L-type Ca-channels: similar Q10 of Ca-, Ba- and Na-conductance points to the importance of ion-channel interaction

The role of feedback and modulation in determining temperature resiliency in the lobster cardiac nervous system

Changes in ambient temperature affect all biological processes. However, these effects are process specific and often vary non-linearly. It is thus a non-trivial problem for neuronal circuits to maintain coordinated, functional output across a range of temperatures. The cardiac nervous systems in two species of decapod crustaceans, Homarus americanus and Cancer borealis, can maintain function across a wide but physiologically relevant temperature range. However, the processes that underlie temperature resilience in neuronal circuits and muscle systems are not fully understood. Here, we demonstrate that the non-isolated cardiac nervous system (i.e., the whole heart: neurons, effector organs, intrinsic feedback systems) in the American lobster, H. americanus, is more sensitive to warm temperatures than the isolated cardiac ganglion (CG) that controls the heartbeat. This was surprising as modulatory processes known to stabilize the output from the CG are absent when the ganglion is isolated. One source of inhibitory feedback in the intact cardiac neuromuscular system is nitric oxide (NO), which is released in response to heart contractions. We hypothesized that the greater temperature tolerance observed in the isolated CG is due to the absence of NO feedback. Here, we demonstrate that applying an NO donor to the isolated CG reduces its temperature tolerance. Similarly, we show that the NO synthase inhibitor L-nitroarginine (LNA) increases the temperature tolerance of the non-isolated nervous system. This is sufficient to explain differences in temperature tolerance between the isolated CG and the whole heart. However, in an intact lobster, the heart and CG are modulated by an array of endogenous peptides and hormones, many of which are positive regulators of the heartbeat. Many studies have demonstrated that excitatory modulators increase temperature resilience. However, this neuromuscular system is regulated by both excitatory and inhibitory peptide modulators. Perfusing SGRNFLRFamide, a FLRFamide-like peptide, through the heart increases the non-isolated nervous system’s tolerance to high temperatures. In contrast, perfusing myosuppressin, a peptide that negatively regulates the heartbeat frequency, decreases the temperature tolerance. Our data suggest that, in this nervous system, positive regulators of neural output increase temperature tolerance of the neuromuscular system, while modulators that decrease neural output decrease temperature tolerance.

Thermal modulation of epicardial Ca2+ dynamics uncovers molecular mechanisms of Ca2+ alternans

Ca²⁺ alternans (Ca-Alts) are alternating beat-to-beat changes in the amplitude of Ca²⁺ transients that frequently occur during tachycardia, ischemia, or hypothermia that can lead to sudden cardiac death. Ca-Alts appear to result from a variation in the amount of Ca²⁺ released from the sarcoplasmic reticulum (SR) between two consecutive heartbeats. This variable Ca²⁺ release has been attributed to the alternation of the action potential duration, delay in the recovery from inactivation of RYR Ca²⁺ release channel (RYR2), or an incomplete Ca²⁺ refilling of the SR. In all three cases, the RYR2 mobilizes less Ca²⁺ from the SR in an alternating manner, thereby generating an alternating profile of the Ca²⁺ transients. We used a new experimental approach, fluorescence local field optical mapping (FLOM), to record at the epicardial layer of an intact heart with subcellular resolution. In conjunction with a local cold finger, a series of images were recorded within an area where the local cooling induced a temperature gradient. Ca-Alts were larger in colder regions and occurred without changes in action potential duration. Analysis of the change in the enthalpy and Q₁₀ of several kinetic processes defining intracellular Ca²⁺ dynamics indicated that the effects of temperature change on the relaxation of intracellular Ca²⁺ transients involved both passive and active mechanisms. The steep temperature dependency of Ca-Alts during tachycardia suggests Ca-Alts are generated by insufficient SERCA-mediated Ca²⁺ uptake into the SR. We found that Ca-Alts are heavily dependent on intra-SR Ca²⁺ and can be promoted through partial pharmacologic inhibition of SERCA2a. Finally, the FLOM experimental approach has the potential to help us understand how arrhythmogenesis correlates with the spatial distribution of metabolically impaired myocytes along the myocardium.

Coupling between fast and slow oscillator circuits in Cancer borealis is temperature-compensated

Coupled oscillatory circuits are ubiquitous in nervous systems. Given that most biological processes are temperature-sensitive, it is remarkable that the neuronal circuits of poikilothermic animals can maintain coupling across a wide range of temperatures. Within the stomatogastric ganglion (STG) of the crab, Cancer borealis, the fast pyloric rhythm (~1 Hz) and the slow gastric mill rhythm (~0.1 Hz) are precisely coordinated at ~11°C such that there is an integer number of pyloric cycles per gastric mill cycle (integer coupling). Upon increasing temperature from 7°C to 23°C, both oscillators showed similar temperature-dependent increases in cycle frequency, and integer coupling between the circuits was conserved. Thus, although both rhythms show temperature-dependent changes in rhythm frequency, the processes that couple these circuits maintain their coordination over a wide range of temperatures. Such robustness to temperature changes could be part of a toolbox of processes that enables neural circuits to maintain function despite global perturbations.

How to Build a Fast and Highly Sensitive Sound Detector That Remains Robust to Temperature Shifts

Frogs must have sharp hearing abilities during the warm summer months to successfully find mating partners. This study aims to understand how frog hair cell ribbon-type synapses preserve both sensitivity and temporal precision during temperature changes. Under room (∼24°C) and high (∼32°C) temperature, we performed in vitro patch-clamp recordings of hair cells and their afferent fibers in amphibian papillae of either male or female bullfrogs. Afferent fibers exhibited a wide heterogeneity in membrane input resistance (R_in) from 100 mΩ to 1000 mΩ, which may contribute to variations in spike threshold and firing frequency. At higher temperatures, most fibers increased their frequency of spike firing due to an increase in spontaneous EPSC frequencies. Hair cell resting membrane potential (V_rest) remained surprisingly stable during temperature increases, because Ca²⁺ influx and K⁺ outflux increased simultaneously. This increase in Ca²⁺ current likely enhanced spontaneous EPSC frequencies. These larger "leak currents" at V_rest also lowered R_in and produced higher electrical resonant frequencies. Lowering R_in will reduce the hair cells receptor potential and presumably moderate the systems sensitivity. Using membrane capacitance measurements, we suggest that hair cells can partially compensate for this reduced sensitivity by increasing exocytosis efficiency and the size of the readily releasable pool of synaptic vesicles. Furthermore, paired recordings of hair cells and their afferent fibers showed that synaptic delays shortened and multivesicular release becomes more synchronous at higher temperatures, which should improve temporal precision. Together, our results explain many previous in vivo observations on the temperature dependence of spikes in auditory nerves.SIGNIFICANCE STATEMENT The vertebrate inner ear detects and transmits auditory information over a broad dynamic range of sound frequency and intensity. It achieves remarkable sensitivity to soft sounds and precise frequency selectivity. How does the ear of cold-blooded vertebrates maintain its performance level as temperature changes? More specifically, how does the hair cell to afferent fiber synapse in bullfrog amphibian papilla adjust to a wide range of physiological temperatures without losing its sensitivity and temporal fidelity to sound signals? This study uses in vitro experiments to reveal the biophysical mechanisms that explain many observations made from in vivo auditory nerve fiber recordings. We find that higher temperature facilitates vesicle exocytosis and electrical tuning to higher sound frequencies, which benefits sensitivity and selectivity.

Differential contribution of calcium channels to α1-adrenoceptor-mediated contraction is responsible for diverse responses to cooling between rat tail and iliac arteries

Our previous studies have shown that α₁-adrenoceptors, in addition to α₂-adrenoceptors, are involved in enhanced contraction of cutaneous blood vessels during cooling. The present study aimed to elucidate the mechanism underlying it. In tail and iliac arteries isolated from rats, isometric contraction was measured using a myograph and the phosphorylation level of myosin phosphatase target subunit 1 (MYPT1) was quantified by western blotting. The phenylephrine-induced contraction was enhanced by cooling to 24 °C in tail arteries, but was suppressed in iliac arteries. Endothelium denudation or treatment with iberiotoxin enhanced the phenylephrine-induced contraction in tail arteries at 37 °C; however, neither affected the contraction at 24 °C. The phenylephrine-induced contraction at 37 °C was largely suppressed by nifedipine in iliac arteries, but only slightly in tail arteries. The Rho kinase inhibitor H-1152 largely suppressed the phenylephrine-induced contraction at 24 °C, but only slightly at 37 °C, in both arteries. The phosphorylation level of MYPT1 at Thr855 in tail arteries was increased by the cooling. Taken together, these results suggest the following mechanism in regard to cooling-induced enhancement of α₁-adrenoceptor-mediated contraction in tail arteries: Cooling enhances the contraction of tail arteries via α₁-adrenoceptor stimulation by reducing endothelium-dependent, large-conductance Ca²⁺-activated K⁺ channel-mediated relaxation and by inducing Rho kinase-mediated Ca²⁺ sensitization, although the latter occurs even in iliac arteries. A smaller contribution of voltage-dependent Ca²⁺ channels, which are largely suppressed by cooling, to α₁-adrenoceptor-mediated contraction in tail arteries seems to be more crucially involved in the appearance of the enhanced contractile response to cooling.

Temperature-Related Reaction Norms of Gene Expression: Regulatory Architecture and Functional Implications

The environment has profound effects on the expression of many traits and reaction norms describe the expression dynamics of a trait across a broad range of environmental conditions. Here, we analyze gene expression in Drosophila melanogaster across four different developmental temperatures (13-29 °C). Gene expression is highly plastic with 83.3% of the genes being differentially expressed. We distinguished three components of plasticity: 1) Dynamics of gene expression intensity (sum of change), 2) direction of change, and 3) curvature of the reaction norm (linear vs. quadratic). Studying their regulatory architecture we found that all three plasticity components were most strongly affected by the number of different transcription factors (TFs) binding to the target gene. More TFs were found in genes with less expression changes across temperatures. Although the effect of microRNAs was weaker, we consistently noted a trend in the opposite direction. The most plastic genes were regulated by fewer TFs and more microRNAs than less plastic genes. Different patterns of plasticity were also reflected by their functional characterization based on gene ontology. Our results suggest that reaction norms provide an important key to understand the functional requirements of natural populations exposed to variable environmental conditions.

Intracellular Ca2+ release underlies the development of phase 2 in mouse ventricular action potentials

The ventricular action potential (AP) is characterized by a fast depolarizing phase followed by a repolarization that displays a second upstroke known as phase 2. This phase is generally not present in mouse ventricular myocytes. Thus we performed colocalized electrophysiological and optical recordings of APs in Langendorff-perfused mouse hearts founding a noticeable phase 2. Ryanodine as well as nifedipine reduced phase 2. Our hypothesis is that a depolarizing current activated by Ca(2+) released from the sarcoplasmic reticulum (SR) rather than the "electrogenicity" of the L-type Ca(2+) current is crucial in the generation of mouse ventricular phase 2. When Na(+) was partially replaced by Li(+) in the extracellular perfusate or the organ was cooled down, phase 2 was reduced. These results suggest that the Na(+)/Ca(2+) exchanger functioning in the forward mode is driving the depolarizing current that defines phase 2. Phase 2 appears to be an intrinsic characteristic of single isolated myocytes and not an emergent property of the tissue. As in whole heart experiments, ventricular myocytes impaled with microelectrodes displayed a large phase 2 that significantly increases when temperature was raised from 22 to 37°C. We conclude that mouse ventricular APs display a phase 2; however, changes in Ca(2+) dynamics and thermodynamic parameters also diminish phase 2, mostly by impairing the Na(+)/Ca(2+) exchanger. In summary, these results provide important insights about the role of Ca(2+) release in AP ventricular repolarization under physiological and pathological conditions.

Diurnal in vivo and rapid in vitro effects of estradiol on voltage-gated calcium channels in gonadotropin-releasing hormone neurons

A robust surge of gonadotropin-releasing hormone (GnRH) release triggers the luteinizing hormone surge that induces ovulation. The GnRH surge is attributable to estradiol feedback, but the mechanisms are incompletely understood. Voltage-gated calcium channels (VGCCs) regulate hormone release and neuronal excitability, and may be part of the surge-generating mechanism. We examined VGCCs of GnRH neurons in brain slices from a model exhibiting daily luteinizing hormone surges. Mice were ovariectomized (OVX), and a subset was treated with estradiol implants (OVX+E). OVX+E mice exhibit negative feedback in the A.M. and positive feedback in the P.M. GnRH neurons express prominent high-voltage-activated (HVA) and small low-voltage-activated (LVA) macroscopic (whole-cell) Ca currents (I(Ca)). LVA-mediated currents were not altered by estradiol or time of day. In contrast, in OVX+E mice, HVA-mediated currents varied with time of day; HVA currents in cells from OVX+E mice were lower than those in cells from OVX mice in the A.M. but were higher in the P.M. These changes were attributable to diurnal alternations in L- and N-type components. There were no diurnal changes in any aspect of HVA-mediated I(Ca) in OVX mice. Acute in vitro treatment of cells from OVX and OVX+E mice with estradiol rapidly increased HVA currents primarily through L- and R-type VGCCs by activating estrogen receptor beta and GPR30, respectively. These results suggest multiple mechanisms contribute to the overall feedback regulation of HVA-mediated I(Ca) by estradiol. In combination with changes in synaptic inputs to GnRH neurons, these intrinsic changes in GnRH neurons may play critical roles in estradiol feedback.

Mechanisms and distribution of ion channels in retinal ganglion cells: using temperature as an independent variable

Trains of action potentials of rat and cat retinal ganglion cells (RGCs) were recorded intracellularly across a temperature range of 7-37 degrees C. Phase plots of the experimental impulse trains were precision fit using multicompartment simulations of anatomically reconstructed rat and cat RGCs. Action potential excitation was simulated with a "Five-channel model" [Na, K(delayed rectifier), Ca, K(A), and K(Ca-activated) channels] and the nonspace-clamped condition of the whole cell recording was exploited to determine the channels' distribution on the dendrites, soma, and proximal axon. At each temperature, optimal phase-plot fits for RGCs occurred with the same unique channel distribution. The "waveform" of the electrotonic current was found to be temperature dependent, which reflected the shape changes in the experimental action potentials and confirmed the channel distributions. The distributions are cell-type specific and adequate for soma and dendritic excitation with a safety margin. The highest Na-channel density was found on an axonal segment some 50-130 microm distal to the soma, as determined from the temperature-dependent "initial segment-somadendritic (IS-SD) break." The voltage dependence of the gating rate constants remains invariant between 7 and 23 degrees C and between 30 and 37 degrees C, but undergoes a transition between 23 and 30 degrees C. Both gating-kinetic and ion-permeability Q10s remain virtually constant between 23 and 37 degrees C (kinetic Q10s = 1.9-1.95; permeability Q10s = 1.49-1.64). The Q10s systematically increase for T <23 degrees C (kinetic Q10 = 8 at T = 8 degrees C). The Na channels were consistently "sleepy" (non-Arrhenius) for T <8 degrees C, with a loss of spiking for T <7 degrees C.

Effects of temperature on pacemaker potentials in the mouse small intestine

The effects of temperature on the generation of pacemaker potentials recorded from myenteric interstitial cells of Cajal (ICC-MY) distributed in the mouse small intestine were investigated using intracellular recording techniques. In response to increasing temperatures in the range of 26-40 degrees C, the frequency and maximum rate of rise (dV/dt (max)) of pacemaker potentials were increased while their duration was decreased. The resting membrane potential and amplitude of the pacemaker potentials were not affected by change in temperature. Elevation of temperature decreased the amplitude, duration, and rise time of unitary potentials generated spontaneously during intervals between the pacemaker potentials. Metabolic inhibition (KCN and iodoacetic acid) decreased the frequency of pacemaker potentials with no alteration to the amplitude and dV/dt (max). Cyclopiazonic acid (3 muM), an inhibitor of the internal Ca(2+) pump, abolished pacemaker potentials in low-temperature conditions (<29 degrees C) but not at high-temperature conditions (>38 degrees C). These results suggest that the primary and plateau components of pacemaker potentials have different temperature sensitivities: the primary component is highly temperature-sensitive and is activated at higher temperatures, while the plateau component is formed by activation of temperature-insensitive mechanisms. The results also suggest that the mitochondria-induced intracellular Ca(2+) handling system seems to be involved in the initiation of the generation of pacemaker potentials but not in their configuration.

Voltage-dependent calcium entry underlies propagation of slow waves in canine gastric antrum

Electrical slow waves in gastrointestinal (GI) muscles are generated by interstitial cells of Cajal (ICC), and these events actively propagate through networks of ICC within the walls of GI organs. The mechanism by which spontaneously active pacemaker sites throughout ICC networks are entrained to produce orderly propagation of slow waves is unresolved. A three-chambered partition bath was used to test the effects of agents that affect metabolism, membrane potential and voltage-dependent Ca(2+) entry on slow wave propagation in canine antral smooth muscle strips. Slow waves evoked by electrical field stimulation actively propagated from end to end of antral muscle strips with a constant latency between two points of recording. When the central chamber of the bath was perfused with low-temperature solutions, mitochondrial inhibitors, reduced extracellular Ca(2+) or blockers of voltage-dependent Ca(2+) channels, active propagation failed. Depolarization or hyperpolarization of the tissue within the central chamber also blocked propagation. Blockade of propagation by reduced extracellular Ca(2+) and inhibitors of dihydropyridine-resistant Ca(2+) channels suggests that voltage-dependent Ca(2+) entry may be the 'entrainment factor' that facilitates active propagation of slow waves in the gastric antrum.

Synaptic modulation by a neuropeptide depends on temperature and extracellular calcium

The crayfish neuropeptide DRNFLRFamide increases transmitter release from synaptic terminals onto muscle cells. As temperature decreases from 20 to 8 degrees C, the size of excitatory junctional potentials (EJPs) decreases, and the peptide becomes more effective at increasing EJP amplitude. The goal of the present study was to determine whether the enhanced effectiveness of the peptide is strictly a temperature-related effect, or whether it is related to the fact that the EJPs are smaller at low temperature, allowing a greater range for EJP amplitude to increase. Decreasing temperature reduced the number of quanta of transmitter released per nerve impulse (assessed by recording synaptic currents) and increased input resistance in muscle fibers. As in earlier work, the ability of the peptide to increase EJP amplitude was enhanced by decreasing temperature. However, the peptide was also more effective at increasing EJP amplitude when transmitter output was lowered by reducing the ratio of calcium to magnesium ions in the bath. Thus the effectiveness of the peptide may be related to the level of output from the synaptic terminals.

Temperature-sensitive intracellular Mg2+ block of L-type Ca2+ channels in cardiac myocytes

We examined the concentration-dependent blocking effects of intracellular Mg2+ on L-type Ca2+ channels in cardiac myocytes using the whole cell patch-clamp technique. The increase of L-type Ca2+ channel current (I(Ca)) (due to relief of Mg2+ block) occurred in two temporal phases. The rapid phase (runup) transiently appeared early (<5 min) in dialysis of the low-Mg2+ solution; the slow phase began later in dialysis (>10 min). Runup was not blocked by intracellular GTP (GTP(i)). The late phase of the I(Ca) increase (late I(Ca)) was suppressed by GTP(i) (0.4 mM) and was observed in myocytes of the guinea pig or frog at higher (32 or 24 degrees C, respectively) rather than lower temperatures (24 or 17.5 degrees C, respectively). At pMg = 6.0, raising the temperature from 24 to 32 degrees C evoked late I(Ca) with a Q10 of 14.5. Restoring the temperature to 24 degrees C decreased I(Ca) with a Q10 of only 2.4. The marked difference in the Q10 values indicated that late I(Ca) (pMg = 5-6) is an irreversible phenomenon. Phosphorylation suppressed the intracellular [Mg2+] dependency of late I(Ca). This effect of phosphorylation together with the inhibitory action of GTP(i) on Mg2+-dependent blocking of I(Ca) are common properties of mammalian and amphibian cardiomyocytes.

Temperature dependence of macroscopic L-type calcium channel currents in single guinea pig ventricular myocytes

INTRODUCTION

Lowering temperature greatly reduces calcium influx through calcium channels. Studies on a number of tissues demonstrate that the peak inward current, ICa, exhibits Q10 values ranging from 1.8 to 3.5; however, it remains unclear which component(s) of calcium channel gating may give rise to this large temperature sensitivity. Components of gating that may affect channel availability include phosphorylation and changes in [Ca2+]i, processes that vary in pertinence depending on the channel examined. This study addresses this problem by examining the temperature sensitivity (from 34 degrees to 14 degrees C) of cardiac ICa under control conditions, during attenuation or activation of protein kinase A (PKA) activity, and when intracellular [Ca2+] has been elevated.

METHODS AND RESULTS

ICa was studied using the whole cell configuration of the patch champ technique. In control, lowering temperature from 34 degrees to 24 degrees C resulted in a shift in the potential for maximum slope (Va) and the peak current (Ymax) toward more positive membrane potentials. The Q10 values for the decrease in Ymax and the macroscopic slope conductance (Gmax), which reflects the number of available channels, were 3.15 +/- 0.19 and 2.57 +/- 0.13, respectively. At 0 mV the Ca2+ current decayed biexponentially, and the two time constants (tau 1 and tau 2) showed Q10 values of 1.79 +/- 0.21 and 2.06 +/- 0.38, while their contribution to the total current (I1 and I2) showed a Q10 of 5.99 +/- 0.83 and 1.61 +/- 0.22. In myocytes loaded with inhibitors of the PKA cycle sufficient to inhibit the increase of ICa to 1 microM isoprenaline, the Q10 values for some of the kinetic parameters were increased with the Q10 for I1 increasing to 17.06 +/- 3.48. Stimulation of ICa by exposing myocytes to 1 microM isoprenaline reduced the temperature sensitivity of Ymax, Gmax and I1, yielding respective values of 2.00 +/- 0.18, 1.85 +/- 0.07, and 2.04 +/- 0.15. Raising [Ca2+]i to enhance Ca2+i-dependent inactivation, while affecting inactivation and activation kinetics, affected temperature sensitivity little compared to control. The Q10 for time to peak changed little under experimental conditions (2.3 to 2.4)

CONCLUSIONS

Increasing the phosphorylated states of calcium channels, but not Ca2+i-dependent inactivation, reduces temperature sensitivity of certain gating parameters. The data suggest that the rate of the transitions between the unavailable and also between the various closed states are changed in the opposite direction to that induced by PKA-dependent phosphorylation. Processes, e.g., inhibitory mechanisms, may be involved to maintain channels in unavailable or "unphosphorylated" states, and it may be these that contribute to the high Q10 of macroscopic channel currents.

Stretch effects on whole-cell currents of guinea-pig urinary bladder myocytes

1. By means of two patch-pipettes, isolated urinary bladder myocytes were longitudinally stretched up to 20% beyond slack length (delta L = 20%). 2. Experiments were conducted using both voltage and current clamp configurations. In current clamped cells at 23 degrees C, delta L depolarized the membrane from -50 to ca -15 mV, the amplitude of depolarization increasing with the extent of delta L. At 36 degrees C, delta L induced action potentials or increased the frequency of spontaneous action potentials. 3. In voltage clamped cells at a holding potential of -50 mV, stretch induced an inward current (Iin) and increased the input conductance. Both effects increased with delta L. They were blocked by 40 microM gadolinium, suggesting stretch activation of non-selective cation channels (SACs) as the underlying mechanism. 4. Stretch-induced difference currents rectified outwardly and reversed at a reversal potential (Erev) of -28 +/- 10 mV. Twenty millimolar [TEA]o suppressed the rectification and shifted Erev to 0 +/- 1 mV. The result suggests that stretch can activate not only SACs but also TEA-sensitive K+ channels. 5. Stretch changed the net current due to clamp steps to 0 mV as though it increased the potassium current (IK) and reduced the calcium current (ICa). While 20 mM intracellular BAPTA did not modify the stretch-induced whole-cell inward current (Iin) at -50 mV, it suppressed the stretch effects on IK and ICa as if these effects were mediated by an increase in the subsarcolemmal Ca2+ concentration. 6. The results support the hypothesis that longitudinal stretch can activate SACs and Ca2+ influx through them. In non-clamped cells, stretch can also modulate Ca2+ influx through L-type Ca2+ channels via changes in membrane potential.

Ca channel kinetics during the spontaneous heart beat in embryonic chick ventricle cells

The ability of Ca ions to inhibit Ca channels presents one of the most intriguing problems in membrane biophysics. Because of this negative feedback, Ca channels can regulate the current that flows through them. The kinetics of the channels depend on voltage, and, because the voltage controls the current, a strong interaction exists between voltage dependence and Ca dependence. In addition to this interaction, the proximity of pores and the local concentration of ions also determine how effectively the Ca ions influence channel kinetics. The present article proposes a model that incorporates voltage-dependent kinetics, current-dependent kinetics, and channel clustering. We have based the model on previous voltage-clamp data and on Ca and Ba action currents measured during the action potential in beating heart cells. In general we observe that great variability exists in channel kinetics from patch to patch: Ba or Ca currents have low or high amplitudes and slow or fast kinetics during essentially the same voltage regime, either applied step-protocols or spontaneous cell action potentials. To explain this variability, we have postulated that Ca channels interact through shared ions. The model we propose expands on our previous model for Ba currents. We use the same voltage-dependent rate constants for the Ca currents that we did for the Ba currents. However, we vary the current-dependent rate constants according to the species of the conducting ion. The model reproduces the main features of our data, and we use it to predict Ca channel kinetics under physiological conditions. Preliminary reports of this work have appeared (DeFelice et al., 1991, Biophys. J. 59:551a; Risso et al., 1992, Biophys. J. 61:248a).

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

Entry of Ca through voltage-dependent Ca channels is an important regulator of the function of smooth muscle, cardiac muscle, and neurons. Although Ca channels have been extensively studied since the first descriptions of Ca action potentials (P. Fatt and B. Katz. J. Physiol. Lond. 120: 171-204, 1953), the permeation rate of Ca through single Ca channels has not been measured directly under physiological conditions. Instead, single Ca channels have typically been examined using high concentrations (80-110 mM) of another divalent charge carrier, Ba, so as to maximize the amplitude of the single-channel currents. Calculations of unitary currents at 2 mM Ca indicated that the single-channel currents would be immeasurably small (i.e., < 0.1 pA). We provide here the first direct measurements of single Ca channel currents at a physiological Ca concentration. Contrary to earlier estimates, we have found that currents through single Ca channels in arterial smooth muscle are 0.1-0.3 pA at 2 mM Ca and physiological membrane potentials. These relatively large unitary currents permit direct measurement of Ca channel properties under conditions that do not distort their function. Our data also indicate that Ca permeates these channels at relatively high rates in physiological Ca concentrations and membrane potentials.

Ca2+ influx through ATP-gated channels increments [Ca2+]i and inactivates ICa in myocytes from guinea-pig urinary bladder

1. Whole-cell patch clamp was combined with microspectrofluometry (Indo-1) to study the effects of bath applied ATP on membrane currents and cytoplasmic Ca2+ concentration ([Ca2+]i) in single smooth muscle cells of the guinea-pig urinary bladder. Experiments were carried out at 22 degrees C and in 3.6 mM [Ca2+]o. Superimposed K+ currents were reduced by Cs+ dialysis from the patch electrode. 2. At -60 mV, ATP induced an inward current (Ins,ATP) that peaked within 0.4 s and then decayed. Ins,ATP was activated half-maximally by 1.1 microM-ATP and saturated at 50 microM-ATP to -1.1 +/- 0.2 nA (mean +/- S.E.M.). At 3.6 mM [Ca2+]o, Ins,ATP had a reversal potential (Erev) of -5 +/- 2 mV. From the shifts in Erev during changes in [Na+]o or [Ca2+]o we estimated that approximately 7% of Ins,ATP is carried by Ca2+ ions. 3. ATP (50 microM) increased [Ca2+]i transiently from resting 130 +/- 40 nM to 730 +/- 100 nM. At 22 degrees C, [Ca2+]i rose at a rate proportional to the instantaneous current amplitude of Ins,ATP. This relation was lost, however, after warming to 36 degrees C which increased the peak Ins,ATP (Q10 = 1.25) but reduced the peak of the ATP induced [Ca2+]i transient (Q10 = 0.75). We suggest that warming to 36 degrees C stimulated Ca2+ sequestration and Ca2+ efflux to such a degree that peak [Ca2+]i was attenuated significantly. 4. The contribution of Ca2+ ions to Ins,ATP was evaluated from a comparison of the increments in [Ca2+]i due to Ins,ATP and due to L-type Ca2+ channel current (ICa). For the same increment, Ins,ATP had to transport 19 times more charge than ICa. This number suggests that 5.8 +/- 0.8% of Ins,ATP is carried by Ca2+ ions which can be translated into a permeability ratio of PNa:PCa approximately 1:1. 5. During bath application of ATP, peak ICa was inhibited by 80 +/- 15%. Inhibition of ICa diminished to 20 +/- 8% after cell dialysis with 40 mM-EGTA, and it was 19 +/- 7% when extracellular Ca2+ had been substituted by Ba2+. These results are in agreement with the hypothesis of 'ICa inactivation by Ca2+'. Depletion of intracellular Ca2+ stores by pre-treatment with 20 mM-caffeine did not attenuate significantly the ATP-induced rise in [Ca2+]i or the ATP-induced inhibition of ICa. 6. The ATP-induced [Ca2+]i transients and the reduction of peak ICa recovered along a similar time course.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of temperature on the rise in intracellular sodium caused by calcium depletion in ferret ventricular muscle and the mechanism of the alleviation of the calcium paradox by hypothermia

The effects of temperature over the range 37-10 degrees C on the responses of isolated ferret ventricular trabeculae to the depletion and repletion of bathing calcium has been investigated. Cooling is found to reduce the rate of rise of intracellular sodium activity (measured with an ion-sensitive microelectrode) induced by depletion of bathing divalent cations, without affecting the prolonged depolarization. The rate of rise of [Na]i (with the sodium pump inhibited) shows a dependence on temperature (energy of activation, 67 kJ.mol-1; Q10, 2.3-2.8) that increases with increasing temperature. This contrasts with previously published data for the sodium pump for which the temperature dependence falls with increasing temperature. These results offer an explanation for the alleviation of the calcium paradox caused by cooling during the period of calcium depletion, which is similar to other procedures that offer protection by limiting the rise in [Na]i by having effects on the L-type calcium channel.