveness.

	REMOVAL OF EXTRACELLULAR CA2+ DURING PRESSURE REDUCTION

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To date, few studies have addressed the role of enhanced uptake or removal of intracellular Ca²⁺ during myogenic vasodilatation. It is generally assumed that a decrease in pressure leads to a reduction in the mechanically mediated stimulus for Ca²⁺ availability and that Ca²⁺ is removed by sequestration and extrusion mechanisms. To date, the possibility that a decrease in pressure activates Ca²⁺ removal systems, for example, through the stimulation of cyclases and generation of cyclic nucleotides and/or modulation of SR Ca²⁺ uptake by molecules such as phospholamban (53) has not been extensively studied. In a preliminary report, Wellman et al. (110), using cerebral vessels from a phospholamban knockout mouse, showed that Ca²⁺ sparks and subsequent K_Ca channel activity are increased by a protein kinase A-dependent mechanism involving phospholamban. Further evidence that such mechanisms exist in arterioles is provided by studies showing that vasodilatory stimuli, working through increases in cGMP, have been reported to decrease myogenic tone through a mechanism involving a decrease in smooth muscle Ca²⁺ sensitivity (103). It does appear, however, that changes in arteriolar smooth muscle intracellular Ca²⁺ and diameter following a reduction in intraluminal pressure are less consistent than those following a pressure increase (unpublished observations), suggesting that this phase of myogenic responsiveness deserves consideration in its own right.

	ROLE OF CA2+ IN PRESSURE-INDUCED RESPONSES OTHER THAN CONTRACTION

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Because it is likely that a change in intraluminal pressure activates multiple responses (for example contraction and control of ion channels and other signaling mechanisms that may underlie longer term adaptive responses), it is conceivable that changes in Ca²⁺ influence more than one pathway. For example, an increase in intraluminal pressure is also known to activate tyrosine phosphorylation (69) and the expression of protooncogenes such as c-Myc and c-Fos (3), the former being implicated in both the regulation of ion channels and signaling mechanisms that underlie growth responses. The involvement of Ca²⁺ in such processes could occur via either a direct effect or an indirect effect secondary to the contractile response. For example, a mechanical stimulus that results in a Ca²⁺-dependent contraction may decrease the drive toward a growth or adaptive response. Consistent with the idea that a pressure stimulus initiates parallel, but interacting, signaling pathways is the finding that protooncogene expression is attenuated in myogenically active vessels (4), that tyrosine phosphorylation is more extensive in passive arterioles, and that the time course of tyrosine phosphorylation (including activation of mitogen-activated protein kinase) differs from that of myosin light chain phosphorylation and myogenic contraction (69).

	SITE-SPECIFIC DIFFERENCES IN CA2+ HANDLING: INFLUENCE OF VESSEL DIAMETER/ORDER

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An intriguing question that is yet to be definitively answered at a cellular level is, Are there differences in Ca²⁺ handling mechanisms at different sites along the arterial tree? For example, could variations in the distribution or type of Ca²⁺ channels confer a difference in mechanosensitivity as a function of vessel diameter? In support of this proposition, it is known that there are longitudinal gradients in myogenic reactivity with smaller vessels tending to be more myogenically reactive (14).

Using a combination of electrophysiological and molecular approaches, Morita et al. (68) recently reported that there is an increasing density of nifedipine-insensitive Ca²⁺ channels toward terminal arterial branches of the mesenteric vasculature. These channels were further characterized by high-voltage activation and a greater sensitivity for inhibition to Cd²⁺ compared with Ni²⁺ but were not found to be N-, P/Q-, or R-type Ca²⁺ channels. On the basis of these data, it was concluded that small arterioles possess novel VGCCs that contribute to the regulation of vascular tone. Although these data could be suggestive of a role for T channels, available pharmacological data and the fact that resting membrane potential in cannulated arterioles is approximately 40 mV (where T channels would be mostly inactivated) argue against a role for T channels in myogenic responsiveness. The possibility of unique channels or splice variants demonstrating novel regulation needs to be considered.

	RELEVANCE OF CELL COUPLING WITHIN THE ARTERIOLAR WALL TO CA2+ SIGNALING

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The recent interest in the propagation of vasodilator and vasoconstrictor responses from the site of origin has fueled interest in the coupling and communication between cells within the vessel wall (92, 118). The demonstration of gap junctions between vascular cells of the arteriolar wall (88) suggests the possibility of exchange of small-molecular-mass (<1,000 kDa) signaling molecules to either propagate or modulate vasomotor responses. For example, coupling of smooth muscle to endothelial cells might be expected to allow movement of Ca²⁺ from smooth muscle to endothelial cells during a pressure stimulus. The increase in endothelial cell Ca²⁺ could conceivably stimulate nitric oxide production and attenuate myogenic vasoconstriction. Although this is consistent with the findings of Dora et al. (22) and Yashiro and Duling (122) showing agonist-induced Ca²⁺ movement from smooth muscle to endothelium, a number of studies in pressurized vessels without flow (but presumably exhibiting pressure-induced increases in smooth muscle Ca²⁺) found that vessel diameter is not altered by removal of the endothelium. In addition to the possibility of direct transfer of Ca²⁺, ionic coupling between endothelium and vascular smooth muscle could result in hyperpolarization of the muscle layer with subsequent inhibition of voltage-gated Ca²⁺ entry and a reduction in the level of myogenic tone.

In addition to communication between different cell types within the vessel wall, gap junctional coupling between smooth muscle cells could allow propagation of a myogenic response upstream or downstream from the site of origin. Propagation of myogenic tone was initially suggested by Folkow (27) and has been recently demonstrated by data from Rivers (86). The longitudinal transfer of second messengers, or direct evidence of an ionic propagation, during myogenic constriction is yet to be demonstrated.

	SUMMARY AND CONCLUSIONS

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It is clear that intracellular Ca²⁺, in particular pressure-induced Ca²⁺ entry through VGCCs, is fundamental to the arteriolar myogenic response. In contrast, a comprehensive appreciation of the spatiotemporal aspects of Ca²⁺ signaling that underlie myogenic tone and reactivity is not yet available. Although a mechanically induced change in global intracellular Ca²⁺ levels contributes to contraction through Ca²⁺/calmodulin-induced activation of myosin light chain kinase, it is apparent that additional Ca²⁺ regulatory mechanisms contribute to the regulation of arteriolar myogenic tone. Such mechanisms include the modulation of Ca²⁺ sensitivity, Ca²⁺-mediated regulation of ion channels, refilling of intracellular Ca²⁺ stores, and regulation of Ca²⁺ sequestration and extrusion. In contrast to the information gained from earlier studies measuring global changes in intracellular Ca²⁺, such events appear to occur within microdomains and involve significant interactions between compartments (for example, between the SR and regions of the plasma membrane). Studies employing sophisticated imaging approaches and techniques of cell and molecular biology will be required to conclusively delineate the importance of these events.