Single calcium channel currents of arterial smooth muscle at physiological calcium concentrations

Arterial myogenic response and aging

The arterial myogenic response is an inherent property of resistance arteries. Myogenic tone is crucial for maintaining a relatively constant blood flow in response to changes in intraluminal pressure and protects delicate organs from excessive blood flow. Although this fundamental physiological phenomenon has been extensively studied, the underlying molecular mechanisms are largely unknown. Recent studies identified a crucial role of mechano-activated angiotensin II type 1 receptors (AT1R) in this process. The development of myogenic response is affected by aging. In this review, we summarize recent progress made to understand the role of AT1R and other mechanosensors in the control of arterial myogenic response. We discuss age-related alterations in myogenic response and possible underlying mechanisms and implications for healthy aging.

Endothelium-dependent vasorelaxant effects of praeruptorin a in isolated rat thoracic aorta

Praeruptorin A (PA) is a natural coumarin compound from the roots of Radix Peucedani and is commonly used in the treatment of certain respiratory diseases and hypertension. Although previous studies identified relaxant effects of PA on tracheal and arterial preparations, little is known about its vasodilative effects and underlying mechanisms. Here, an organ bath system and tension recording methods were used to prepare and analyze isolated rat thoracic aorta artery rings. Aorta artery rings were pre-contracted with phenylephrine and then incubated with PA, and the possible mechanism of relaxation was investigated by adding inhibitors of nitric oxide synthase (NG-nitro-L-arginine methyl ester, L-NAME), endothelial nitric oxide synthase (L-NG-nitroarginine, L-NNA), cyclooxygenase (indomethacin), guanylyl cyclase (1 H-[1,2,4]oxadiazolo [4,3-a]quinoxalin-1-one, ODQ), and KCa channels (tetraethylammonium, TEA). Our study showed that PA-induced vasodilation was blocked by L-NAME, L-NNA, and ODQ, while CaCl₂-induced vasoconstriction was countered by PA. Thus, PA may exert a vasodilatory effect by influencing the amounts of endothelium-derived relaxing factors through endothelial-dependent NO-cGMP and prostacyclin pathways (such as NO and prostacyclin 2). In the rat thoracic aorta, PA reduces vasoconstriction by inhibiting Ca²⁺ inflow.

Fluctuation Analysis in Nonstationary Conditions: Single Ca2+ Channel Current in Pyramidal Neurons

Fluctuation analysis is a method that allows measurement of the single-channel current of ion channels even when it is too small to be resolved directly with the patch-clamp technique. This is the case for voltage-gated calcium channels. They are present in all mammalian central neurons, controlling presynaptic release of transmitter, postsynaptic signaling, and synaptic integration. The amplitudes of their single-channel currents in a physiological concentration of extracellular calcium, however, are small and not well determined. But measurement of this quantity is essential for estimating numbers of functional voltage-gated calcium channels in the membrane and the size of channel-associated calcium signaling domains, and for understanding the stochastic nature of calcium signaling. Here, we recorded the voltage-gated calcium channel current in nucleated patches from layer 5 pyramidal neurons in rat neocortex, in physiological external calcium (1-2 mM). The ensemble-averaging of current responses required for conventional fluctuation analysis proved impractical because of the rapid rundown of calcium channel currents. We therefore developed a more robust method, using mean current fitting of individual current responses and band-pass filtering. Furthermore, voltage-ramp stimulation proved useful. We validated the accuracy of the method by analyzing simulated data. At an external calcium concentration of 1 mM, and a membrane potential of -20 mV, we found that the average single-channel current amplitude was ∼0.04 pA, increasing to 0.065 pA at 2 mM external calcium, and 0.12 pA at 5 mM. The relaxation time constant of the fluctuations was in the range 0.2-0.8 ms. The results are relevant to understanding the stochastic properties of dendritic Ca²⁺ spikes in neocortical layer 5 pyramidal neurons. With the reported method, single-channel current amplitude of native voltage-gated calcium channels can be resolved accurately despite conditions of unstable rundown.

Arteriolar oxygen reactivity: where is the sensor and what is the mechanism of action?

Arterioles in the peripheral microcirculation are exquisitely sensitive to changes in PO2 in their environment: increases in PO2 cause vasoconstriction while decreases in PO2 result in vasodilatation. However, the cell type that senses O2 (the O2 sensor) and the signalling pathway that couples changes in PO2 to changes in arteriolar tone (the mechanism of action) remain unclear. Many (but not all) ex vivo studies of isolated cannulated resistance arteries and large, first-order arterioles support the hypothesis that these vessels are intrinsically sensitive to PO2 with the smooth muscle, endothelial cells, or red blood cells serving as the O2 sensor. However, in situ studies testing these hypotheses in downstream arterioles have failed to find evidence of intrinsic O2 sensitivity, and instead have supported the idea that extravascular cells sense O2 . Similarly, ex vivo studies of isolated, cannulated resistance arteries and large first-order arterioles support the hypotheses that O2 -dependent inhibition of production of vasodilator cyclooxygenase products or O2 -dependent destruction of nitric oxide mediates O2 reactivity of these upstream vessels. In contrast, most in vivo studies of downstream arterioles have disproved these hypotheses and instead have provided evidence supporting the idea that O2 -dependent production of vasoconstrictors mediates arteriolar O2 reactivity, with significant regional heterogeneity in the specific vasoconstrictor involved. Oxygen-induced vasoconstriction may serve as a protective mechanism to reduce the oxidative burden to which a tissue is exposed, a process that is superimposed on top of the local mechanisms which regulate tissue blood flow to meet a tissue's metabolic demand.

Information processing and synaptic plasticity at hippocampal mossy fiber terminals

Granule cells of the dentate gyrus receive cortical information and they transform and transmit this code to the CA3 area via their axons, the mossy fibers (MFs). Structural and functional complexity of this network has been extensively studied at various organizational levels. This review is focused on the anatomical and physiological properties of the MF system. We will discuss the mechanism by which dentate granule cells process signals from single action potentials (APs), short bursts and longer stimuli. Various parameters of synaptic interactions at different target cells such as quantal transmission, short- and long-term plasticity (LTP) will be summarized. Different types of synaptic contacts formed by MFs have unique sets of rules for information processing during different rates of granule cell activity. We will investigate the complex interactions between key determinants of information transfer between the dentate gyrus and the CA3 area of the hippocampus.

Ca2+ channel nanodomains boost local Ca2+ amplitude

Local Ca(2+) signals through voltage-gated Ca(2+) channels (CaVs) drive synaptic transmission, neural plasticity, and cardiac contraction. Despite the importance of these events, the fundamental relationship between flux through a single CaV channel and the Ca(2+) signaling concentration within nanometers of its pore has resisted empirical determination, owing to limitations in the spatial resolution and specificity of fluorescence-based Ca(2+) measurements. Here, we exploited Ca(2+)-dependent inactivation of CaV channels as a nanometer-range Ca(2+) indicator specific to active channels. We observed an unexpected and dramatic boost in nanodomain Ca(2+) amplitude, ten-fold higher than predicted on theoretical grounds. Our results uncover a striking feature of CaV nanodomains, as diffusion-restricted environments that amplify small Ca(2+) fluxes into enormous local Ca(2+) concentrations. This Ca(2+) tuning by the physical composition of the nanodomain may represent an energy-efficient means of local amplification that maximizes information signaling capacity, while minimizing global Ca(2+) load.

Release from the cone ribbon synapse under bright light conditions can be controlled by the opening of only a few Ca(2+) channels

Light hyperpolarizes cone photoreceptors, causing synaptic voltage-gated Ca(2+) channels to open infrequently. To understand neurotransmission under these conditions, we determined the number of L-type Ca(2+) channel openings necessary for vesicle fusion at the cone ribbon synapse. Ca(2+) currents (I(Ca)) were activated in voltage-clamped cones, and excitatory postsynaptic currents (EPSCs) were recorded from horizontal cells in the salamander retina slice preparation. Ca(2+) channel number and single-channel current amplitude were calculated by mean-variance analysis of I(Ca). Two different comparisons-one comparing average numbers of release events to average I(Ca) amplitude and the other involving deconvolution of both EPSCs and simultaneously recorded cone I(Ca)-suggested that fewer than three Ca(2+) channel openings accompanied fusion of each vesicle at the peak of release during the first few milliseconds of stimulation. Opening fewer Ca(2+) channels did not enhance fusion efficiency, suggesting that few unnecessary channel openings occurred during strong depolarization. We simulated release at the cone synapse, using empirically determined synaptic dimensions, vesicle pool size, Ca(2+) dependence of release, Ca(2+) channel number, and Ca(2+) channel properties. The model replicated observations when a barrier was added to slow Ca(2+) diffusion. Consistent with the presence of a diffusion barrier, dialyzing cones with diffusible Ca(2+) buffers did not affect release efficiency. The tight clustering of Ca(2+) channels, along with a high-Ca(2+) affinity release mechanism and diffusion barrier, promotes a linear coupling between Ca(2+) influx and vesicle fusion. This may improve detection of small light decrements when cones are hyperpolarized by bright light.

Reliability and heterogeneity of calcium signaling at single presynaptic boutons of cerebellar granule cells

Activity-dependent elevation of calcium within presynaptic boutons regulates many aspects of synaptic transmission. Here, we examine presynaptic residual calcium (Ca(res)) transients in individual presynaptic boutons of cerebellar granule cells at near-physiological temperatures using two-photon microscopy. Properties of Ca(res) under conditions of zero-added buffer were determined by measuring Ca(res) transients while loading boutons to a steady-state indicator concentration. These experiments revealed that, in the absence of exogenous calcium buffers, a single action potential evokes transients of Ca(res) that vary widely in different boutons both in amplitude (400-900 nM) and time course (25-55 ms). Variation in calcium influx density, endogenous buffer capacity, and calcium extrusion density contribute to differences in Ca(res) among boutons. Heterogeneity in Ca(res) within different boutons suggests that plasticity can be regulated independently at different synapses arising from an individual granule cell. In a given bouton, Ca(res) signals were highly reproducible from trial to trial and failures of calcium influx were not observed. We find that a factor contributing to this reliability is that an action potential opens a large number of calcium channels (20-125) in a bouton. Presynaptic calcium signals were also used to assess the ability of granule cell axons to convey somatically generated action potentials to distant synapses. In response to pairs of action potentials or trains, granule cell boutons showed a remarkable ability to respond reliably at frequencies up to 500 Hz. Thus, individual boutons appear specialized for reliable calcium signaling during bursts of high-frequency activation such as those that are observed in vivo.

Nanodomains of single Ca2+ channels contribute to action potential repolarization in cortical neurons

The precise shape of action potentials in cortical neurons is a key determinant of action potential-dependent Ca2+ influx, as well as of neuronal signaling, on a millisecond scale. In cortical neurons, Ca2+-sensitive K+ channels, or BK channels (BKChs), are crucial for action potential termination, but the precise functional interplay between Ca2+ channels and BKChs has remained unclear. In this study, we investigate the mechanisms allowing for rapid and reliable activation of BKChs by single action potentials in hippocampal granule cells and the impact of endogenous Ca2+ buffers. We find that BKChs are operated by nanodomains of single Ca2+ channels. Using a novel approach based on a linear approximation of buffered Ca2+ diffusion in microdomains, we quantitatively analyze the prolongation of action potentials by the Ca2+ chelator BAPTA. This analysis allowed us to estimate that the mean diffusional distance for Ca2+ ions from a Ca2+ channel to a BKCh is approximately 13 nm. This surprisingly short diffusional distance cannot be explained by a random distribution of Ca2+ channels and renders the activation of BKChs insensitive to the relatively high concentrations of endogenous Ca2+ buffers in hippocampal neurons. These data suggest that tight colocalization of the two types of channels permits hippocampal neurons to regulate global Ca2+ signals by a high cytoplasmic Ca2+ buffer capacity without affecting the fast and brief activation of BKChs required for proper repolarization of action potentials.

Timing and efficacy of transmitter release at mossy fiber synapses in the hippocampal network

It is widely accepted that the hippocampus plays a major role in learning and memory. The mossy fiber synapse between granule cells in the dentate gyrus and pyramidal neurons in the CA3 region is a key component of the hippocampal trisynaptic circuit. Recent work, partially based on direct presynaptic patch-clamp recordings from hippocampal mossy fiber boutons, sheds light on the mechanisms of synaptic transmission and plasticity at mossy fiber synapses. A high Na(+) channel density in mossy fiber boutons leads to a large amplitude of the presynaptic action potential. Together with the fast gating of presynaptic Ca(2+) channels, this generates a large and brief presynaptic Ca(2+) influx, which can trigger transmitter release with high efficiency and temporal precision. The large number of release sites, the large size of the releasable pool of vesicles, and the huge extent of presynaptic plasticity confer unique strength to this synapse, suggesting a large impact onto the CA3 pyramidal cell network under specific behavioral conditions. The characteristic properties of the hippocampal mossy fiber synapse may be important for pattern separation and information storage in the dentate gyrus-CA3 cell network.

Single-channel L-type Ca2+ currents in chicken embryo semicircular canal type I and type II hair cells

Few data are available concerning single Ca channel properties in inner ear hair cells and particularly none in vestibular type I hair cells. By using the cell-attached configuration of the patch-clamp technique in combination with the semicircular canal crista slice preparation, we determined the elementary properties of voltage-dependent Ca channels in chicken embryo type I and type II hair cells. The pipette solutions included Bay K 8644. With 70 mM Ba(2+) in the patch pipette, Ca channel activity appeared as very brief openings at -60 mV. Ca channel properties were found to be similar in type I and type II hair cells; therefore data were pooled. The mean inward current amplitude was -1.3 +/- 0.1 (SD) pA at - 30 mV (n = 16). The average slope conductance was 21 pS (n = 20). With 5 mM Ba(2+) in the patch pipette, very brief openings were already detectable at -80 mV. The mean inward current amplitude was -0.7 +/- 0.2 pA at -40 mV (n = 9). The average slope conductance was 11 pS (n = 9). The mean open time and the open probability increased significantly with depolarization. Ca channel activity was still present and unaffected when omega-agatoxin IVA (2 microM) and omega-conotoxin GVIA (3.2 microM) were added to the pipette solution. Our results show that types I and II hair cells express L-type Ca channels with similar properties. Moreover, they suggest that in vivo Ca(2+) influx might occur at membrane voltages more negative than -60 mV.

Ca2+ -induced Ca2+ release through localized Ca2+ uncaging in smooth muscle

Ca²⁺-induced Ca²⁺ release (CICR) from the sarcoplasmic reticulum (SR) occurs in smooth muscle as spontaneous SR Ca²⁺ release or Ca²⁺ sparks and, in some spiking tissues, as Ca²⁺ release that is triggered by the activation of sarcolemmal Ca²⁺ channels. Both processes display spatial localization in that release occurs at a higher frequency at specific subcellular regions. We have used two-photon flash photolysis (TPFP) of caged Ca²⁺ (DMNP-EDTA) in Fluo-4–loaded urinary bladder smooth muscle cells to determine the extent to which spatially localized increases in Ca²⁺ activate SR release and to further understand the molecular and biophysical processes underlying CICR. TPFP resulted in localized Ca²⁺ release in the form of Ca²⁺ sparks and Ca²⁺ waves that were distinguishable from increases in Ca²⁺ associated with Ca²⁺ uncaging, unequivocally demonstrating that Ca²⁺ release occurs subsequent to a localized rise in [Ca²⁺]_i. TPFP-triggered Ca²⁺ release was not constrained to a few discharge regions but could be activated at all areas of the cell, with release usually occurring at or within several microns of the site of photolysis. As expected, the process of CICR was dominated by ryanodine receptor (RYR) activity, as ryanodine abolished individual Ca²⁺ sparks and evoked release with different threshold and kinetics in FKBP12.6-null cells. However, TPFP CICR was not completely inhibited by ryanodine; Ca²⁺ release with distinct kinetic features occurred with a higher TPFP threshold in the presence of ryanodine. This high threshold release was blocked by xestospongin C, and the pharmacological sensitivity and kinetics were consistent with CICR release at high local [Ca²⁺]_i through inositol trisphosphate (InsP₃) receptors (InsP₃Rs). We conclude that CICR activated by localized Ca²⁺ release bears essential similarities to those observed by the activation of I_Ca (i.e., major dependence on the type 2 RYR), that the release is not spatially constrained to a few specific subcellular regions, and that Ca²⁺ release through InsP₃R can occur at high local [Ca²⁺]_i.

Endogenous Ca2+ buffer concentration and Ca2+ microdomains in hippocampal neurons

Ca2+-binding proteins are ubiquitously expressed throughout the CNS and serve as valuable immunohistochemical markers for certain types of neurons. However, the functional role of most Ca2+-binding proteins has to date remained obscure because their concentration in central neurons is not known. In this study, we investigate the intracellular concentration of the widely expressed Ca2+-binding protein calbindin-D28k in adult hippocampal slices using patch-clamp recordings and immunohistochemistry. First, we show that calbindin-D28k freely exchanges between patch pipette and cytoplasm during whole cell patch-clamp recordings with a time constant of approximately 10 min. Substituting known concentrations of recombinant calbindin-D28k in patch pipettes enabled us to determine the endogenous calbindin-D28k concentration by postrecording immunohistochemistry. Using this calibration procedure, we find that mature granule cells (doublecortin-) contain approximately 40 microm, and newborn granule cells (doublecortin+) contain 0-20 microm calbindin-D28k. CA3 stratum radiatum interneurons and CA1 pyramidal cells enclose approximately 47 and approximately 45 microm calbindin-D28k, respectively. Numerical simulations showed that 40 microm calbindin-D28k is capable of tuning Ca2+ microdomains associated with action potentials at the mouth of single or clustered Ca2+ channels: calbindin-D28k reduces the increment in free Ca2+ at a distance of 100 and 200 nm by 20 and 35%, respectively, and strongly accelerates the collapse of the Ca2+ gradient after cessation of Ca2+ influx. These data suggest that calbindin-D28k equips hippocampal neurons with approximately 160 microm mobile, high-affinity Ca2+-binding sites (kappa(S) approximately 200) that slow and reduce global Ca2+ signals while they enhance the spatiotemporal fidelity of submicroscopic Ca2+ signals.

Basal calcium entry in vascular smooth muscle

Basal calcium leak into smooth muscle was identified 30 years ago yet remains poorly understood. We characterized this leak measuring 45Ca2+ uptake into cultured rat aortic smooth muscle cells. Wash solution (0 degrees C) containing lanthanum (3 mM) removed extracellular tracer and increased cellular 45Ca2+ retention more effectively than EGTA (0.2 mM). Basal Ca2+ entry was 1.45 x 10(9) Ca2+ x cell(-1) x min(-1). This translated to approximately 250 micromol(-1) x min(-1) given cell volumes of 4-15 pl as determined by 3-D image reconstruction. Gadolinium (100 microM) blocked 80% of the leak and exhibited a biphasic concentration-response relation (IC50s=1 microM and 2 mM). Organic ion channel blockers also inhibited approximately 80% of the leak; 45% by nifedipine (10 microM), 7% was exclusively blocked by SKF 96365 (1-[b-[3-(4-Methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole) (50 microM) and 23% was exclusively sensitive to 2-aminoethoxy-diphenylborate (2-APB, 75 microM). Reverse transcriptase polymerase chain reaction revealed TrpC1, 4 and 6 mRNA, and we propose that 2-APB may selectively block TrpC4-containing channels. We conclude that basal Ca2+ entry is mainly due to a basal open probability of excitable Ca2+ -channels.

Local routes revisited: the space and time dependence of the Ca2+ signal for phasic transmitter release at the rat calyx of Held

During the last decade, advances in experimental techniques and quantitative modelling have resulted in the development of the calyx of Held as one of the best preparations in which to study synaptic transmission. Here we review some of these advances, including simultaneous recording of pre- and postsynaptic currents, measuring the Ca2+ sensitivity of transmitter release, reconstructing the 3-D anatomy at the electron microscope (EM) level, and modelling the buffered diffusion of Ca2+ in the nerve terminal. An important outcome of these studies is an improved understanding of the Ca2+ signal that controls phasic transmitter release. This article illustrates the spatial and temporal aspects of the three main steps in the presynaptic signalling cascade: Ca2+ influx through voltage-gated calcium channels, buffered Ca2+ diffusion from the channels to releasable vesicles, and activation of the Ca2+ sensor for release. Particular emphasis is placed on how presynaptic Ca2+ buffers affect the Ca2+ signal and thus the amplitude and time course of the release probability. Since many aspects of the signalling cascade were first conceived with reference to the squid giant presynaptic terminal, we include comparisons with the squid model and revisit some of its implications. Whilst the characteristics of buffered Ca2+ diffusion presented here are based on the calyx of Held, we demonstrate the circumstances under which they may be valid for other nerve terminals at mammalian CNS synapses.

Selective blockade of endothelial Ca2+-activated small- and intermediate-conductance K+-channels suppresses EDHF-mediated vasodilation

1. Activation of Ca(2+)-activated K(+)-channels (K(Ca)) has been suggested to play a key role in endothelium-derived hyperpolarizing factor (EDHF)-mediated vasodilation. However, due to the low selectivity of commonly used K(Ca)-channel blockers it is still elusive which endothelial K(Ca)-subtypes mediate hyperpolarization and thus initiate EDHF-mediated vasodilation. 2. Using the non-cytochrome P450 blocking clotrimazole-derivatives, 1-[(2-chlorophenyl) diphenylmethyl]-1H-pyrazole (TRAM-34) and 2-(2-chlorophenyl)-2,2-diphenylacetonitrile (TRAM-39) as highly selective IK1-inhibitors, we investigated the role of the intermediate-conductance K(Ca) (rIK1) in endothelial hyperpolarization and EDHF-mediated vasodilation. 3. Expression and function of rIK1 and small-conductance K(Ca) (rSK3) were demonstrated in situ in single endothelial cells of rat carotid arteries (CA). rIK1-currents were blocked by TRAM-34 or TRAM-39, while rSK3 was blocked by apamin. In current-clamp experiments, endothelial hyperpolarization in response to acetylcholine was abolished by the combination of apamin and TRAM-34. 4. In phenylephrine-preconstricted CA, acetylcholine-induced NO and prostacyclin-independent vasodilation was almost completely blocked by ChTX, CLT, TRAM-34, or TRAM-39 in combination with the SK3-blocker apamin. Apamin, TRAM-34, and CLT alone or sulphaphenzole, a blocker of the cytochrome P450 isoform 2C9, were ineffective in blocking the EDHF-response. 5. In experiments without blocking NO and prostacyclin synthesis, the combined blockade of SK3 and IK1 reduced endothelium-dependent vasodilation. 6. In conclusion, the use of selective IK1-inhibitors together with the SK3-blocker apamin revealed that activation of both K(Ca), rIK1 and rSK3 is crucial in mediating endothelial hyperpolarization and generation of the EDHF-signal while the cytochrome P450 pathway seems to play a minor or no role in rat CA.

Timing and efficacy of Ca2+ channel activation in hippocampal mossy fiber boutons

The presynaptic Ca2+ signal is a key determinant of transmitter release at chemical synapses. In cortical synaptic terminals, however, little is known about the kinetic properties of the presynaptic Ca2+ channels. To investigate the timing and magnitude of the presynaptic Ca2+ inflow, we performed whole-cell patch-clamp recordings from mossy fiber boutons (MFBs) in rat hippocampus. MFBs showed large high-voltage-activated Ca(2+) currents, with a maximal amplitude of approximately 100 pA at a membrane potential of 0 mV. Both activation and deactivation were fast, with time constants in the submillisecond range at a temperature of approximately 23 degrees C. An MFB action potential (AP) applied as a voltage-clamp command evoked a transient Ca2+ current with an average amplitude of approximately 170 pA and a half-duration of 580 microsec. A prepulse to +40 mV had only minimal effects on the AP-evoked Ca2+ current, indicating that presynaptic APs open the voltage-gated Ca2+ channels very effectively. On the basis of the experimental data, we developed a kinetic model with four closed states and one open state, linked by voltage-dependent rate constants. Simulations of the Ca2+ current could reproduce the experimental data, including the large amplitude and rapid time course of the current evoked by MFB APs. Furthermore, the simulations indicate that the shape of the presynaptic AP and the gating kinetics of the Ca2+ channels are tuned to produce a maximal Ca2+ influx during a minimal period of time. The precise timing and high efficacy of Ca2+ channel activation at this cortical glutamatergic synapse may be important for synchronous transmitter release and temporal information processing.

48-h Hypoxic exposure results in endothelium-dependent systemic vascular smooth muscle cell hyperpolarization

Chronic hypoxia (CH) results in reduced sensitivity to vasoconstrictors in conscious rats that persists upon restoration of normoxia. We hypothesized that this effect is due to endothelium-dependent hyperpolarization of vascular smooth muscle (VSM) cells after CH. VSM cell resting membrane potential was determined for superior mesenteric artery strips isolated from CH rats (PB = 380 Torr for 48 h) and normoxic controls. VSM cells from CH rats studied under normoxia were hyperpolarized compared with controls. Resting vessel wall intracellular Ca(2+) concentration ([Ca(2+)](i)) and pressure-induced vasoconstriction were reduced in vessels isolated from CH rats compared with controls. Vasoconstriction and increases in vessel wall [Ca(2+)](i) in response to the alpha(1)-adrenergic agonist phenylephrine (PE) were also blunted in resistance arteries from CH rats. Removal of the endothelium normalized resting membrane potential, resting vessel wall [Ca(2+)](i), pressure-induced vasoconstrictor responses, and PE-induced constrictor and Ca(2+) responses between groups. Whereas VSM cell hyperpolarization persisted in the presence of nitric oxide synthase inhibition, heme oxygenase inhibition restored VSM cell resting membrane potential in vessels from CH rats to control levels. We conclude that endothelial derived CO accounts for persistent VSM cell hyperpolarization and vasoconstrictor hyporeactivity after CH.

Calcium secretion coupling at calyx of Held governed by nonuniform channel-vesicle topography

Phasic transmitter release at synapses in the mammalian CNS is regulated by local [Ca2+] transients, which control the fusion of readily releasable vesicles docked at active zones (AZs) in the presynaptic membrane. The time course and amplitude of these [Ca2+] transients critically determine the time course and amplitude of the release and thus the frequency and amplitude tuning of the synaptic connection. As yet, the spatiotemporal nature of the [Ca2+] transients and the number and location of release-controlling Ca2+ channels relative to the vesicles, the "topography" of the release sites, have remained elusive. We used a time-dependent model to simulate Ca2+ influx, three-dimensional buffered Ca2+ diffusion, and the binding of Ca2+ to the release sensor. The parameters of the model were constrained by recent anatomical and biophysical data of the calyx of Held. Comparing the predictions of the model with previously measured release probabilities under a variety of experimental conditions, we inferred which release site topography is likely to operate at the calyx: At each AZ one or a few clusters of Ca2+ channels control the release of the vesicles. The distance of a vesicle to the cluster(s) varies across the multiple release sites of a single calyx (ranging from 30 to 300 nm; average approximately 100 nm). Assuming this topography, vesicles in different locations are exposed to different [Ca2+] transients, with peak amplitudes ranging from 0.5 to 40 microm (half-width approximately 400 microsec) during an action potential. Consequently the vesicles have different release probabilities ranging from <0.01 to 1. We demonstrate how this spatially heterogeneous release probability creates functional advantages for synaptic transmission.

Ca2+ transport properties and determinants of anomalous mole fraction effects of single voltage-gated Ca2+ channels in hair cells from bullfrog saccule

We studied the permeation properties of two distinct single voltage-gated Ca2+ channels in bullfrog saccular hair cells to assess the roles of the channels as physiological Ca2+ transporters and multi-ion pores. By varying the permeant ions (Ba2+, Ca2+) and concentrations (2-70 mM), we estimated the affinity constant (K(D)) of the two channels as follows (mM): L-type channel, K(D,Ba) = 7.4 +/- 1.0, K(D,Ca) = 7.1 +/- 2.2 (n = 7); non-L-type channel, K(D,Ba) = 5.3 +/- 3.2, K(D,Ca) = 2.0 +/- 1.0 (n = 8). Using ionic concentrations close to physiological conditions (2 mM Ca2+ and 1.0 mM Mg2+), the conductance of the L-type channel was approximately 2 pS. We determined the mechanisms by which ions traverse the pore of these single Ca2+ channels, using mixtures of Ba2+ and Ca2+ at total concentrations above (70 mM) or close to (5 mM) the K(D) of the channels. We found evidence for an anomalous mole fraction effect (AMFE) only when the total divalent ion concentration was 5 mM, consistent with a multi-ion pore. We show that AMFE arises from the boundaries between the pore and bulk solution in the atria of the channel, which is derived from the presence of depletion zones that become apparent at low divalent cation concentrations. The present findings provide an explanation as to why previous whole-cell Ca2+ currents that were recorded in quasi-physiological Ca2+ concentrations (approximately 2-5 mM) showed clear AMFE, whereas single Ca2+ channel currents that were recorded routinely at high Ca2+ concentrations (20-110 mM) did not.

Potentiation of the cardiac L-type Ca(2+) channel (alpha(1C)) by dihydropyridine agonist and strong depolarization occur via distinct mechanisms

A defining property of L-type Ca(2+) channels is their potentiation by both 1,4-dihydropyridine agonists and strong depolarization. In contrast, non-L-type channels are potentiated by neither agonist nor depolarization, suggesting that these two processes may by linked. In this study, we have tested whether the mechanisms of agonist- and depolarization-induced potentiation in the cardiac L-type channel (alpha(1C)) are linked. We found that the mutant L-type channel GFP-alpha(1C)(TQ-->YM), bearing the mutations T1066Y and Q1070M, was able to undergo depolarization-induced potentiation but not potentiation by agonist. Conversely, the chimeric channel GFP-CACC was potentiated by agonist but not by strong depolarization. These data indicate that the mechanisms of agonist- and depolarization-induced potentiation of alpha(1C) are distinct. Since neither GFP-CACC nor GFP-CCAA was potentiated significantly by depolarization, no single repeat of alpha(1C) appears to be responsible for depolarization-induced potentiation. Surprisingly, GFP-CACC displayed a low estimated open probability similar to that of the alpha(1C), but could not support depolarization-induced potentiation, demonstrating that a relatively low open probability alone is not sufficient for depolarization-induced potentiation to occur. Thus, depolarization-induced potentiation may be a global channel property requiring participation from all four homologous repeats.

Invited review: mechanisms of calcium handling in smooth muscles

The concentration of cytoplasmic Ca(2+) regulates the contractile state of smooth muscle cells and tissues. Elevations in global cytoplasmic Ca(2+) resulting in contraction are accomplished by Ca(2+) entry and release from intracellular stores. Pathways for Ca(2+) entry include dihydropyridine-sensitive and -insensitive Ca(2+) channels and receptor and store-operated nonselective channels permeable to Ca(2+). Intracellular release from the sarcoplasmic reticulum (SR) is accomplished by ryanodine and inositol trisphosphate receptors. The impact of Ca(2+) entry and release on cytoplasmic concentration is modulated by Ca(2+) reuptake into the SR, uptake into mitochondria, and extrusion into the extracellular solution. Highly localized Ca(2+) transients (i.e., sparks and puffs) regulate ionic conductances in the plasma membrane, which can provide feedback to cell excitability and affect Ca(2+) entry. This short review describes the major transport mechanisms and compartments that are utilized for Ca(2+) handling in smooth muscles.

Calcium dynamics associated with action potentials in single nerve terminals of pyramidal cells in layer 2/3 of the young rat neocortex

Calcium dynamics associated with a single action potential (AP) were studied in single boutons of the axonal arbor of layer 2/3 pyramidal cells in the neocortex of young (P14-16) rats. We used fluorescence imaging with two-photon excitation and Ca2+-selective fluorescence indicators to measure volume-averaged Ca2+ signals. These rapidly reached a peak (in about 1 ms) and then decayed more slowly (tens to hundreds of milliseconds). Single APs and trains of APs reliably evoked Ca2+ transients in en passant boutons located on axon collaterals in cortical layers 2/3, 4 and 5, indicating that APs propagate actively and reliably throughout the axonal arbor. Branch point failures are unlikely to contribute to differences in synaptic efficacy and reliability in the connections made by layer 2/3 pyramidal cells. AP-evoked Ca2+ transients in boutons were mediated by voltage-dependent Ca2+ channels (VDCCs), predominantly by the P/Q- and N-subtypes. Ca2+ transients were, on average, of significantly larger amplitude in boutons than in the flanking segments of the axon collateral. Large amplitude Ca2+ transients in boutons were spatially restricted to within <= 3 m of axonal length. Single AP-evoked Ca2+ transients varied up to 10-fold across different boutons even if they were located on the same axon collateral. In contrast, variation of Ca2+ transients evoked by successive APs in a given single bouton was small (coefficient of variation, c.v. <= 0.21). Amplitudes of AP-evoked Ca2+ signals did not correlate with the distance of boutons from the soma. In contrast, AP-evoked Ca2+ signals in spines of basal dendrites decreased slightly (correlation coefficient, r2 = -0.27) with distance from the soma. Measurements with the low-affinity Ca2+ indicator Magnesium Green suggest that the volume-averaged residual free [Ca2+]i in a bouton increases on average by 500 nM following a single AP. Higher concentrations of indicator caused, on average, a decrease in the amplitude and an increase in the decay time constant of Ca2+ transients. Assuming a single-compartment model the concentration dependence of decay time constants suggests a low endogenous Ca2+ binding ratio close to 140, indicating that of the total Ca2+ influx ( approximately 2 fC) less than 1% remained free. The indicator concentration dependence of decay time constants further suggests that the residual free Delta[Ca2+]i associated with an AP decays with a time constant of about 60 ms (35 C) reflecting a high Ca2+ extrusion rate of about 2600 s(-1). The results show that AP-evoked volume-averaged Ca2+ transients in single boutons are evoked reliably and, on average, have larger amplitudes than Ca2+ transients in other subcellular compartments of layer 2/3 pyramidal cells. A major functional signature is the large variation in the amplitude of Ca2+ transients between different boutons. This could indicate that local interactions between boutons and different target cells modify the spatiotemporal Ca2+ dynamics in boutons and cause target cell-specific differences in their transmitter release properties.

The spatial relationship between Ca2+ channels and Ca2+-activated channels and the function of Ca2+-buffering in avian sensory neurons

In order to learn about the endogenous Ca2+-buffering in the cytoplasm of chick dorsal root ganglion (DRG) neurons and the distance separating the ryanodine receptor Ca2+ release channels (RyRs) from the plasma membrane, we monitored the amplitude and time course of Ca2+-activated Cl- currents (I(ClCa)) in protocols that manipulated Ca2+-buffering. I(ClCa)was activated by Ca2+ influx via voltage-gated Ca2+ channels or by Ca2+ release via RyRs activated by 10 mM caffeine. I(ClCa)was measured in neurons at 20 degrees C and 35 degrees C using the amphotericin perforated patch technique that preserves endogenous Ca2+-buffering, or at 20 degrees C in neurons dialyzed with pipette solutions designed to replace the endogenous Ca2+ buffers. The amplitude of I(ClCa)activated by Ca2+ influx or Ca2+ at 20 degrees C was similar in the amphotericin neurons and neurons dialyzed with an 'unbuffered' pipette solution containing 10 mM citrate and 3 mM ATP as the only Ca2+ binding molecules. Thus, endogenous mobile Ca2+ buffers are relatively unimportant in chick DRG neurons. Warming the neurons from 20 degrees C to 35 degrees C increased the amplitude and the rate of deactivation of I(ClCa)consistent with an increased rate of Ca2+ buffering by fixed endogenous Ca2+-buffers. Dialysis with 2 mM EGTA/0.1 microM free Ca2+ reduced the amplitude and increased the rate of deactivation of I(ClCa)activated by Ca2+ influx and abolished I(ClCa)activated by Ca2+ release. Dialysis with 2 mM BAPTA/0.1 microM free Ca2+ abolished I(ClCa)activated by Ca2+ influx or release. Dialysis with 42 mM HEEDTA/0.5 microM free Ca2+ caused the persistent activation of I(ClCa). Calculations using a Ca2+-diffusion model suggest that the voltage-gated Ca2+ channels and the Ca2+-activated Cl- channels are separated by 50-400 nm and that the RyRs are more than 600 nm from the plasma membrane.

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.

Oxygen induces electromechanical coupling in arteriolar smooth muscle cells: a role for L-type Ca2+ channels

We tested whether O2-induced vasomotor responses of arterioles correspond to changes in membrane potential (Em) of cells in the arteriolar wall. The cheek pouches of anesthetized male hamsters were prepared for intravital microscopy and intracellular recording. Microelectrodes containing Lucifer yellow dye were used to label smooth muscle cells (SMC) or endothelial cells (EC) during arteriolar responses to O2. During low- PO2 superfusion (approximately 20 Torr; arteriolar diameter 55 +/- 2 micron), Em of SMC and EC averaged -37 and -36 mV, respectively. High-PO2 superfusion ( approximately 150 Torr) depolarized SMC (to -15 +/- 1 mV) with vasoconstriction (to 24 +/- 2 micron) and diameter cycled with Em of SMC during vasomotion. In contrast, the Em of EC did not change with PO2 nor during vasomotion, yet Em depolarized by 21 +/- 2 mV when the extracellular K+ concentration ([K+]o) was raised to 55 mM. Superfusion with diltiazem (10 microM) or nifedipine (1 microM) abolished vasomotor and electrical responses to PO2 in SMC but did not eliminate depolarizations to elevated [K+]o. We conclude that, under physiological conditions, electrical and mechanical responses of arteriolar SMC to changes in PO2 are mediated through L-type Ca2+ channels without corresponding electrical activity in EC.

Calcium influx and transmitter release in a fast CNS synapse

Calcium entry through presynaptic calcium channels controls the release of neurotransmitter. It is not known whether the putative calcium sensor that triggers this rapid neurotransmitter release is close enough to be activated by the large increase in the Ca2+ concentration (calcium 'domain') reached within nanometres of a single calcium channel or whether many channels have to open. We tested this in a calyx-type synapse in the rat medial nucleus of the trapezoid body. We compared the quantal content of postsynaptic currents with the presynaptic calcium current that flows during an action potential, and the results suggest that more than 60 calcium channels open for each vesicle that is released. In addition, we dialysed terminals with the slow calcium buffer EGTA, which reduced phasic transmitter release at concentrations as low as 1 mM. These results indicate that the distance that calcium ions must diffuse to reach the calcium sensor is relatively long, and that therefore Ca2+ entry through multiple calcium channels is needed to release a vesicle.

Ca2+ currents in cerebral artery smooth muscle cells of rat at physiological Ca2+ concentrations

Single Ca2+ channel and whole cell currents were measured in smooth muscle cells dissociated from resistance-sized (100-microns diameter) rat cerebral arteries. We sought to quantify the magnitude of Ca2+ channel currents and activity under the putative physiological conditions of these cells: 2 mM [Ca2+]o, steady depolarizations to potentials between -50 and -20 mV, and (where possible) without extrinsic channel agonists. Single Ca2+ channel conductance was measured over a broad range of Ca2+ concentrations (0.5-80 mM). The saturating conductance ranged from 1.5 pS at 0.5 mM to 7.8 pS at 80 mM, with a value of 3.5 pS at 2 mM Ca (unitary currents of 0.18 pA at -40 mV). Both single channel and whole cell Ca2+ currents were measured during pulses and at steady holding potentials. Ca2+ channel open probability and the lower limit for the total number of channels per cell were estimated by dividing the whole-cell Ca2+ currents by the single channel current. We estimate that an average cell has at least 5,000 functional channels with open probabilities of 3.4 x 10(-4) and 2 x 10(-3) at -40 and -20 mV, respectively. An average of 1-10 (-40 mV and -20 mV, respectively) Ca2+ channels are thus open at physiological potentials, carrying approximately 0.5 pA steady Ca2+ current at -30 mV. We also observed a very slow reduction in open probability during steady test potentials when compared with peak pulse responses. This 4-10-fold reduction in activity could not be accounted for by the channel's normal inactivation at our recording potentials between -50 and -20 mV, implying that an additional slow inactivation process may be important in regulating Ca2+ channel activity during steady depolarization.

K+ currents in human coronary artery vascular smooth muscle cells

K+ channels and their currents are important in vascular tone regulation and are potential therapeutic targets; however, K+ channels in human coronary artery vascular smooth muscle cells (VSMCs) have received little attention. We examined K+ currents in freshly isolated VSMCs from human coronary arteries (n=368 from 32 human hearts) with conventional patch-clamp or perforated-patch techniques with nystatin. We detected four different K+ currents: (1) the delayed rectifier K+ current, IK(dr); (2) the Ca2+-activated K+ current, IK(Ca); (3) the nonrectifying noninactivating outward ATP-dependent K+ current, IK(ATP); and (4) the spontaneous transient outward K+ current, IK(STOC). K+ channels underlying spontaneous transient outward currents probably represent a single clustered population of Ca2+-activated K+ channels functionally associated with Ca2+ release channels in the sarcoplasmic reticulum. Inwardly rectifying K+ currents were not observed. K+ currents were unevenly distributed in that they were not uniformly exhibited by all cells. The most prominent K+ currents were IK(Ca) (100%) and IK(dr) (46%). IK(STOC)s, which have not been previously described in humans, were present in 67% of VSMCs. IK(ATP) was small under physiological conditions; however, IK(ATP) increased markedly after cell stimulation with exogenous or endogenous coronary vasodilators. Thus, IK(ATP) may be particularly relevant in ischemia and could be of special importance as a therapeutic target. We conclude that human coronary VSMCs have unique K+ currents that differ sufficiently from those of other species, thus making the investigation of human material clinically relevant. The findings suggest potential avenues for further therapeutic research.

Calcium channels in cerebral arteries

Electrophysiological evidence shows the existence of voltage-operated Ca2+ channels of the L- and, in some cases, T- and B-, type in the smooth muscle cells of major cerebral arteries and arterioles. Current intensity through L-type Ca2+ channels is higher in cerebral than in peripheral arteries, which points to a greater dependence on extracellular Ca2+ of contractile responses in cerebral arteries. The increase in cytosolic Ca2+ concentration is the key event leading both to maintenance of basal cerebrovascular tone and to contraction of cerebral arteries in response to depolarization and agonist-receptor interaction. Such an increase results from increased transmembrane influx of Ca2+ through L-type Ca2+ channels, as well as from the release of Ca2+ from intracellular Ca2+ stores. Ca2+ entry modulators (dihydropyridines, phenylalkylamines, benzothiazepines, and diphenylpiperazines) bind to allosterically coupled sites in the Ca2+ channel, thus inhibiting (Ca2+ entry blockers) or stimulating (Ca2+ entry activators) Ca2+ influx and, therefore, contractile responses of the cerebral arteries. In vivo, Ca2+ entry blockers increase pial vascular caliber and cerebral blood flow by their direct action on the cerebroarterial wall. However, such an action also takes place on several peripheral vascular beds, which leads to hypotension. Therefore, the brain cannot be considered a "privileged" organ when the vasodilatatory action of Ca2+ entry blockers is considered. Since increased cytosolic Ca2+ concentration (and, therefore, activation of Ca2+ channels) plays a crucial role in the pathogenesis of ischemic brain damage (e.g., acute stroke and subarachnoid hemorrhage), Ca2+ entry blockers could be useful cytoprotective drugs. However, with the exception of nimodipine in the management of subarachnoid hemorrhage, clinical trials have yielded results that are not so promising as one could expect from those obtained in experimental research.

Voltage window for sustained elevation of cytosolic calcium in smooth muscle cells

Action potentials activate voltage-dependent calcium channels and attendant increases in cytosolic calcium concentration ([Ca2+]i) in many excitable cells. The role of these channels in the regulation of [Ca2+]i in nonspiking cells that do not depolarize to membrane potentials sufficient to activate a substantial fraction of the available current is less clear. Measurements of the peak activation and steady-state inactivation of L-type calcium currents have predicted the existence of a noninactivating current window over a voltage range where channel inactivation is incomplete. The degree to which such small currents might regulate [Ca2+]i, however, has not been established. Here we demonstrate a "calcium window" in nondialyzed, quiescent smooth muscle cells over a small voltage range near the resting membrane potential. Sustained depolarizations in this voltage range, but not to more positive potentials, resulted in sustained rises in calcium, despite the fact that macroscopic inward currents were < 2 pA. The calcium window corresponded well with the predicted window current determined under the same conditions; the peak of the calcium window occurred at -30 mV, with steady-state rises in [Ca2+]i in some cells at -50 mV. Steady-state rises in [Ca2+]i following depolarization were completely blocked by nisoldipine and were augmented and shifted to more negative potentials by BAY K8644. Voltage-dependent calcium channels thus regulate steady-state calcium levels in nonspiking cells over a voltage range where macroscopic currents are only barely detectable. This voltage range is bounded at negative potentials by calcium channel activation and at more positive potentials by channel inactivation.

Calcium signaling in restricted diffusion spaces

One- and two-dimensional models of Ca2+ diffusion and regulation were developed and used to study the magnitudes and the spatial and temporal characteristics of the Ca2+ transients that are likely to develop in smooth muscle cells in restricted diffusion spaces between the plasma membrane and intracellular organelles. Simulations with the models showed that high [Ca2+] (on the order of several microM) can develop in such spaces and persist for 100-200 ms. These Ca2+ transients could: 1) facilitate the coupling of Ca2+ influx to intracellular Ca2+ release; 2) provide a mechanism for the regulation of stored Ca2+ that does not affect the contractile state of smooth muscle; 3) locally activate specific signal transduction pathways, before, or without activating other Ca2+ dependent pathways in the central cytoplasm of the cell. The latter possibility suggests that independent enzymatic processes in cells could be differentially regulated by the same intracellular second messenger.

The role of the membrane potential of endothelial and smooth muscle cells in the regulation of coronary blood flow

In the mammalian heart the supply of oxygen and energy-rich substrates through the coronary arterioles is continuously adapted to the variations of cardiac work. The coronary resistance arteries and the surrounding myocardium form a functional unit with multiple interactions between coronary endothelial cells, smooth muscle cells, perivascular nerves, and cardiac muscle cells. We describe the mechanisms underlying the electrical and chemical communication between the different cell types, the ionic channels contributing to the resting potential of endothelial and smooth muscle cells, and the mechanisms responsible for modulation of the resting potential. The main conclusion of our analysis is that the membrane potential of coronary endothelial and smooth muscle cells is one of the major determinants of coronary blood flow, and that modulation of the membrane potential provides a way to dilate or constrict coronary resistance arteries. It is proposed that the membrane potential of the myo-endothelial regulatory unit, i.e., of the endothelial cells and the underlying smooth muscle cells in the terminal arterioles, may function as an integrator of the numerous local and global vasodilator and constrictor signals that provide for the adaptation of coronary blood flow to the metabolic demands of the heart.

Calcium currents elicited by voltage steps and steady voltages in myocytes isolated from the rat basilar artery

1. Whole-cell patch clamp methods were used to record Ca2+ channel currents from isolated rat basilar arterial myocytes either in response to voltage steps or at steady holding potentials. Inward currents were rapidly and reversibly blocked by 2 mM Co2+, and this solution was routinely used to subtract leakage currents. 2. Peak currents measured in response to voltage steps were -85 +/- 23 pA (n = 7) in physiological Ca2+ (1.8 mM) and -256 +/- 39 pA (n = 19) in 10 mM Ba2+. The time course of activation and inactivation were unaffected by changing the holding potential from -88 to -48 mV, and thus we found no evidence for a component of current flowing through transient Ca2+ channels. 3. Activation curves were constructed by dividing the macroscopic current by the single Ca2+ channel current. The product of the number of functional channels and the open-state probability (NPo) measured in this way rose to around 2000. Its dependence on voltage was fitted by a Boltzmann function with half-activation at -8.6 mV in physiological Ca2+ concentrations and a steepness factor in the range 6-8 mV. 4. In physiological solution, block by external Mg2+ reduced peak current through Ca2+ channels by 17%. 5. Steady-state currents were measured by holding the cell at a fixed voltage and rapidly applying Co2+ to block the current through Ca2+ channels. Steady-state currents could be detected at voltages as negative as -58 mV, and persisted for more than 15 min, suggesting that Ca2+ channels in this artery may provide a steady-state Ca2+ influx that contributes to resting contractile tone.

Measuring kinetics of complex single ion channel data using mean-variance histograms

The measurement of single ion channel kinetics is difficult when those channels exhibit subconductance events. When the kinetics are fast, and when the current magnitudes are small, as is the case for Na+, Ca2+, and some K+ channels, these difficulties can lead to serious errors in the estimation of channel kinetics. I present here a method, based on the construction and analysis of mean-variance histograms, that can overcome these problems. A mean-variance histogram is constructed by calculating the mean current and the current variance within a brief "window" (a set of N consecutive data samples) superimposed on the digitized raw channel data. Systematic movement of this window over the data produces large numbers of mean-variance pairs which can be assembled into a two-dimensional histogram. Defined current levels (open, closed, or sublevel) appear in such plots as low variance regions. The total number of events in such low variance regions is estimated by curve fitting and plotted as a function of window width. This function decreases with the same time constants as the original dwell time probability distribution for each of the regions. The method can therefore be used: 1) to present a qualitative summary of the single channel data from which the signal-to-noise ratio, open channel noise, steadiness of the baseline, and number of conductance levels can be quickly determined; 2) to quantify the dwell time distribution in each of the levels exhibited. In this paper I present the analysis of a Na+ channel recording that had a number of complexities. The signal-to-noise ratio was only about 8 for the main open state, open channel noise, and fast flickers to other states were present, as were a substantial number of subconductance states. "Standard" half-amplitude threshold analysis of these data produce open and closed time histograms that were well fitted by the sum of two exponentials, but with apparently erroneous time constants, whereas the mean-variance histogram technique provided a more credible analysis of the open, closed, and subconductance times for the patch. I also show that the method produces accurate results on simulated data in a wide variety of conditions, whereas the half-amplitude method, when applied to complex simulated data shows the same errors as were apparent in the real data. The utility and the limitations of this new method are discussed.