Opening of dihydropyridine calcium channels in skeletal muscle membranes by inositol trisphosphate

Malignant hyperthermia

Malignant hyperthermia (MH) is an uncommon, life-threatening, acute pharmacogenetic disorder of the skeletal muscle cell. It manifests in susceptible individuals as a hypermetabolic response on exposure to halogenated volatile anaesthetics and depolarizing muscle relaxants. There may also be a relationship between susceptibility to MH, heat stroke and exercise-induced rhabdomyolysis. The pathophysiology of the crisis involves an uncontrolled release of cytoplasmic free calcium from the sarcoplasmic reticulum leading to activation of energy-producing biochemical pathways. Organ system failure and rhabdomyolysis may occur as a result of high fever, hyperkalaemia and acidosis. The ryanodine receptor, the calcium-release channel of the sarcoplasmic reticulum, is the primary locus for malignant hypothermia susceptibility. Multiple mutations in the gene for the ryanodine receptor protein are causative. Other genes may also be involved. A classical fulminant crisis presents with a rising end-tidal carbon dioxide, skeletal muscle rigidity, tachycardia, hyperthermia and acidosis. Mortality may be as high as 70% if the syndrome is not recognized and treated. Immediate discontinuation of triggering agents, oxygenation, and correction of acidosis and electrolyte abnormalities, cooling and dantrolene are essential for treatment of the syndrome. Thanks to clinical and research investigations, widespread education and the introduction of dantrolene sodium, the mortality from MH is less than 5%. This chapter provides an overview and an update of MH.

Activation of a Large-conductance Ca2+-Dependent K+ Channel by Stimulation of Glutamate Phosphoinositide-coupled Receptors in Cultured Cerebellar Granule Cells

Trans-1-amino-cyclopentyl-1,3-dicarboxylic acid (trans-ACPD), a specific agonist of the glutamate phosphoinositide-coupled receptor (Qp receptor), increased the amplitude of the outward K+ current recorded in the whole-cell configuration of the patch-clamp technique in mouse cultured cerebellar granule cells. This effect was abolished by buffering internal Ca2+ with BAPTA [1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid]. Activation of a large-conductance K+ channel was observed when trans-ACPD or quisqualic acid (QA), another Qp receptor agonist, was applied outside the cell-attached patch pipettes. No activation was observed with alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), a specific agonist of ionotropic non-N-methyl-d-aspartate (non-NMDA) receptors. The effects of trans-ACPD or QA were potentiated in the presence of external Ca2+. The channel was also directly activated by both micromolar concentrations of internal Ca2+ and membrane depolarization. Its unitary conductance was 100 - 115 pS under asymmetrical K+ and 195 - 235 pS under high symmetrical K+ conditions. In the absence of agonist, the channel was blocked by 1 mM external tetraethylammonium. This is the first description of a large conductance Ca2+-activated K+ channel in cultured cerebellar granule cells. It possesses properties similar to those of the so-called 'big K+ channel' described in other preparations. Our cell-attached experiments demonstrated an indirect coupling between Qp receptors and this channel. The most likely hypothesis is that the second messenger system inositol 1,4,5-triphosphate (IP3)-Ca2+ was involved in the coupling process. This hypothesis was further strengthened by our whole-cell experiments. On the basis of the voltage- and Ca2+-sensitivities of the studied channel, we estimated an increase of 350 to 570 nM in internal Ca2+ concentration when Qp receptors were stimulated by 100 microM trans-ACPD. Under physiological conditions, stimulation of Qp receptors by the endogenous neurotransmitter should lead to similar K+ channel activation and therefore would tend to reduce the efficacy of ionotropic glutamate synaptic receptor stimulation responsible for cell excitation.

Regulation of splanchnic perfusion

In splanchnic arteries, vasodilators decrease the cytosolic free calcium concentration ([Ca(2+)]i) by inducing Ca(2+)sequestration and extrusion, by limiting the entry of extracellular Ca(2+)via L-type Ca(2+)channels and by decreasing the agonist-induced mobilization of intracellular Ca(2+). Cyclic GMP kinase and membrane hyperpolarization are important mediators of the decrease in [Ca(2+)]i. Vasoconstrictors increase [Ca(2+)]i by mobilizing intracellular Ca(2+)from the sarcoplasmic reticulum and by stimulating extracellular Ca(2+)entry via L-type Ca(2+)channels. Activation of G proteins, inositol trisphosphate, diacylglycerol and membrane depolarization are important mediators of the increase in [Ca(2+)]i.

Ts65Dn mouse, a Down syndrome model, exhibits elevated myo-inositol in selected brain regions and peripheral tissues

myo-Inositol is elevated in the Down syndrome (DS; trisomy 21) brain and may play a role in mental retardation. In the present study, we examined brain regions and peripheral tissues of Ts65Dn mouse, a recently characterized genetic model of DS, for abnormal myo-inositol accumulation. A GC/MS technique was used to quantitate myo-inositol and other polyol species (ribitol, arabitol, xylitol, and 1,5-anhydrosorbitol) in tissues from the Ts65Dn mice and control diploid mice. myo-Inositol was found to be elevated in frontal cortex, hippocampus, and brain stem but not in cerebellum of the Ts65Dn mouse. Among peripheral organs examined, liver and skeletal muscle were found to excessively accumulate myo-inositol. In all tissues, concentrations of polyol internal controls were normal. The Ts65Dn mouse is useful to study the possible effect of elevated myo-inositol on cellular processes.

ANG II and vasopressin stimulate calcium entry in dispersed smooth muscle cells of preglomerular arterioles

Calcium signaling mechanisms were examined in vessel segments and dispersed single smooth muscle cells (SMC) of interlobular arteries and afferent arterioles (< 50 microns diameter) from the rat kidney. These resistance vessels were isolated from rat kidneys, using an iron oxide-sieving technique with subsequent collagenase digestion. Individual cells were identified by their characteristic oval appearance and positive staining for smooth muscle-specific alpha-actin and heavy chain myosin SM-1 and SM-2. Cytosolic calcium concentration ([Ca2+]i) was measured using fura 2 ratiometric fluorescence at 340 and 380 nm wavelength with a microscope-based photometer. Angiotensin II (ANG II) and arginine vasopressin (AVP), at concentrations of 10(-10)-10(-6) M, produced dose-dependent increases in [Ca2+]i; maximum increases were 221 +/- 49 nM for ANG II and 237 +/- 49 nM for AVP. The temporal response patterns for both agonists were characterized by a square-shaped, immediate step increase in [Ca2+]i to a near maximum level that was maintained through the recording period of 150-200 s. Responses of individual dispersed SMC and short vessel segments were similar. Losartan antagonized the action of ANG II, indicating mediation by AT1 receptors on preglomerular arteriolar SMC. The V1-selective antagonist [d(CH2)5Tyr(Me)2Tyr(NH2)9]AVP completely inhibited AVP-induced [Ca2+]i changes. The importance of calcium entry in hormone-induced changes in [Ca2+]i was demonstrated by the finding that neither ANG II nor AVP elicited a [Ca2+]i response in media rendered nominally calcium free by addition of ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. Calcium entry occurred primarily through L-type, voltage-gated calcium channels as the dihydropyridine, nifedipine, completely prevented or reversed [Ca2+]i changes normally elicited by either hormone. Our results provide new information about the similarity of calcium signaling in single SMC and short segments freshly isolated from renal interlobular arteries and afferent arterioles. The observations indicate that AT1 and V1 receptors are coupled to signal transduction pathways leading to rapid changes in [Ca2+]i. Calcium mobilization appears to play a minor to nonexistent role under the experimental conditions. The predominant mechanism involves calcium entry through dihydropyridine-sensitive, voltage-gated calcium channels in single SMC from these resistance vessels.

Investigation of the effect of inositol trisphosphate in skinned skeletal muscle fibres with functional excitation-contraction coupling

The effect of inositol trisphosphate (IP3) was investigated in mechanically skinned fibres which had the endogenous level of sarcoplasmic reticulum (SR) Ca2+ and in which the normal excitation-contraction (E-C) coupling mechanism was still functional. Application of 50 or 100 microM IP3 failed to induce a detectable force response in any such skinned fibre from either the extensor digitorum longus muscle of the rat or iliofibularis muscle of the toad, irrespective of whether the fibre was: (a) in its normally polarized, resting state; (b) chronically depolarized to inactivate the voltage sensors; (c) paralysed with D600; or (d) depolarized to threshold for force activation. Furthermore, the size of the response to subsequent depolarization or exposure to caffeine (2mM) or reduced myoplasmic [Mg2+] indicated that little if any Ca2+ had been lost from the SR during the period of IP3 exposure (> or = 1 min). Also, IP3 did not induce a detectable force response when SR Ca2+ uptake was potently inhibited with 20 microM TBQ. Exposure to IP3 (50 microM) slightly potentiated the peak force response to depolarization in toad fibres, and this was probably because of an accompanying small increase in Ca2+ sensitivity of the contractile apparatus. These results appear inconsistent with the proposal that IP3 acts as the second messenger in E-C coupling in skeletal muscle.

Inositol 1,4,5-trisphosphate-induced Ca2+ release is regulated by cytosolic Ca2+ in intact skeletal muscle

Microinjection of inositol 1,4,5-trisphosphate (InsP3) into intact skeletal muscle fibers isolated from frogs (Rana temporaria) increased resting cytosolic Ca2+ concentration ([Ca2+]i) as measured by double-barreled Ca2+-selective microelectrodes. In contrast, microinjection of inositol 1-phosphate, inositol 1,4-biphosphate, and inositol 1,4,5,6-tetrakisphosphate did not induce changes in [Ca2+]i. Incubation in low-Ca2+ solution, or in the presence of L-type Ca2+ channel blockers did not affect InsP3-induced release of cytosolic Ca2+. Neither ruthenium red, a blocker of ryanodine receptor Ca2+-release channels, nor cytosolic Mg2+, a known inhibitor of the Ca2+-induced Ca2+-release process, modified the InsP3-induced release of cytosolic Ca2+. However, heparin, a blocker of InsP3 receptors, inhibited InsP3-induced release of cytosolic Ca2+. Also, pretreatment with dantrolene or azumulene, two inhibitors of cytosolic Ca2+ release, reduced [Ca2+]i, and prevented InsP3 from inducing release of cytosolic Ca2+. Incubation in caffeine or lengthening of the muscle increased [Ca2+]i and enhanced the ability of InsP3 to induce release of cytosolic Ca2+. These results indicate that InsP3, at physiological concentrations, induces Ca2+ release in intact muscle fibers, and suggest that the InsP3-induced Ca2+ release is regulated by [Ca2+]i. A Ca2+-dependent effect of InsP3 on cytosolic Ca2+ release could be of importance under physiological or pathophysiological conditions associated with alterations in cytosolic Ca2+ homeostasis.

The effects of lithium on muscle contractile function in humans

A side effect of lithium (Li+) treatment is fatigue. Li+ decreases inositol triphosphate (IP3) accumulation and IP3 may play a role in excitation-contraction (E-C) coupling in skeletal muscle. Li+ carbonate (600 mg b.i.d. x 6 days) was administered in a randomized, double-blind fashion to 12 males to measure the effect upon muscle contractile function: peak twitch torque (PTT), time to PTT, half-relaxation time, maximal voluntary contraction strength (MVC), percent motor unit activation, M-wave characteristics, and tetanic torque (3 min at 15 and 50 Hz). Li+ resulted in a significant decrease in 15- and 50Hz tetanic torque (P<0.00l), MVC, and resting PTT (P<0.05). There were no effects of Li+ upon any of the other measured variables. Li+ had a negative effect upon E-C coupling and did not affect central motor unit recruitment. Elucidation of the role of IP3 in E-C coupling may help to understand fatigue in some neuromuscular disorders.

Effect of pentachlorophenol on calcium accumulation in barnacle muscle cells

1. The effect of extracellularly applied pentachlorophenol (PCP) was studied on the membrane potential (Vm) and Ca2+ uptake in isolated single skeletal muscle cells of Balanus nubilus. 2. When compared with the controls, 0.1 mM PCP induced a significant (P < 0.05) increase in Ca2+ uptake accompanied by membrane depolarization (9 mV at 45 min incubation). This depolarization was reduced by 11% of extracellular Ca2+ (Cao2+) was replaced by Tris+ and by 50% if extracellular Na+ was also replaced by Tris+. 3. The Ca2+ channel blocker, verapamil (0.1 mM), completely inhibited the PCP-induced Ca2+ uptake as well as the membrane depolarization either in the absence or presence of Cao2+. Experiments on voltage-clamped cells show that the PCP-induced Ca2+ uptake was independent of the PCP-induced depolarization. 4. The results indicate that PCP induces activation of a verapamil-sensitive Ca2+ influx pathway (presumably L-type Ca2+ channels) independent of Vm. The permeation of Ca2+, Na+ and Tris+ through this pathway produces membrane depolarization in the following order of effectiveness: Ca2+ > Na+ > Tris+.

Propofol regulation of calcium entry pathways in cultured A10 and rat aortic smooth muscle cells

1. We have investigated the effect of propofol, an intravenous anaesthetic, on the intracellular calcium concentration ([Ca2+]i), Ca2+ entry pathways and on inositol phosphate formation in vascular smooth muscle cells. [Ca2+]i and Ca2+ flux were monitored with the Ca(2+)-sensitive fluorescent dye, fura-2, and by 45Ca2+ uptake. Production of labelled inositol phosphates was analysed by anion-exchange chromatography. 2. Treatment of the cells with endothelin-1 (ET-1) increased formation of inositol phosphates and elevated [Ca2+]i due to both release of Ca2+ from intracellular pools and prolonged entry of Ca2+ from outside the cell. Propofol reduced production of inositol phosphates mediated by ET-1 and arginine vasopressin which activate phospholipase C. 3. The sustained Ca2+ entry stimulated by ET-1 was found to occur through the activation of L-type Ca channels. This was inhibited by propofol in a dose-dependent manner. 4. Activation of protein kinase C (PKC) by phorbol esters activated a pharmacologically-similar channel and produced a similar change in [Ca2+]i due to Ca2+ entry. The entry was blocked by an L-type channel antagonist, nicardipine and by the anaesthetic drug, propofol. 5. Treatment of the cells with thapsigargin, a selective inhibitor of the sarcoplasmic reticulum Ca(2+)-ATPase, also elevated [Ca2+]i by inducing the release of intracellular Ca2+ and the continued entry of extracellular Ca2+ through a nicardipine-insensitive Ca channel. Neither release nor entry induced by thapsigargin was affected by propofol. 6. These findings suggest that propofol selectively inhibits Ca2+ entry through the L-type channel induced by ET-1 and phorbol esters but has no effects on Ca2+ entry via the nicardipine-insensitive channel and on Ca2+ release from intracellular pools initiated by thapsigargin. This may represent one of the mechanisms responsible for propofol-induced vasodilatation.

Inositol 1,4,5-trisphosphate receptor in skeletal muscle: differential expression in myofibres

The role of inositol 1,4,5-trisphosphate as a second messenger in signal transduction has been well established in many cell types. However, conflicting reports have led to a controversy regarding the role, if any, of inositol 1,4,5-trisphosphate signalling in skeletal muscle. Indeed, expression of the inositol 1,4,5-trisphosphate receptor has not previously been demonstrated in skeletal muscle. In the present study we used in situ hybridization, immunohistochemistry, and [3H]-inositol 1,4,5-trisphosphate binding to demonstrate that rat skeletal muscle fibres contain inositol 1,4,5-trisphosphate receptors. RNAse protection and partial sequencing suggested that the inositol 1,4,5-trisphosphate receptors expressed in skeletal muscle was most similar to the non-neuronal form of the type 1 inositol 1,4,5-trisphosphate receptor. While in situ hybridization showed inositol 1,4,5-trisphosphate receptor mRNA in all types of skeletal myofibres, immunodetectable inositol 1,4,5-trisphosphate receptor protein and specific [3H]-inositol 1,4,5-trisphosphate binding sites were preferentially expressed in slow oxidative (type I) and fast oxidative-glycolytic (type IIA) fibres, but not in fast glycolytic (type IIB) fibres. These findings indicate that an inositol 1,4,5-trisphosphate receptor is preferentially expressed in oxidative fibres of skeletal muscle.

The role of phosphoinositide metabolism in Ca2+ signalling of skeletal muscle cells

1. The mobilization of Ca2+ from intracellular stores by D-myo-inositol 1,4,5-triphosphate[Ins(1,4,5)P3] is now widely accepted as the primary link between plasma membrane receptors that stimulate phospholipase C and the subsequent increase in intracellular free Ca2+ that occurs when such receptors are activated (Berridge, 1993). Since the observations of Volpe et al. (1985) which showed that Ins(1,4,5)P3 could induce Ca2+ release from isolated terminal cisternae membranes and elicit contracture of chemically skinned muscle fibres, research has focused on the role of Ins(1,4,5)P3 in the generation of SR Ca2+ transients and in the mechanism of excitation-contraction coupling (EC-coupling). 2. The mechanism of signal transduction at the triadic junction during EC-coupling is unknown. Asymmetric charge movement and mechanical coupling between highly specialized triadic proteins has been proposed as the primary mechanism for voltage-activated generation of SR Ca2+ signals and subsequent contraction. Ins(1,4,5)P3 has also been proposed as the major signal transduction molecule for the generation of the primary Ca2+ transient produced during EC-coupling. 3. Investigations on the generation of Ca2+ transients by Ins(1,4,5)P3 have been conducted on ion channels incorporated into lipid bilayers, skinned and intact fibres and isolated membrane vesicles. Ins(1,4,5)P3 induces SR Ca2+ release and the enzymes responsible for its synthesis and degradation are present in muscle tissue. However, the sensitivity of the Ca2+ release mechanism to Ins(1,4,5)P3 is highly dependent on experimental conditions and on membrane potential. 4. While Ins(1,4,5)P3 may not be the major signal transduction molecule for the generation of the primary Ca2+ signal produced during voltage-activated contraction, this inositol polyphosphate may play a functional role as a modulator of EC-coupling and/or of the processes of myoplasmic Ca2+ regulation occurring on a time scale of seconds, during the events of contraction.

Effects of heparin on excitation-contraction coupling in skeletal muscle toad and rat

1. Intracellularly applied heparin was found to cause a novel, use-dependent block of excitation-contraction (E-C) coupling in skinned skeletal muscle fibres of the toad. After one to four depolarizations in the presence of 100 micrograms ml-1 heparin, no further depolarization-induced responses could be elicited, even though addition of caffeine or lowering [Mg2+] could still induce massive Ca2+ release. This effect could not be reversed by extensive wash-out of the heparin (> 15 min). 2. Heparin (100 micrograms ml-1) did not abolish subsequent depolarization-induced responses if applied while the voltage sensors were in either their resting or inactivated states, that is (a) while a fibre remained fully polarized, (b) when a fibre was already chronically depolarized or (c) after a fibre had been depolarized in the presence of D600 (gallopamil) and then repolarized. 3. When a toad fibre was depolarized in heparin, with the associated Ca2+ release blocked by the presence of 10 mM intracellular Mg2+, subsequent E-C coupling was abolished. Heparin did not interrupt E-C coupling when Ca2+ release was triggered in the absence of any depolarization, by either caffeine or low [Mg2+]. Thus, the opening of the Ca2+ release channels was neither necessary nor sufficient for heparin to abolish E-C coupling. 4. Heparin had direct effects on the contractile apparatus in toad fibres, increasing the Ca2+ sensitivity and decreasing the maximum Ca(2+)-activated force. These effects could only be partly reversed by extensive wash-out of heparin. 5. At 100 micrograms ml-1, both low molecular weight heparin and pentosanpolysulphate, another highly sulphated polysaccharide, were less effective than heparin in blocking the depolarization-induced response and in changing the properties of the contractile apparatus, and these effects could be substantially reversed by wash-out. Two other polyanions, de-N-sulphated heparin (100 micrograms ml-1), which lacked N-sulphate groups, and polyglutamate (500 micrograms ml-1), had no measurable effect on either E-C coupling or the contractile apparatus. 6. In skinned fibres of the extensor digitorum longus muscle of the rat, 100 micrograms ml-1 heparin had little or no effect on E-C coupling and on the Ca2+ sensitivity of the contractile apparatus, but caused a larger reduction of the maximum Ca(2+)-activated force than in skinned fibres of the toad. 7. These results indicate that heparin blocks E-C coupling in toad muscle if, and only if, it is present when the voltage sensors are activated by depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)

The action of amyotrophic lateral sclerosis immunoglobulins on mammalian single skeletal muscle Ca2+ channels

1. The planar phospholipid bilayer technique was used to study the T-tubule skeletal muscle dihydropyridine (DHP)-sensitive calcium (Ca2+) channel. To improve the signal-to-noise ratio, Ca2+ channel activity was recorded using both 800-50 and 500-50 mM NaCl gradients. 2. Ca2+ channels were characterized by their cation selectivity and pharmacological profile. The mean open time for channels identified by these techniques was increased by the DHP agonist Bay K 8644 (2 microM), while it was decreased by the DHP antagonist nifedipine (5 microM). Nifedipine also reduced Ca2+ channel amplitude levels. 3. Immunoglobulins G (IgG) from three amyotrophic lateral sclerosis (ALS) patients (n = 14 experiments), one myasthenia gravis (MG) patient (n = 3 experiments) and one healthy individual (n = 4 experiments), were tested on Ca2+ channel activity at a final concentration of 3 mg/ml. 4. Channel mean open time, mean closed time and time integral for the current were not modified by normal IgG (n = 4 experiments). Similarly, MG IgG did not reduce channel activity (n = 3 experiments). 5. ALS IgG reduced the mean open time of DHP-sensitive Ca2+ channel activity in twelve out of fourteen experiments. In addition, in five out of twelve experiments, ALS IgG stabilized the channel to a smaller amplitude level. 6. ALS IgG reduced Ca2+ channel activity in a side-selective fashion, probably corresponding to the external side of the channel. 7. These results suggest that ALS IgG action on DHP-sensitive Ca2+ channels is not mediated by second messengers, thus favouring a direct mechanism for interaction with the DHP receptor complex.

Plasma membrane inositol 1,4,5-trisphosphate-activated channels mediate signal transduction in lobster olfactory receptor neurons

Inositol 1,4,5-trisphosphate (IP3) selectively evokes an inward (excitatory) current in cultured lobster olfactory receptor neurons (ORNs) and directly activates two types of channels in cell-free patches of plasma membrane from the ORNs. The IP3-activated channels have kinetic properties of odor-activated channels in the ORNs and pharmacological properties of intracellular IP3-activated channels in other systems. An antibody directed against an intracellular, cerebellar IP3 receptor recognizes a protein with a molecular weight similar to the mammalian receptor in the ORNs. The antibody selectively increases odor-evoked inward currents and IP3-activated unitary currents in the ORNs. The data provide further evidence for IP3 as an olfactory second messenger and implicate at least one and possibly two novel plasma membrane IP3 receptors in olfactory transduction.

Inositol phosphate regulation of voltage-dependent calcium channels in cerebellar granule neurons

The effects of intracellularly applied inositol phosphates on voltage-dependent calcium channel currents were assessed in rat cerebellar neurons using the whole-cell recording configuration of the patch-clamp technique. Intraneuronal perfusion of 10 microM inositol 1,4,5-trisphosphate (IP3) increased the amplitude of currents elicited by depolarization from a holding potential of -40 mV. IP3 did not modify current activation, but shifted the steady-state inactivation curve toward more positive values. The dose-response curve indicated an EC50 of 0.5 microM for IP3. Inositol 1,3,4,5-tetrakisphosphate (IP4), but not inositol 4,5,-bisphosphate, mimicked the effect of IP3. The effect of IP3 persisted in the presence of 100 micrograms/ml heparin and did not depend on intracellular calcium mobilization, as similar responses were not produced by 10 mM caffeine or by intrapipette calcium buffering at pCa 6 instead of pCa 7.7. Preincubation with omega-conotoxin led to a 55% inhibition of barium current; however, inhibition was reversed by IP3, which reestablished the control current amplitude. These results imply that IP3 and IP4 can elicit calcium entry by modifying both the gating characteristics and the pharmacological properties of voltage-dependent calcium channels.

Inositol trisphosphate (InsP3) causes contraction in skeletal muscle only under artificial conditions: evidence that Ca2+ release can result from depolarization of T-tubules

It has been proposed that in striated muscle inositol 1,4,5-trisphosphate (InsP3) may serve as a chemical transmitter linking membrane depolarization to Ca(2+)-release from the sarcoplasmic reticulum. Key to that hypothesis of excitation-concentration (EC) coupling was the observation that skinned muscle fibres contract on the application of InsP3. Yet skinned fibres do not always respond in this way, and in our hands intact fibres do not contract when InsP3 (1 microM-1 mM) is microinjected into them. Glycerol-shocked fibres do contract, however, and so do intact fibres that have been depolarized to about -50 mV by increasing [K+]0. These observations and related pharmacological evidence support the hypothesis that InsP3 causes a low-level depolarizing current to cross the T-tubular membrane. This current is sufficient to depolarize the T-tubules to the threshold for contraction only when the tubules are sealed over or when they are already close to the threshold. The InsP3-induced Ca2+ release sometimes observed in skinned muscle fibres and in vesicles derived from junctional sarcoplasmic reticulum probably often results from an action on sealed-over transverse tubules; in such situations it is an artifact of cell disruption. The fact that high concentrations of InsP3 do not cause contraction in normal muscle fibres is strong evidence against the hypothesis that InsP3 plays a central role in EC coupling in skeletal muscle.

Thapsigargin stimulates Ca2+ entry in vascular smooth muscle cells: nicardipine-sensitive and -insensitive pathways

We have investigated the role of the sarcoplasmic reticulum Ca2+ pool in regulating Ca2+ entry in vascular smooth muscle cells using a receptor-independent means of mobilizing the intracellular Ca2+ pool. Thapsigargin (TG) has been shown to inhibit the endoplasmic reticulum Ca(2+)-ATPase, mobilize intracellular Ca2+, and activate Ca2+ entry in nonmuscle tissues. When smooth muscle cells were treated with 0.2 microM TG, cytosolic Ca2+ concentrations rose gradually over 8 min to a peak value of 365 +/- 18 nM. Cytosolic Ca2+ remained elevated for at least 20 min and was supported by continued entry of extracellular Ca2+. TG also stimulated entry of Mn2+ and 45Ca2+ from outside the cell. Importantly, TG-induced Ca2+ entry and Mn2+ entry were found to occur through mechanisms that were independent of L-type Ca2+ channel activation because influx was not inhibited by concentrations of nicardipine that were found to block either endothelin- or 100 mM extracellular K(+)-induced cation influx. The mechanism through which TG activates cation entry appears to involve mobilization of the inositol 1,4,5-trisphosphate-responsive intracellular Ca2+ pool. In permeabilized cells, TG prevented ATP-stimulated Ca2+ uptake into the sarcoplasmic reticulum and slowly released sequestered Ca2+. The Ca2+ pool involved was responsive to inositol 1,4,5-trisphosphate. However, TG did not initiate the formation of inositol polyphosphates. Thus TG mobilizes the sarcoplasmic reticulum Ca2+ pool and activates Ca2+ entry through a nicardipine-insensitive Ca2+ channel in vascular smooth muscle. The mechanism is independent of inositol polyphosphate formation.

Regulation of extracellular calcium entry in endothelial cells: role of intracellular calcium pool

We have investigated the role of the intracellular Ca2+ pool in regulating Ca2+ entry into vascular endothelial cells. The intracellular Ca2+ pool was mobilized using either thapsigargin (TG) or 2',5'-di(tert-butyl)-1,4-benzohydroquinone (BHQ), inhibitors of the endoplasmic reticulum Ca(2+)-adenosinetriphosphatase (ATPase). Mobilization of intracellular Ca2+ stores with either inhibitor depleted intracellular Ca2+ and greatly reduced subsequent mobilization of the inositol 1,4,5-trisphosphate (IP3)-sensitive intracellular Ca2+ pool by bradykinin. However, bradykinin-induced mobilization of the IP3-sensitive intracellular Ca2+ pool only partially reduced the subsequent response of cells to TG and BHQ. Mobilization of the intracellular Ca2+ pool by either TG or BHQ led to a concentration-dependent elevation of cytosolic Ca2+ concentrations ([Ca2+]i) without initiating inositol polyphosphate formation. In contrast to the rapidly developing, transient rise in Ca2+ concentration initiated by bradykinin, maximal concentrations of TG and BHQ stimulated a slowly developing, prolonged elevation of [Ca2+]i that required extracellular Ca2+ and could be blocked by extracellular Ni2+. Extracellular Ca2+ entered the cell through an activated cation entry pathway, since bradykinin, TG, and BHQ stimulated Mn2+ and 45Ca2+ entry. Bradykinin-stimulated 45Ca2+ uptake reached a peak within 2 min, whereas 45Ca2+ influx initiated by TG or BHQ continued for at least 8 min. Importantly, the [Ca2+]i response after low concentrations of BHQ was more transient than that seen after TG. The return of [Ca2+]i to basal values after low concentrations of BHQ was associated with reversal of Ca(2+)-ATPase inhibition and refilling of the IP3-sensitive Ca2+ pool. The continued elevation of [Ca2+]i and prolonged Ca2+ entry seen with TG was associated with continued Ca(2+)-ATPase inhibition and an empty IP3-sensitive Ca2+ pool. We conclude that mobilization of intracellular Ca2+ stores induces Ca2+ entry in endothelial cells which continues until the intracellular Ca2+ pool is refilled.

Characteristics and function of Ca(2+)- and inositol 1,4,5-trisphosphate-releasable stores of Ca2+ in neurons

Molecular, biochemical and physiological evidence for the existence of releasable Ca2+ stores in neurons is strong. There are two separate molecules that function as release channels from those Ca2+ stores, the RyanR and InsP3R, and both have multiple regulatory sites for positive and negative control. Perhaps most intriguing is the biphasic, concentration-dependent action of cytosolic Ca2+ on both channels, first to stimulate release then, at higher concentration, to depress release. Whether the InsP3R and RyanR channels regulate Ca2+ release from different or identical functional compartments will need to be defined for each neuron type and perhaps even for each intracellular region within neurons since the evidence for functional separation of stores is mixed. The identification of Ca2+ storage and releasing capacity throughout all subcellular regions of neurons and the increasing evidence for a role for Ca2+ stores in neuronal plasticity suggests that the further characterization of the functional properties of Ca2+ stores will be an increasingly important and expanding area of interest in neurobiology.

Masses of inositol phosphates in resting and tetanically stimulated vertebrate skeletal muscles

The masses of inositol phosphates have been determined in isolated skeletal muscles from Xenopus laevis (sartorius, tibialis anterior and iliofibularis) and rat (gastrocnemius and soleus) which were quick-frozen in the resting state and at different stages of an isometric (Xenopus) or isotonic (rat) tetanus. The isomeric spectrum of inositol phosphates detected was similar to that in other tissues and cell types. The total sarcoplasmic concentrations of the isomers Ins-(1,4,5,6)P4/Ins(3,4,5,6)P4 (0.2-0.9 microM), Ins(1,3,4,6)P4 (not detectable), Ins(1,3,4,5,6)P5 (about 1 microM) and InsP6 (3.2-4.6 microM) were lower than in other cell types. Variations in these concentrations were due to the muscle type rather than to the donor species. The putative second messenger Ins(1,4,5)P3, as well as its dephosphorylation product Ins(1,4)P2, were present at surprisingly high total myoplasmic resting concentrations, ranging from 1.2 to 2.5 microM and 3.5 to 6.9 microM respectively. Upon tetanic stimulation these two inositol phosphates in particular exhibited significantly increased total sarcoplasmic concentrations, up to 4.2 microM and 11.3 microM respectively, with a time scale of seconds. From the initial rate of increase in the total sarcoplasmic concentrations of Ins(1,4,5)P3 and its rapidly formed metabolic products, a minimal phosphoinositidase C (PIC) activity in tetanically activated Xenopus skeletal muscle of about 1.7-2.6 microM/s can be estimated. This PIC activity observed in vivo seems to be far too low to account for a functional role for Ins(1,4,5)P3 as a chemical transmitter in the fast excitation-contraction coupling (ECC) process in skeletal muscle. The presence of Ins(1,3,4,5)P4 in all muscle types is indicative of a Ca(2+)-activated Ins(1,4,5)P3 3-kinase activity. The rapid transient increases in Ins(1,3,4)P3 and Ins(1,3)P2 in isometrically contracting Xenopus muscles suggest that corresponding Ins(1,3,4,5)P4 phosphatases are operating in skeletal muscle as well. In all muscles investigated except rat soleus, the fructose 1,6-bisphosphate [Fru(1,6)P2] concentration increased substantially during a tetanus, up to about 2 mM. This increase is correlated with a simultaneous decrease in phosphocreatine, whereas the energy charge of the muscles was essentially unaffected by the applied tetani. The time course of the rise in Fru(1,6)P2 was used to model changes in the free concentrations of high-affinity aldolase-binding inositol phosphates during the course of a tetanus. These calculations demonstrate that the free concentration of Ins(1,4,5)P3 and other aldolase-bound inositol phosphates can increase much faster and to a larger extent than the corresponding total concentrations as a result of their competitive displacement from aldolase-binding sites by the rapidly rising concentration of Fru(1,6)P2.

Inositol 1,4,5-trisphosphate directly opens diphtheria toxin channels

Inositol 1,4,5-trisphosphate (IP3) is a soluble second messenger, which acts by cooperatively releasing Ca2+ into the cytosol from non-mitochondrial stores, probably by activating Ca(2+)-permeable channels. We demonstrate that submicromolar concentrations of IP3 can directly open the Ca(2+)-permeable diphtheria toxin channel, and that this occurs by IP3 binding to a specific site on the toxin protein. This provides a model for IP3-induced Ca2+ release and suggests that IP3-induced channel opening may play a role in diphtheria intoxication and in protein translocation across membranes.

Variety of Ca(2+)-permeable channels in human carcinoma A431 cells

Patch-clamp methods were used to search for and characterize channels that mediate calcium influx through the plasma membrane of human carcinoma A431 cells. Here we present four Ca(2+)-permeable channel types referred to as SG, G, 1 and BI. With 105 mM Ca2+ as the charge carrier, at 30-33 degrees C their mean unitary conductances (in pS) are: 1.3 (SG), 2.4 (G), 3.7 (I) and 12.8 (BI). SG and G channels are activated by nonhydrolyzable analogues of guanosine 5-triphosphate (GTP) applied to the inside of the membrane, suggesting an involvement of G-proteins in the control of their activity. I and BI channels are activated by inositol 1,4,5-trisphosphate (InsP3). G, I, BI and possibly SG channels are activated from the extracellular side of the membrane by epidermal growth factor (EGF) and histamine. It is assumed that all identified Ca2+ channels take part in the generation of the agonist-induced intracellular Ca2+ signal. The variety of Ca-channel types seems to be necessary to tune cell responses according to the respective type and level of an external signal, on the one hand, and to the functional state of the cell, on the other.

Inositol 1,4,5-trisphosphate increases myoplasmic [Ca2+] in isolated muscle fibers. Depolarization enhances its effects

Inositol 1,4,5-trisphosphate (InsP3) has been proposed as an intracellular messenger which mobilizes calcium from the sarcoplasmic reticulum, during excitation-contraction coupling in skeletal muscle. We have measured the myoplasmic free calcium concentration ([Ca2+]i) by means of calcium selective microelectrodes in intact fibers isolated from Leptodactylus insularis microinjected with InsP3. In muscle fibers bathed in normal Ringer, the mean resting [Ca2+]i was 0.11 +/- 0.01 microM (M +/- SEM, n = 30). The microinjection of 0.3, 0.5 and 1 microM InsP3 induced transient increments in the [Ca2+]i to 0.35 +/- 0.02 microM (n = 9), to 0.53 +/- 0.03 microM (n = 11) and 0.94 +/- 0.06 microM (n = 10) respectively. Microinjection of 0.3, 0.5 and 1 microM InsP3 in muscle fibers incubated in low Ca2+ solution induced increments in [Ca2+]i similar to those observed in fibers bathed with normal Ringer. The microinjection of 0.3, 0.5 and 1 microM InsP3 in muscle fibers partially depolarized with 10 mM [K+]o induced transient enhancements of the resting [Ca2+]i that were greater than the transients observed in the normally polarized muscle. In partially depolarized fibers microinjected with 0.3, 0.5 and 1 microM InsP3, the [Ca2+]i was changed to 1.45 +/- 0.14 microM (n = 20), to 3.37 +/- 0.34 microM (n = 7) and to 7.43 +/- 0.70 microM (n = 6) respectively. In all partially depolarized fibers these increments in [Ca2+]i were associated with local contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of endothelin-mediated calcium mobilization in vascular smooth muscle cells by isoproterenol

We have investigated the effect of isoproterenol on endothelin-induced Ca2+ mobilization in A10 vascular smooth muscle cells. Endothelin (ET) stimulates a rapid and sustained elevation of intracellular Ca2+ mediated by production of inositol phosphates, release of intracellular Ca2+, and activation of a plasmalemmal Ca2+ influx pathway. This influx pathway appears to be a L-type channel because it is inhibited by nicardipine and activated by BAY K 8644. Depolarization of the cells, by elevating extracellular K+, activated a pharmacologically similar channel and produced a similar change in intracellular Ca2+ concentration. Preincubation of cells with isoproterenol reduced the peak Ca2+ response to endothelin and blocked the sustained elevation. However, isoproterenol did not alter K(+)-induced Ca2+ entry. Thus it appears that ET-induced entry is mediated by intracellular signals and not by depolarization. With the use of cells incubated in Ca2(+)-free medium containing 1 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, isoproterenol was shown to inhibit Ca2+ release from intracellular pools by 36 +/- 3%. Furthermore, isoproterenol pretreatment or addition of adenosine 3',5'-cyclic monophosphate (cAMP) to saponin-permeabilized cells inhibited inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]-induced Ca2+ release from intracellular sites. Similar effects were seen with forskolin. Propranolol reversed the inhibitory effects of isoproterenol. Isoproterenol pretreatment also inhibited the rapid formation of Ins(1,4,5)P3 and [2-3H]inositol 1,3,4,5-tetrakisphosphate stimulated by endothelin and reduced the sustained formation of these compounds. Finally, isoproterenol and forskolin led to a greater than 10-fold increase in intracellular cAMP levels. This stimulation of adenylate cyclase by isoproterenol was completely blocked by propranolol. It appears then that the beta-agonist isoproterenol interacts with a beta-adrenergic receptor, elevates cAMP, and thereby alters endothelin-induced Ca2+ mobilization. Inhibition of Ins(1,4,5)P3 formation, reduction in the responsiveness of the Ins(1,4,5)P3 intracellular receptor, and perhaps inhibition of ET-induced Ca2+ entry appear to be involved.

Ca2+ and activation mechanisms in skeletal muscle

It has been known for a number of years that calcium ions play a crucial role in excitation-contraction (e-c) coupling (Sandow, 1952). The majority of the calcium required for this process is derived, at least in vertebrate striated muscle fibres, from discrete intracellular stores located at sites within the cell: the terminal cysternae (tc)/junctional SR of the sarcoplasmic reticulum (SR) (Fig. 1 a). These storage sites not only form a compartment that is distinct from the sarcoplasm of the fibre, but they are also closely associated with the contractile elements, the myofibrils. The SR release sites are activated following the spread of electrical activity (Huxley and Taylor, 1958) along the transverse (T) tubular system (Eisenberg and Gage, 1967; Adrian et al. 1969a, b; Peachey, 1973) from the surface membrane (Bm).

Structure and function of inositol trisphosphate receptors

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) is a soluble intracellular messenger formed rapidly after activation of a variety of cell-surface receptors that stimulate phosphoinositidase C activity. The initial response to Ins(1,4,5)P3 is a rapid Ca2+ efflux from nonmitochondrial intracellular stores which are probably specialized subcompartments of the endoplasmic reticulum, although their exact identities remain unknown. This initial response is followed by more complex Ca2+ signals: regenerative Ca2+ waves propagate across the cell, repetitive Ca2+ spikes occur, and stimulated Ca2+ entry across the plasma membrane contributes to the sustained Ca2+ signal. The mechanisms underlying these complex Ca2+ signals are unknown, although Ins(1,4,5)P3 is clearly involved. The intracellular receptor that mediates Ins(1,4,5)P3-stimulated Ca2+ mobilization has been purified and functionally reconstituted, and its amino acid sequence deduced from its cDNA sequence. These studies demonstrate that the Ins(1,4,5)P3 receptor has an integral Ca2+ channel separated from the Ins(1,4,5)P3 binding site by a long stretch of residues some of which form binding sites for allosteric regulators, and some of which are substrates for phosphorylation. In this review, we discuss the ligand recognition characteristics of Ins(1,4,5)P3 receptors, and their functional properties in their native environment and after purification, and we relate these properties to what is known of the structure of the receptor. In addition to regulation by Ins(1,4,5)P3, the Ins(1,4,5)P3 receptor is subject to many additional regulatory influences which include Ca2+, adenine nucleotides, pH and phosphorylation by protein kinases. Many of the functional and structural characteristics of the Ins(1,4,5)P3 receptor show striking similarities to another intracellular Ca2+ channel, the ryanodine receptor. These properties of the Ins(1,4,5)P3 are discussed, and their possible roles in contributing to the complex Ca2+ signals evoked by extracellular stimuli are considered.

Structural and functional aspects of calcium homeostasis in eukaryotic cells

The maintenance of a low cytosolic free-Ca2+ concentration, ([Ca2+]i) is a common feature of all eukaryotic cells. For this purpose a variety of mechanisms have developed during evolution to ensure the buffering of Ca2+ in the cytoplasm, its extrusion from the cell and/or its accumulation within organelles. Opening of plasma membrane channels or release of Ca2+ from intracellular pools leads to elevation of [Ca2+]i; as a result, Ca2+ binds to cytosolic proteins which translate the changes in [Ca2+]i into activation of a number of key cellular functions. The purpose of this review is to provide a comprehensive description of the structural and functional characteristics of the various components of [Ca2+]i homeostasis in eukaryotes.

Neuromodulation: associative and nonlinear adaptation

Neuromodulation, the interaction between at least two chemical messengers in the nervous system, serves as a mechanism by which biochemical association can occur. A simple, yet compelling, hypothesis is that the criteria for expression of associative learning and memory are subserved by biochemical events which are also associative in nature. A neuromodulatory interaction that has been linked to memory function and which has been the subject of biochemical inquiry is the interaction between the catecholamine, norepinephrine (NE) and the neuropeptide, vasopressin (AVP). Studies described in this report show that vasopressin acts to potentiate norepinephrine (NE)-induced cyclic adenosine monophosphate (cAMP) accumulation in the hippocampus by a calcium-dependent mechanism. Results of these studies are considered in the context of the nonlinear properties of synergism and conditionality and in the context of the associative learning requirements of spatial and temporal coupling. Secondly, the calcium dependency of AVP-induced neuromodulation is considered in relation to the calcium dependency for induction of associative long-term potentiation. Lastly, the potential for changes in neuronal morphology in response to neuromodulatory events is considered. By using vasopressin potentiation of noradrenalin-induced cAMP formation as a model system, I have applied the theoretical framework of associative learning and memory to test the hypothesis that neuromodulation can serve as a biochemical analog of associative cognitive events.

Whole-cell calcium current in guinea-pig ventricular myocytes dialysed with guanine nucleotides

1. Whole-cell calcium current (ICa) was recorded in guinea-pig ventricular myocytes superfused with Na+,K(+)-free solution and dialysed with a substrate-free solution (minimum intracellular solution, MICS). A dual tight-seal pipette method was often used to permit pressure-enhanced dialysis of a test solution after a given pre-dialysis. 2. In dual-pipette experiments, test dialysates contained 100 mM-GTP-gamma-S (guanosine 5'-O-(3-thiotriphosphate] or 100 microM-GMP-PNP (guanyl-5'-imidodiphosphate). These non-hydrolysable analogues of guanosine triphosphate (GTP) enhanced ICa amplitude (+ 10 mV) by 20-40%. Dialysates containing 100 microM-GTP or GDP-beta-S (guanosine 5'-O-(2-thiodiphosphate] were ineffective, and pre-dialysis with GDP-beta-S blocked stimulation by GTP-gamma-S. 3. Non-hydrolysable GTP analogues slowed the inactivation of ICa and shifted the voltage eliciting maximum ICa by 5-10 mV in the negative direction. 4. ICa enhancement by GTP analogues was attributed to the activation of three GTP-binding regulatory (G) proteins (Gi, Gp and Gs). In single-pipette experiments, the inactivation of Gi by pre-treatment with pertussis toxin did not block enhancement, and a Gp-activating regimen (external acetylcholine-internal GTP) was without effect. Thus, it is probable that the effects of GTP analogues on ICa were primarily mediated by Gs activation. 5. PI-MICS dialysates contained phosphorylation-pathway inhibitors and were used to inhibit Ca2+ channel phosphorylation via the adenyl cyclase pathway. These were deemed effective since forskolin (1-5 microM) doubled ICa during control dialysis but was without effect after 8 min PI-MICS dialysis. However, 0.1 microM-isoprenaline increased ICa by 35% in myocytes totally unresponsive to forskolin, suggesting that beta-adrenergic receptor occupation can stimulate ICa even when the phosphorylation pathway is blocked. 6. After prolonged dialysis of myocytes with PI-MICS, ICa was still enhanced by pressure-assisted dialysis of 100 microM-GTP-gamma-S or GMP-PNP. We conclude that activated Gs has a direct effect on cardiac Ca2+ channels.

Tansley Review No. 21 Plant ion channels: whole-cell and single channel studies

Ion channels are proteins which catalyse rapid, passive, electrogenic uniport of ions through pores spanning an otherwise poorly permeable lipid bilayer. Among other processes, fluxes through ion channels are responsible for action potentials - large, transient changes in membrane potential which have been known of in plants for over 100 years. Much disparate information on ion channels in plant cells has accumulated over the past few years. In an attempt to synthesize these data, the properties of at least 18 different ion channels are collated in this review. Channels are initially classified according to ion selectivity (Ca²⁺ , Cl^- , K⁺ and H⁺ ); then gating characteristics (i.e. control of opening and closing), unitary conductance and pharmacology are used to distinguish further different sub-types of channels. To provide a background for this overview, the fundamental properties which define ion channels in animal cells, namely conduction, selectivity and gating, are described. Appropriate techniques for the study of ion channels are also assessed. The review concludes with a discussion on the role of ion channels in plant cells, although any comment on functions beyond turgor regulation and general statements about signalling remains largely speculative. The study of ion channels in plant cells is still at an early stage and it is hoped that this review will provide a framework upon which further work in both algae and vascular plants can be based. CONTENTS Summary 305 I. Introduction: plant electrophysiology 306 II. A general description of ion channels 306 III. Ion channels in plants 310 IV. Ca²⁺ channels 313 V. Cl^- channels 315 VI. K⁺ channels in the plasma membrane 318 VII. K⁺ channels in the tonoplast 322 VIII. Channels in thylakoids 324 IX. H⁺ channels 324 X. Functions of channels 325 XI. Conclusions 328 Acknowledgements 328 References 329.

Molecular interactions of the junctional foot protein and dihydropyridine receptor in skeletal muscle triads

Isolated triadic proteins were employed to investigate the molecular architecture of the triad junction in skeletal muscle. Immunoaffinity-purified junctional foot protein (JFP), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), aldolase and partially purified dihydropyridine (DHP) receptor were employed to probe protein-protein interactions using affinity chromatography, protein overlay and crosslinking techniques. The JFP, an integral protein of the sarcoplasmic reticulum (SR) preferentially binds to GAPDH and aldolase, peripheral proteins of the transverse (T)-tubule. No direct binding of JFP to the DHP receptor was detected. The interactions of JFP with GAPDH and aldolase appear to be specific since other glycolytic enzymes associated with membranes do not bind to the JFP. The DHP receptor, an integral protein of the T-tubule, also binds GAPDH and aldolase. A ternary complex between the JFP and the DHP receptor can be formed in the presence of GAPDH. In addition, the DHP receptor binds to a previously undetected Mr 95 K protein which is distinct from the SR Ca2+ pump and phosphorylase b. The Mr 95 K protein is an integral protein of the junctional domain of the SR terminal cisternae. It is also present in the newly identified "strong triads" (accompanying paper). From these findings, we propose a new model for the triad junction.

Glutamate induces calcium waves in cultured astrocytes: long-range glial signaling

The finding that astrocytes possess glutamate-sensitive ion channels hinted at a previously unrecognized signaling role for these cells. Now it is reported that cultured hippocampal astrocytes can respond to glutamate with a prompt and oscillatory elevation of cytoplasmic free calcium, visible through use of the fluorescent calcium indicator fluo-3. Two types of glutamate receptor--one preferring quisqualate and releasing calcium from intracellular stores and the other preferring kainate and promoting surface-membrane calcium influx--appear to be involved. Moreover, glutamate-induced increases in cytoplasmic free calcium frequently propagate as waves within the cytoplasm of individual astrocytes and between adjacent astrocytes in confluent cultures. These propagating waves of calcium suggest that networks of astrocytes may constitute a long-range signaling system within the brain.

Molecular target sizes of inositol 1,4,5-trisphosphate receptors in liver and cerebellum

Ins(1,4,5)P3 is the intracellular messenger that mediates the effects of many cell-surface receptors on intracellular Ca2+ stores. Although radioligand-binding studies have identified high-affinity Ins(1,4,5)P3-binding sites in many tissues, these have not yet been convincingly shown to be the receptors that mediate Ca2+ mobilization, nor is it clear whether there are differences in these binding sites between tissues. Here we report that Ins(1,4,5)P3 binds to a single class of high-affinity sites in both permeabilized hepatocytes (KD = 7.8 +/- 1.1 nM) and cerebellar membranes (KD = 6.5 +/- 2.4 nM), and provide evidence that these are unlikely to reflect binding to either of the enzymes known to metabolize Ins(1,4,5)P3. Furthermore, the rank order of potency of synthetic inositol phosphate analogues in displacing specifically bound Ins(1,4,5)P3 is the same as their rank order of potency in stimulating mobilization of intracellular Ca2+ stores, suggesting that the Ins(1,4,5)P3-binding site may be the physiological receptor. Radiation inactivation of the Ins(1,4,5)P3-binding sites of liver and cerebellum reveals that they have similar molecular target sizes: 257 +/- 36 kDa in liver and 258 +/- 20 kDa in cerebellum. We conclude that an Ins(1,4,5)P3-binding protein with a molecular target size of about 260 kDa is probably the receptor that mediates Ca2+ mobilization in hepatocytes, and our limited data provide no evidence to distinguish this from the cerebellar Ins(1,4,5)P3-binding protein.

Internal and external effects of dihydropyridines in the calcium channel of skeletal muscle

The agonist effect of the dihydropyridine (DHP) (-)Bay K 8644 and the inhibitory effects of nine antagonist DHPs were studied at a constant membrane potential of 0 mV in Ca channels of skeletal muscle transverse tubules incorporated into planar lipid bilayers. Four phenylalkylamines (verapamil, D600, D575, and D890) and d-cis-diltiazem were also tested. In Ca channels activated by 1 microM Bay K 8644, the antagonists nifedipine, nitrendipine, PN200-110, nimodipine, and pure enantiomer antagonists (+)nimodipine, (-)nimodipine, (+)Bay K 8644, inhibited activity in the concentration range of 10 nM to 10 microM. Effective doses (ED50) were 2 to 10 times higher when HDPs were added to the internal side than when added to the external side. This sidedness arises from different structure-activity relationships for DHPs on both sides of the Ca channel since the ranking potency of DHPs is PN200-110 greater than (-)nimodipine greater than nifedipine approximately S207-180 on the external side while PN200-110 greater than S207-180 greater than nifedipine approximately (-)nimodipine on the internal side. A comparison of ED50's for inhibition of single channels by DHPs added to the external side and ED50's for displacement of [3H]PN200-110 bound to the DHP receptor, revealed a good quantitative agreement. However, internal ED50's of channels were consistently higher than radioligand binding affinities by up to two orders of magnitude. Evidently, Ca channels of skeletal muscle are functionally coupled to two DHP receptor sites on opposite sides of the membrane.

Multiple types of Ca2+ currents in single canine Purkinje cells

Whole-cell Ca2+ channel currents were recorded from isolated single canine Purkinje and ventricular cells to determine whether there were multiple types of Ca2+ channels in these two cell types, as in many other excitable tissues. The experimental conditions were such that currents other than Ca2+ channel currents were largely suppressed. The charge carrier was either Ca2+ or Ba2+ (5mM). In every canine Purkinje cell studied (n = 36), we saw T and L Ca2+ channel currents that are similar to their counterparts in other tissues. Neither current was affected by tetrodotoxin (30 microM), but both were reduced by Mn2+ (5mM). Ni2+ (50 microM) blocked T more than L current. Nisoldipine (1 microM) apparently abolished the L current but also decreased the T current by 50%. Substitution of Ba2+ for Ca2+ augmented and prolonged L current but did not affect T current significantly. At 36 degrees C and with 5 mM [Ca2+]o, T current inactivated over a voltage range from -70 to -30 mV whereas L current inactivated between -30 and +20 mV. T current was detectable in only some of the ventricular cells studied (8 out of 12). In these cells the ratio of maximal T current to maximal L current (0.2 +/- 0.1, n = 8) was lower than the T/L ratio in Purkinje cells (0.6 +/- 0.2, n = 6). The density of peak L current in ventricular cells (7.5 +/- 1.7 pA/pF, n = 8) was higher than that in Purkinje cells (4.4 +/- 3.4 pA/pF, n = 6). Therefore, in ventricular cells the L current is the main Ca2+ current whereas in Purkinje cells, the T current also contributes significantly to membrane electrical activity. In Purkinje cells, beta-adrenoceptor stimulation by isoproterenol (1 microM) increased L current but did not affect T current. On the other hand, in 70% (7 out of 10) of the Purkinje cells, alpha-adrenoceptor stimulation by 10 microM norepinephrine (in the presence of 2 microM propranolol) increased the T current. Our observations show that the distribution of the two types of Ca2+ channels in canine ventricle is heterogeneous and that the two types of Ca2+ channels are modulated by catecholamines by different receptors.

Intestinal secretagogues increase cytosolic free Ca2+ concentration and K+ conductance in a human intestinal epithelial cell line

A human intestinal epithelial cell line (Intestine 407) is known to retain receptors for intestinal secretagogues such as acetylcholine (ACh), histamine, serotonin (5-HT) and vasoactive intestinal peptide (VIP). The cells were also found to possess separate receptors for secretin and ATP, the stimulation of which elicited transient hyperpolarizations coupled to decreased membrane resistances. These responses were reversed in polarity at the K+ equilibrium potential. The hyperpolarizing responses to six agonists were reversibly inhibited by quinine or quinidine. By means of Ca2(+)-selective microelectrodes, increases in the cytosolic free Ca2+ concentration were observed in response to individual secretagogues. The time course of Ca2+ responses coincided with that of hyperpolarizing responses. The responses to ACh and 5-HT were abolished by a reduction in the extracellular Ca2+ concentration down to pCa 7 or by application of Co2+. Thus, in Intestine 407 cells, not only the intestinal secretagogues, which are believed to act via increased cytosolic Ca2+ (ACh, 5-HT and histamine), but also those which elevate cyclic AMP (VIP, secretin and ATP) induce increases in cytosolic Ca2+, thereby activating the K+ conductance. It is likely that the origin of increased cytosolic Ca2+ is mainly extracellular for ACh- and 5-HT-induced responses, whereas histamine, VIP, secretin and ATP mobilize Ca2+ from the internal compartment.

Calcium and T lymphocyte activation

A prolonged (at least 2-4 hr) elevation of [Ca2+]i accompanies early T cell activation by TCR/CD3-specific ligands. Ca2+ is generally thought to be an essential second messenger for early activation, but the precise molecular events contingent upon the Ca2+ signal remain to be determined. The Ca2+ signal can be separated into an early transient peak due to InsP3-released Ca2+ from intracellular stores, and a sustained plateau due to altered transmembrane Ca2+ flux. Patch clamp studies have identified an InsP3-activated, Ca2+ permeable channel in the plasma membrane of T lymphocytes that may be responsible for the sustained elevation of [Ca2+]i during continuous TCR/CD3 occupancy. The Ca2+ signal can be further resolved at the level of the single cell into a series of repetitive oscillations between peak and trough levels with a period of 16-20 s. The oscillations may be part of a frequency-encoded signaling system. Several nonlinear internal feedback controls may contribute to the periodic nature of the Ca2+ signal: PKC-mediated phosphorylation of the CD3 gamma subunit, which is a feedback inhibitor of TCR/CD3 function; amplification of Ca2+ release from endoplasmic reticulum by a highly cooperative step in the opening of Ca2+ channels by InsP3, and Ca2+-dependent feedback enhancement of PLC function; autoregulatory negative feedback on Ca2+ influx by Ca2+, both by a direct effect on the plasma membrane Ca2+ channel and by induction of membrane hyperpolarization secondary to Ca2+-activated K+ efflux. In addition, several other internal feedback controls on TCR/CD3 function, by CD4-induced tyrosine-specific phosphorylation of the CD3 zeta subunit, or on the Ca2+ signal, by extracellular Cl- or by GM1 gangliosides, are also postulated. The question of whether a G protein couples TCR/CD3 to PI hydrolysis and to Ca2+ mobilization is unresolved, although some indirect evidence for the involvement of GTP binding proteins in T cell activation has recently been obtained with cholera toxin. There is also preliminary evidence that TCR/CD3 may structurally conform to G protein coupled receptors, i.e., having a core structure of seven alpha helical transmembrane spanning segments, a ligand recognition site, loci for regulatory phosphorylation, and a putative nucleotide binding site.