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M T Nelson

Publications and source records attributed to M T Nelson.

At least 37 records · Page 2Linked to original sources
PubMed2000

Targeted disruption of Kir2.1 and Kir2.2 genes reveals the essential role of the inwardly rectifying K(+) current in K(+)-mediated vasodilation.

The molecular bases of inwardly rectifying K(+) (Kir) currents and K(+)-induced dilations were examined in cerebral arteries of mice that lack the Kir2.1 and Kir2.2 genes. The complete absence of the open reading frame in animals homozygous for the targeted allele was confirmed. Kir2.1(-/-) animals die 8 to 12 hours after birth, apparently due to a complete cleft of the secondary palate. In contrast, Kir2.2(-/-) animals are viable and fertile. Kir currents were observed in cerebral artery myocytes isolated from control neonatal animals but were absent in myocytes from Kir2.1(-/-) animals. Voltage-dependent K(+) currents were similar in cells from neonatal control and Kir2.1(-/-) animals. An increase in the extracellular K(+) concentration from 6 to 15 mmol/L caused Ba(2+)-sensitive dilations in pressurized cerebral arteries from control and Kir2.2 mice. In contrast, arteries from Kir2.1(-/-) animals did not dilate when the extracellular K(+) concentration was increased to 15 mmol/L. In summary, Kir2.1 gene expression in arterial smooth muscle is required for Kir currents and K(+)-induced dilations in cerebral arteries.

15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5↗
PubMed2000

Coupling of Ca(2+) to CREB activation and gene expression in intact cerebral arteries from mouse : roles of ryanodine receptors and voltage-dependent Ca(2+) channels.

Pathological changes of the vasculature are characterized by changes in Ca(2+) handling and alterations in gene expression. In neurons and other cell types, [Ca(2+)](i) often drives changes in gene expression. However, the relationship between Ca(2+) signaling and gene expression in vascular smooth muscle is not well understood. This study examines the ability of Ca(2+) influx through voltage-dependent, L-type Ca(2+) channels (VDCCs) and Ca(2+) release through ryanodine receptors (RyRs) to activate the transcription factor, cAMP-responsive element binding protein (CREB), and increase c-fos levels in intact cerebral arteries. Membrane depolarization increased the fraction of nuclei staining for phosphorylated CREB (P-CREB) and levels of c-fos mRNA in intact mouse cerebral arteries. Ryanodine, which inhibits RyRs, increased P-CREB staining and c-fos levels. Forskolin, an activator of adenylyl cyclase, and sodium nitroprusside, an NO donor, increased P-CREB and c-fos levels. Nisoldipine, an inhibitor of VDCCs, reversed the effects of depolarization and ryanodine on P-CREB and c-fos levels, but not the effects of forskolin or sodium nitroprusside. Inhibition of Ca(2+)/calmodulin-dependent protein kinase (CaM kinase) blocked increases in P-CREB and c-fos levels seen with membrane depolarization, suggesting that CaM kinase has an important role in the pathway leading from Ca(2+) influx to CREB-mediated changes in c-fos levels. Our data suggest that membrane depolarization increases [Ca(2+)](i) through activation of VDCCs, leading to increased P-CREB and c-fos, and that RyRs have a profound effect on this pathway by indirectly regulating Ca(2+) entry through VDCCs. These results provide the first evidence of Ca(2+) regulation of CREB and c-fos in arterial smooth muscle.

Animals↗
PubMed2000

Ca2+ channels, Ca2+ sparks, and regulation of arterial smooth muscle function.

In cardiac, skeletal, and arterial muscle, transient, spatially localized elevations in [Ca2+]i, termed "Ca2+ sparks", have been observed using confocal laser scanning microscopy. Ca2+ sparks are thought to represent "elementary" Ca2+ release events, which arise from one or more ryanodine receptor (RyR) channels in the sarcoplasmic reticulum (SR). In striated muscle, Ca2+ sparks are thought to be key elements of excitation-contraction coupling. In arterial smooth muscle, Ca2+ sparks have been suggested to oppose myogenic vasoconstriction and to influence vasorelaxation. Using a developmental model, we have investigated whether RyRs causing Ca2+ sparks and activation of Ca(2+)-activated K+ (KCa) channels (STOCs) function as "elementary" Ca2+ release units that regulate arterial myogenic tone. Whereas increases in the global [Ca2+]i induce sustained constriction of arterial smooth muscle, Ca2+ sparks induce vasodilation through the local activation of KCa channels. In cerebral arteries, the global bulk [Ca2+]i and a Ca2+ spark frequency < 10(-2) Hz/cell do not cause sufficient KCa channel activity to regulate membrane potential of smooth muscle cells and myogenic tone. The frequency of Ca2+ sparks and STOCs is regulated by agents that modulate protein kinase G and protein kinase A activity. Our findings suggest that "elementary" Ca2+ release units may represent novel, important therapeutic targets for regulating function of the intact arterial smooth muscle tissue.

Animals↗
PubMed2000

Ca2+ channels, 'quantized' Ca2+ release, and differentiation of myocytes in the cardiovascular system.

The application of confocal microscopy to cardiac and skeletal muscle has resulted in the observation of transient, spatially localized elevations in [Ca2+]i, termed 'Ca2+ sparks'. Ca2+ sparks are thought to represent 'elementary' Ca2+ release events, which arise from one or more ryanodine receptor (RyR) channels in the sarcoplasmic reticulum. In cardiac muscle, Ca2+ sparks appear to be key elements of excitation-contraction coupling, in which the global [Ca2+]i transient is thought to involve the recruitment of Ca2+ sparks, each of which is controlled locally by single coassociated L-type Ca2+ channels. Recently, Ca2+ sparks have been detected in smooth muscle cells of arteries. In this review, we analyse the complex relationship of Ca2+ influx and Ca2+ release with local, subcellular Ca2+ microdomains in light of recent studies on Ca2+ sparks in cardiovascular cells. We performed a comparative analysis of 'elementary' Ca2+ release units in mouse, rat and human arterial smooth muscle cells, using measurements of Ca2+ sparks and plasmalemmal K(Ca) currents activated by Ca2+ sparks (STOCs). Furthermore, the appearance of Ca2+ sparks during ontogeny of arterial smooth muscle is explored. Using intact pressurized arteries, we have investigated whether RyRs causing Ca2+ sparks (but not smaller 'quantized' Ca2+ release events, e.g. hypothetical 'Ca2+ quarks') function as key signals that, through membrane potential and global cytoplasmic [Ca2+], oppose arterial myogenic tone and influence vasorelaxation. We believe that voltage-dependent Ca2+ channels and local RyR-related Ca2+ signals are important in differentiation, proliferation, and gene expression. Our findings suggest that 'elementary' Ca2+ release units may represent novel potent therapeutic targets for regulating function of intact arterial smooth muscle tissue.

Animals↗
PubMed2000

Calcium sparks in smooth muscle.

Local intracellular Ca(2+) transients, termed Ca(2+) sparks, are caused by the coordinated opening of a cluster of ryanodine-sensitive Ca(2+) release channels in the sarcoplasmic reticulum of smooth muscle cells. Ca(2+) sparks are activated by Ca(2+) entry through dihydropyridine-sensitive voltage-dependent Ca(2+) channels, although the precise mechanisms of communication of Ca(2+) entry to Ca(2+) spark activation are not clear in smooth muscle. Ca(2+) sparks act as a positive-feedback element to increase smooth muscle contractility, directly by contributing to the global cytoplasmic Ca(2+) concentration ([Ca(2+)]) and indirectly by increasing Ca(2+) entry through membrane potential depolarization, caused by activation of Ca(2+) spark-activated Cl(-) channels. Ca(2+) sparks also have a profound negative-feedback effect on contractility by decreasing Ca(2+) entry through membrane potential hyperpolarization, caused by activation of large-conductance, Ca(2+)-sensitive K(+) channels. In this review, the roles of Ca(2+) sparks in positive- and negative-feedback regulation of smooth muscle function are explored. We also propose that frequency and amplitude modulation of Ca(2+) sparks by contractile and relaxant agents is an important mechanism to regulate smooth muscle function.

Animals↗
PubMed2000

Pharmacology and modulation of K(ATP) channels by protein kinase C and phosphatases in gallbladder smooth muscle.

ATP-sensitive K(+) (K(ATP)) channels exhibit pharmacological diversity, which is critical for the development of novel therapeutic agents. We have characterized K(ATP) channels in gallbladder smooth muscle to determine how their pharmacological properties compare to K(ATP) channels in other types of smooth muscle. K(ATP) currents were measured in myocytes isolated from gallbladder and mesenteric artery. The potencies of pinacidil, diazoxide, and glibenclamide were similar in gallbladder and vascular smooth muscle, suggesting that the regions of the channel conferring sensitivity to these agents are conserved among smooth muscle types. Activators of protein kinase C (PKC), however, were less effective at inhibiting K(ATP) currents in myocytes from gallbladder than mesenteric artery. The phosphatase inhibitor okadaic acid increased the efficacy of PKC activators and revealed ongoing basal activation of K(ATP) channels by protein kinase A in gallbladder. These results suggest that phosphatases and basal kinase activity play an important role in controlling K(ATP) channel activity.

Adenosine Triphosphate↗
PubMed2000

Intracellular calcium events activated by ATP in murine colonic myocytes.

ATP is a candidate enteric inhibitory neurotransmitter in visceral smooth muscles. ATP hyperpolarizes visceral muscles via activation of small-conductance, Ca(2+)-activated K(+) (SK) channels. Coupling between ATP stimulation and SK channels may be mediated by localized Ca(2+) release. Isolated myocytes of the murine colon produced spontaneous, localized Ca(2+) release events. These events corresponded to spontaneous transient outward currents (STOCs) consisting of charybdotoxin (ChTX)-sensitive and -insensitive events. ChTX-insensitive STOCs were inhibited by apamin. Localized Ca(2+) transients were not blocked by ryanodine, but these events were reduced in magnitude and frequency by xestospongin C (Xe-C), a blocker of inositol 1,4,5-trisphosphate receptors. Thus we have termed the localized Ca(2+) events in colonic myocytes "Ca(2+) puffs. " The P(2Y) receptor agonist 2-methylthio-ATP (2-MeS-ATP) increased the intensity and frequency of Ca(2+) puffs. 2-MeS-ATP also increased STOCs in association with the increase in Ca(2+) puffs. Pyridoxal-phospate-6-azophenyl-2',4'-disculfonic acid tetrasodium, a P(2) receptor inhibitor, blocked responses to 2-MeS-ATP. Spontaneous Ca(2+) transients and the effects of 2-MeS-ATP on Ca(2+) puffs and STOCs were blocked by U-73122, an inhibitor of phospholipase C. Xe-C and ryanodine also blocked responses to 2-MeS-ATP, suggesting that, in addition to release from IP(3) receptor-operated stores, ryanodine receptors may be recruited during agonist stimulation to amplify release of Ca(2+). These data suggest that localized Ca(2+) release modulates Ca(2+)-dependent ionic conductances in the plasma membrane. Localized Ca(2+) release may contribute to the electrical responses resulting from purinergic stimulation.

Adenosine Triphosphate↗
PubMed2000

Differential regulation of Ca(2+) sparks and Ca(2+) waves by UTP in rat cerebral artery smooth muscle cells.

Uridine 5'-triphosphate (UTP), a potent vasoconstrictor that activates phospholipase C, shifted Ca(2+) signaling from sparks to waves in the smooth muscle cells of rat cerebral arteries. UTP decreased the frequency of Ca(2+) sparks and transient Ca(2+)-activated K(+) (K(Ca)) currents and increased the frequency of Ca(2+) waves. The UTP-induced reduction in Ca(2+) spark frequency did not reflect a decrease in global cytoplasmic Ca(2+), Ca(2+) influx through voltage-dependent Ca(2+) channels (VDCC), or Ca(2+) load of the sarcoplasmic reticulum (SR), since global Ca(2+) was elevated, blocking VDCC did not prevent the effect, and SR Ca(2+) load did not decrease. However, blocking protein kinase C (PKC) with bisindolylmaleimide I did prevent UTP reduction of Ca(2+) sparks and transient K(Ca) currents. UTP decreased the effectiveness of caffeine, which increases the Ca(2+) sensitivity of ryanodine-sensitive Ca(2+) release (RyR) channels, to activate transient K(Ca) currents. This work supports the concept that vasoconstrictors shift Ca(2+) signaling modalities from Ca(2+) sparks to Ca(2+) waves through the concerted actions of PKC on the Ca(2+) sensitivity of RyR channels, which cause Ca(2+) sparks, and of inositol trisphosphate (IP(3)) on IP(3) receptors to generate Ca(2+) waves.

Animals↗
PubMed2000

Actions of histamine on muscle and ganglia of the guinea pig gallbladder.

Histamine is an inflammatory mediator present in mast cells, which are abundant in the wall of the gallbladder. We examined the electrical properties of gallbladder smooth muscle and nerve associated with histamine-induced changes in gallbladder tone. Recordings were made from gallbladder smooth muscle and neurons, and responses to histamine and receptor subtype-specific compounds were tested. Histamine application to intact smooth muscle produced a concentration-dependent membrane depolarization and increased excitability. In the presence of the H(2) antagonist ranitidine, the response to histamine was potentiated. Activation of H(2) receptors caused membrane hyperpolarization and elimination of spontaneous action potentials. The H(2) response was attenuated by the ATP-sensitive K(+) (K(ATP)) channel blocker glibenclamide in intact and isolated smooth muscle. Histamine had no effect on the resting membrane potential or excitability of gallbladder neurons. Furthermore, neither histamine nor the H(3) agonist R-alpha-methylhistamine altered the amplitude of the fast excitatory postsynaptic potential in gallbladder ganglia. The mast cell degranulator compound 48/80 caused a smooth muscle depolarization that was inhibited by the H(1) antagonist mepyramine, indicating that histamine released from mast cells can activate gallbladder smooth muscle. In conclusion, histamine released from mast cells can act on gallbladder smooth muscle, but not in ganglia. The depolarization and associated contraction of gallbladder smooth muscle represent the net effect of activation of both H(1) (excitatory) and H(2) (inhibitory) receptors, with the H(2) receptor-mediated response involving the activation of K(ATP) channels.

Adenosine Triphosphate↗
PubMed2000

Regulation of urinary bladder smooth muscle contractions by ryanodine receptors and BK and SK channels.

This study examines the roles of voltage-dependent Ca(2+) channels (VDCC), ryanodine receptors (RyRs), large-conductance Ca(2+)-activated K(+) (BK) channels, and small-conductance Ca(2+)-activated K(+) (SK) channels in the regulation of phasic contractions of guinea pig urinary bladder smooth muscle (UBSM). Nisoldipine (100 nM), a dihydropyridine inhibitor of VDCC, abolished spontaneous UBSM contractions. Ryanodine (10 microM) increased contraction frequency and thereby integrated force and, in the presence of the SK blocker apamin, had a greater effect on integrated force than ryanodine alone. Blocking BK (iberiotoxin, 100 nM) or SK (apamin, 100 nM) channels increased contraction amplitude and duration but decreased frequency. The contractile response to iberiotoxin was more pronounced than to apamin. The increases in contraction amplitude and duration to apamin were substantially augmented with ryanodine pretreatment. These results indicate that BK and SK channels have prominent roles as negative feedback elements to limit UBSM contraction amplitude and duration. RyRs also appear to play a significant role as a negative feedback regulator of contraction frequency and duration, and this role is influenced by the activity of SK channels.

Action Potentials↗
PubMed1999

Kir2.1 encodes the inward rectifier potassium channel in rat arterial smooth muscle cells.

1. The molecular nature of the strong inward rectifier K+ channel in vascular smooth muscle was explored by using isolated cell RT-PCR, cDNA cloning and expression techniques. 2. RT-PCR of RNA from single smooth muscle cells of rat cerebral (basilar), coronary and mesenteric arteries revealed transcripts for Kir2.1. Transcripts for Kir2.2 and Kir2.3 were not found. 3. Quantitative PCR analysis revealed significant differences in transcript levels of Kir2.1 between the different vascular preparations (n = 3; P < 0.05). A two-fold difference was detected between Kir2.1 mRNA and beta-actin mRNA in coronary arteries when compared with relative levels measured in mesenteric and basilar preparations. 4. Kir2.1 was cloned from rat mesenteric vascular smooth muscle cells and expressed in Xenopus oocytes. Currents were strongly inwardly rectifying and selective for K+. 5. The effect of extracellular Ba2+, Ca2+, Mg2+ and Cs2+ ions on cloned Kir2.1 channels expressed in Xenopus oocytes was examined. Ba2+ and Cs+ block were steeply voltage dependent, whereas block by external Ca2+ and Mg2+ exhibited little voltage dependence. The apparent half-block constants and voltage dependences for Ba2+, Cs+, Ca2+ and Mg2+ were very similar for inward rectifier K+ currents from native cells and cloned Kir2.1 channels expressed in oocytes. 6. Molecular studies demonstrate that Kir2.1 is the only member of the Kir2 channel subfamily present in vascular arterial smooth muscle cells. Expression of cloned Kir2.1 in Xenopus oocytes resulted in inward rectifier K+ currents that strongly resemble those that are observed in native vascular arterial smooth muscle cells. We conclude that Kir2.1 encodes for inward rectifier K+ channels in arterial smooth muscle.

Animals↗
PubMed1999

Functional coupling of ryanodine receptors to KCa channels in smooth muscle cells from rat cerebral arteries.

The relationship between Ca2+ release ("Ca2+ sparks") through ryanodine-sensitive Ca2+ release channels in the sarcoplasmic reticulum and KCa channels was examined in smooth muscle cells from rat cerebral arteries. Whole cell potassium currents at physiological membrane potentials (-40 mV) and intracellular Ca2+ were measured simultaneously, using the perforated patch clamp technique and a laser two-dimensional (x-y) scanning confocal microscope and the fluorescent Ca2+ indicator, fluo-3. Virtually all (96%) detectable Ca2+ sparks were associated with the activation of a spontaneous transient outward current (STOC) through KCa channels. A small number of sparks (5 of 128) were associated with currents smaller than 6 pA (mean amplitude, 4.7 pA, at -40 mV). Approximately 41% of STOCs occurred without a detectable Ca2+ spark. The amplitudes of the Ca2+ sparks correlated with the amplitudes of the STOCs (regression coefficient 0.8; P < 0.05). The half time of decay of Ca2+ sparks (56 ms) was longer than the associated STOCs (9 ms). The mean amplitude of the STOCs, which were associated with Ca2+ sparks, was 33 pA at -40 mV. The mean amplitude of the "sparkless" STOCs was smaller, 16 pA. The very significant increase in KCa channel open probability (>10(4)-fold) during a Ca2+ spark is consistent with local Ca2+ during a spark being in the order of 1-100 microM. Therefore, the increase in fractional fluorescence (F/Fo) measured during a Ca2+ spark (mean 2.04 F/Fo or approximately 310 nM Ca2+) appears to significantly underestimate the local Ca2+ that activates KCa channels. These results indicate that the majority of ryanodine receptors that cause Ca2+ sparks are functionally coupled to KCa channels in the surface membrane, providing direct support for the idea that Ca2+ sparks cause STOCs.

Animals↗
PubMed1999

Cholesterol inhibits spontaneous action potentials and calcium currents in guinea pig gallbladder smooth muscle.

Elevated cholesterol decreases agonist-induced contractility and enhances stone formation in the gallbladder. The current study was conducted to determine if and how the electrical properties and ionic conductances of gallbladder smooth muscle are altered by elevated cholesterol. Cholesterol was delivered as a complex with cyclodextrin, and effects were evaluated with intracellular recordings from intact gallbladder and whole cell patch-clamp recordings from isolated cells. Cholesterol significantly attenuated the spontaneous action potentials of intact tissue. Furthermore, calcium-dependent action potentials and calcium currents were reduced in the intact tissue and in isolated cells, respectively. However, neither membrane potential hyperpolarizations induced by the ATP-sensitive potassium channel opener, pinacidil, nor voltage-activated outward potassium currents were affected by cholesterol. Hyperpolarizations elicited by calcitonin gene-related peptide were reduced by cholesterol enrichment, indicating potential changes in receptor ligand binding and/or second messenger interactions. These data indicate that excess cholesterol can contribute to gallbladder stasis by affecting calcium channel activity, whereas potassium channels remained unaffected. In addition, cholesterol enrichment may also modulate receptor ligand behavior and/or second messenger interactions.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1999

Gender differences in coronary artery diameter reflect changes in both endothelial Ca2+ and ecNOS activity.

Elevation of nitric oxide (NO) release from the vascular endothelium may contribute to some of the gender-associated differences in coronary artery function. The mechanisms by which gender affects NO release from the endothelium of coronary arteries are not known. In this study, endothelial function was examined in pressurized coronary arteries from female and male rats. Diameter and endothelial cell intracellular Ca2+ concentration ([Ca2+]i) in intact arteries, as well as enzymatic activity of endothelial constitutive nitric oxide synthase (ecNOS) in arterial lysates, was measured. Elevation of intravascular pressure to 60 mmHg constricted coronary arteries from female animals less than coronary arteries from male animals (18% and 31% constriction, respectively). The increased arterial diameter of coronary arteries from females was associated with elevated endothelial [Ca2+]i (female 174 nM, male 90 nM; P < 0.001). Elevation of Ca2+ activated ecNOS with a similar slope and half-activation constant ( approximately 160 nM) for both female and male coronary arteries. However, at [Ca2+] > 100 nM, ecNOS activity was significantly higher in coronary arteries from female rats compared with their male equivalents (P < 0.01). Maximal activity for ecNOS at saturating Ca2+ (300 nM) was 37% higher in coronary arteries from female animals compared with male animals (P < 0.05). Thus elevated [Ca2+]i in the endothelium of female coronary arteries alone is predicted to increase the production of NO (by nearly 2-fold). This gender difference combined with increased ecNOS activity at a given [Ca2+] in females indicates that tonic NO production should be nearly threefold greater in female coronary arteries compared with male coronary arteries. We conclude that, in the regulation of endothelial Ca2+ and ecNOS, gender differences contribute significantly to the overall decrease in myogenic tone observed in coronary arteries of females.

Animals↗
PubMed1999

Increased Ca2+ sensitivity as a key mechanism of PKC-induced constriction in pressurized cerebral arteries.

The effects of activating protein kinase C (PKC) with indolactam V (Indo-V) and 1,2-dioctanoyl-sn-glycerol (DOG) on smooth muscle intracellular Ca2+ concentrations ([Ca2+]i) and arterial diameter were determined using ratiometric Ca2+ imaging and video edge detection of pressurized rat posterior cerebral arteries. Elevation of intraluminal pressure from 10 to 60 mmHg resulted in an increase in [Ca2+]i from 74 +/- 5 to 219 +/- 8 nM and myogenic constriction. Application of Indo-V (0.01-3 microM) or DOG (0.1-30 microM) induced constriction and decreased [Ca2+]i to 140 +/- 11 and 127 +/- 12 nM, respectively, at the highest concentrations used. In the presence of Indo-V, the dihydropyridine Ca2+-channel-blocker nisoldipine produced nearly maximum dilation and decreased [Ca2+]i to 97 +/- 7 nM. In alpha-toxin-permeabilized arteries, the constrictor effects of Indo-V and DOG were not observed in the absence of Ca2+. Both PKC activators significantly increased the degree of constriction of permeabilized arteries at different [Ca2+]i. We conclude that 1) Indo-V- or DOG-induced constriction of pressurized arteries requires Ca2+ influx through voltage-dependent Ca2+ channels, and 2) PKC-induced constriction of pressurized rat cerebral arteries is associated with a decrease in [Ca2+]i, suggesting an increase in the Ca2+ sensitivity of the contractile process.

Animals↗
PubMed1998

Ontogeny of local sarcoplasmic reticulum Ca2+ signals in cerebral arteries: Ca2+ sparks as elementary physiological events.

Ca2+ release through ryanodine receptors (RyRs) in the sarcoplasmic reticulum is a key element of excitation-contraction coupling in muscle. In arterial smooth muscle, Ca2+ release through RyRs activates Ca2+-sensitive K+ (KCa) channels to oppose vasoconstriction. Local Ca2+ transients ("Ca2+ sparks"), apparently caused by opening of clustered RyRs, have been observed in smooth and striated muscle. We explored the fundamental issue of whether RyRs generate Ca2+ sparks to regulate arterial smooth muscle tone by examining the function of RyRs during ontogeny of arteries in the brain. In the present study, Ca2+ sparks were measured using the fluorescent Ca2+ indicator fluo-3 combined with laser scanning confocal microscopy. Diameter and arterial wall [Ca2+] measurements obtained from isolated pressurized arteries were also used in this study to provide functional insights. Neonatal arteries (<1 day postnatal), although still proliferative, have the molecular components for excitation-contraction coupling, including functional voltage-dependent Ca2+ channels, RyRs, and KCa channels and also constrict to elevations in intravascular pressure. Despite having functional RyRs, Ca2+ spark frequency in intact neonatal arteries was approximately 1/100 of adult arteries. In marked contrast to adult arteries, neonatal arteries did not respond to inhibitors of RyRs and KCa channels. These results support the hypothesis that RyRs organize during postnatal development to cause Ca2+ sparks, and RyRs must generate Ca2+ sparks to regulate the function of the intact tissue.

Animals↗
PubMed1998

Regulation of arterial diameter and wall [Ca2+] in cerebral arteries of rat by membrane potential and intravascular pressure.

1. The regulation of intracellular [Ca2+] in the smooth muscle cells in the wall of small pressurized cerebral arteries (100-200 micron) of rat was studied using simultaneous digital fluorescence video imaging of arterial diameter and wall [Ca2+], combined with microelectrode measurements of arterial membrane potential. 2. Elevation of intravascular pressure (from 10 to 100 mmHg) caused a membrane depolarization from -63 +/- 1 to -36 +/- 2 mV, increased arterial wall [Ca2+] from 119 +/- 10 to 245 +/- 9 nM, and constricted the arteries from 208 +/- 10 micron (fully dilated, Ca2+ free) to 116 +/- 7 micron or by 45 % ('myogenic tone'). 3. Pressure-induced increases in arterial wall [Ca2+] and vasoconstriction were blocked by inhibitors of voltage-dependent Ca2+ channels (diltiazem and nisoldipine) or to the same extent by removal of external Ca2+. 4. At a steady pressure (i.e. under isobaric conditions at 60 mmHg), the membrane potential was stable at -45 +/- 1 mV, intracellular [Ca2+] was 190 +/- 10 nM, and arteries were constricted by 41 % (to 115 +/- 7 micron from 196 +/- 8 micron fully dilated). Under this condition of -45 +/- 5 mV at 60 mmHg, the voltage sensitivity of wall [Ca2+] and diameter were 7.5 nM mV-1 and 7.5 micron mV-1, respectively, resulting in a Ca2+ sensitivity of diameter of 1 mum nM-1. 5. Membrane potential depolarization from -58 to -23 mV caused pressurized arteries (to 60 mmHg) to constrict over their entire working range, i.e. from maximally dilated to constricted. This depolarization was associated with an elevation of arterial wall [Ca2+] from 124 +/- 7 to 347 +/- 12 nM. These increases in arterial wall [Ca2+] and vasoconstriction were blocked by L-type voltage-dependent Ca2+ channel inhibitors. 6. The relationship between arterial wall [Ca2+] and membrane potential was not significantly different under isobaric (60 mmHg) and non-isobaric conditions (10-100 mmHg), suggesting that intravascular pressure regulates arterial wall [Ca2+] through changes in membrane potential. 7. The results are consistent with the idea that intravascular pressure causes membrane potential depolarization, which opens voltage-dependent Ca2+ channels, acting as 'voltage sensors', thus increasing Ca2+ entry and arterial wall [Ca2+], which leads to vasoconstriction.

Animals↗