Voltage dependence of inositol 1,4,5-trisphosphate-induced Ca2+ release in peeled skeletal muscle fibers

Generation and propagation of gastric slow waves

1. Mechanisms underlying the generation and propagation of gastrointestinal slow wave depolarizations have long been controversial. The present review aims to collate present knowledge on this subject with specific reference to slow waves in gastric smooth muscle. 2. At present, there is strong agreement that interstitial cells of Cajal (ICC) are the pacemaker cells that generate slow waves. What has been less clear is the relative role of primary types of ICC, including the network in the myenteric plexus (ICC-MY) and the intramuscular network (ICC-IM). It is concluded that both ICC-MY and ICC-IM are likely to serve a major role in slow wave generation and propagation. 3. There has been long-standing controversy as to how slow waves 'propagate' circumferentially and down the gastrointestinal tract. Two mechanisms have been proposed, one being action potential (AP)-like conduction and the other phase wave-based 'propagation' resulting from an interaction of coupled oscillators. Studies made on single bundle gastric strips indicate that both mechanisms apply with relative dominance depending on conditions; the phase wave mechanism is dominant under circumstances of rhythmically generating slow waves and the AP-like propagation is dominant when the system is perturbed. 4. The phase wave mechanism (termed Ca(2+) phase wave) uses cyclical Ca(2+) release as the oscillator, with coupling between oscillators mediated by several factors, including: (i) store-induced depolarization; (ii) resultant electrical current flow/depolarization through the pacemaker cell network; and (iii) depolarization-induced increase in excitability of downstream Ca(2+) stores. An analogy is provided by pendulums in an array coupled together by a network of springs. These, when randomly activated, entrain to swing at the same frequency but with a relative delay along the row giving the impression of a propagating wave. 5. The AP-like mechanism (termed voltage-accelerated Ca(2+) wave) propagates sequentially like a conducting AP. However, it is different in that it depends on regenerative store Ca(2+) release and resultant depolarization rather than regenerative activation of voltage-dependent channels in the cell membrane. 6. The applicability of these mechanisms to describing propagation in large intact gastrointestinal tissues, where voltage-dependent Ca(2+) entry is also likely to be functional, is discussed.

Role of voltage-dependent modulation of store Ca2+ release in synchronization of Ca2+ oscillations

Slow waves are rhythmic depolarizations that underlie mechanical activity of many smooth muscles. Slow waves result through rhythmic Ca(2+) release from intracellular Ca(2+) stores through inositol 1,4,5-trisphosphate (IP(3)) sensitive receptors and Ca(2+)-induced Ca(2+) release. Ca(2+) oscillations are transformed into membrane depolarizations by generation of a Ca(2+)-activated inward current. Importantly, the store Ca(2+) oscillations that underlie slow waves are entrained across many cells over large distances. It has been shown that IP(3) receptor-mediated Ca(2+) release is enhanced by membrane depolarization. Previous studies have implicated diffusion of Ca(2+) or the second messenger IP(3) across gap junctions in synchronization of Ca(2+) oscillations. In this study, a novel mechanism of Ca(2+) store entrainment through depolarization-induced IP(3) receptor-mediated Ca(2+) release is investigated. This mechanism is significantly different from chemical coupling-based mechanisms, as membrane potential has a coupling effect over distances several orders of magnitude greater than either diffusion of Ca(2+) or IP(3) through gap junctions. It is shown that electrical coupling acting through voltage-dependent modulation of store Ca(2+) release is able to synchronize oscillations of cells even when cells are widely separated and have different intrinsic frequencies of oscillation.

Control of aldosterone secretion: a model for convergence in cellular signaling pathways

Aldosterone secretion by glomerulosa cells is stimulated by angiotensin II (ANG II), extracellular K(+), corticotrophin, and several paracrine factors. Electrophysiological, fluorimetric, and molecular biological techniques have significantly clarified the molecular action of these stimuli. The steroidogenic effect of corticotrophin is mediated by adenylyl cyclase, whereas potassium activates voltage-operated Ca(2+) channels. ANG II, bound to AT(1) receptors, acts through the inositol 1,4,5-trisphosphate (IP(3))-Ca(2+)/calmodulin system. All three types of IP(3) receptors are coexpressed, rendering a complex control of Ca(2+) release possible. Ca(2+) release is followed by both capacitative and voltage-activated Ca(2+) influx. ANG II inhibits the background K(+) channel TASK and Na(+)-K(+)-ATPase, and the ensuing depolarization activates T-type (Ca(v)3.2) Ca(2+) channels. Activation of protein kinase C by diacylglycerol (DAG) inhibits aldosterone production, whereas the arachidonate released from DAG in ANG II-stimulated cells is converted by lipoxygenase to 12-hydroxyeicosatetraenoic acid, which may also induce Ca(2+) signaling. Feedback effects and cross-talk of signal-transducing pathways sensitize glomerulosa cells to low-intensity stimuli, such as physiological elevations of [K(+)] (< or =1 mM), ANG II, and ACTH. Ca(2+) signaling is also modified by cell swelling, as well as receptor desensitization, resensitization, and downregulation. Long-term regulation of glomerulosa cells involves cell growth and proliferation and induction of steroidogenic enzymes. Ca(2+), receptor, and nonreceptor tyrosine kinases and mitogen-activated kinases participate in these processes. Ca(2+)- and cAMP-dependent phosphorylation induce the transfer of the steroid precursor cholesterol from the cytoplasm to the inner mitochondrial membrane. Ca(2+) signaling, transferred into the mitochondria, stimulates the reduction of pyridine nucleotides.

A theoretical model of slow wave regulation using voltage-dependent synthesis of inositol 1,4,5-trisphosphate

A qualitative mathematical model is presented that examines membrane potential feedback on synthesis of inositol 1,4,5-trisphosphate (IP(3)), and its role in generation and modulation of slow waves. Previous experimental studies indicate that slow waves show voltage dependence, and this is likely to result through membrane potential modulation of IP(3). It is proposed that the observed response of the tissue to current pulse, pulse train, and maintained current injection can be explained by changes in IP(3), modulated through a voltage-IP(3) feedback loop. Differences underlying the tissue responses to current injections of opposite polarities are shown to be due to the sequence of events following such currents. Results from this model are consistent with experimental findings and provide further understanding of these experimental observations. Specifically, we find that membrane potential can induce, abolish, and modulate slow wave frequency by altering the excitability of the tissue through the voltage-IP(3) feedback loop.

Role of calcium stores and membrane voltage in the generation of slow wave action potentials in guinea-pig gastric pylorus

1. Intracellular recordings made in single bundle strips of a visceral smooth muscle revealed rhythmic spontaneous membrane depolarizations termed slow waves (SWs). These exhibited 'pacemaker' and 'regenerative' components composed of summations of more elementary events termed spontaneous transient depolarizations (STDs). 2. STDs and SWs persisted in the presence of tetrodotoxin, nifedipine and ryanodine, and upon brief exposure to Ca2+-free Cd2+-containing solutions; they were enhanced by ACh and blocked by BAPTA AM, cyclopiazonic acid and caffeine. 3. SWs were also inhibited in heparin-loaded strips. SWs were observed over a wide range of membrane potentials (e.g. -80 to -45 mV) with increased frequencies at more depolarized potentials. 4. Regular spontaneous SW activity in this preparation began after 1-3 h superfusion of the tissue with physiological saline following the dissection procedure. Membrane depolarization applied before the onset of this activity induced bursts of STD-like events (termed the 'initial' response) which, when larger than threshold levels initiated regenerative responses. The combined initial-regenerative waveform was termed the SW-like action potential. 5. Voltage-induced responses exhibited large variable latencies (typical range 0.3-4 s), refractory periods of approximately 11 s and a pharmacology that was indistinguishable from those of STDs and spontaneous SWs. 6. The data indicate that SWs arise through more elementary inositol 1,4,5-trisphosphate (IP3) receptor-induced Ca2+ release events which rhythmically synchronize to trigger regenerative Ca2+ release and induce inward current across the plasmalemma. The finding that action potentials, which were indistinguishable from SWs, could be evoked by depolarization suggests that membrane potential modulates IP3 production. Voltage feedback on intracellular IP3-sensitive Ca2+ release is likely to have a major influence on the generation and propagation of SWs.

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.

Potentiation of depolarization-induced calcium release from skeletal muscle triads by cyclic ADP-ribose and inositol 1,4,5-trisphosphate

Two second messengers, cyclic ADP-ribose (cADPR) and inositol 1,4,5-trisphosphate (IP3), potentiated the Ca2+ release from sarcoplasmic reticulum induced by transverse tubular membrane depolarization monitored in a triadic vesicle prepared from skeletal muscle. However, without depolarization they could not trigger the Ca2+ release. On the contrary, only cADPR potentiated caffeine-induced Ca2+ release. Because Ca2+ releases potentiated by cADPR and IP3 were inhibited by 1 microM ruthenium red and 100 microM ryanodine, probably these second messengers potentiated the Ca2+ release through ryanodine receptor Ca2+ channels. These results suggest that in skeletal excitation-contraction coupling, cADPR and IP3 play a role as a potentiator or a modifier in vivo, but both modification pathways are different from each other.

Negative inotropic effect of heparin on tension development in rat skinned skeletal muscle fibres

Heparin inhibits inositol trisphosphate receptors, particularly in smooth muscle, but its effect on skeletal muscle is controversial. Our study showed that heparin induced a decrease in the amplitude of 10 mM caffeine-induced contracture in slow and fast saponin-skinned fibres. Moreover, measurements on Triton X-100-skinned fibres in soleus muscle showed that heparin alone decreased maximal Ca2(+)-activated tension and Ca2+ sensitivity of contractile proteins, whereas no significant effect was observed in extensor digitorum longus muscle. However, in the presence of caffeine, heparin decreased maximal Ca2(+)-activated tension in both muscles. It would appear that the heparin-induced decrease in the amplitude of caffeine contracture in rat skeletal muscle was not related to a direct inhibition of Ca2+ release from sarcoplasmic reticulum but to a desensitising effect of heparin and caffeine on myofilaments.

Modulation of Ca2+ channels, charge movement and Ca2+ transients by heparin in frog skeletal muscle fibres

This study is an investigation into the modulatory effects of heparin, a component of the extracellular matrix that binds to dihydropyridine receptors, on contraction and Ca2+ channels in frog skeletal muscle. Using tension and Ca2+ signal measurements in single intact skeletal muscle cells we have found that heparin (100-200 micrograms ml-1) substantially potentiates twitch and tetanic tension (55% and 28%, respectively). In contrast, heparin reduces the amplitude of K+ contractures. Heparin most likely potentiates twitch tension by prolonging action potentials. The ionic basis of this effect was investigated in voltage-clamp experiments. Membrane currents were monitored in voltage-clamped segments of single fibres using the triple Vaseline gap technique. We found that heparin partially blocks delayed rectifier potassium channels. The depressive effects of heparin on K+ contractures prompted us to investigate the effects of heparin on charge movement and Ca2+ currents (ICa) under voltage-clamp. Charge movement was measured using a subtraction procedure that employed a -20 mV control pulse from a holding potential of -100 mV. Heparin depresses the total charge by 25%. We propose that the reduction in the amplitude of potassium contractures is related to a partial blockade of charge movement. Extracellular heparin shifts the ICa-V relation toward more negative voltages and delays the deactivation of tail currents. Double pulse experiments revealed that conditioning depolarizations speed the activation of ICa during test depolarizations. Heparin does not affect this process. The primary action of heparin is to accelerate the activation of ICa during pulses not preceded by conditioning depolarizations. Overall, the kinetic effects of heparin on ICa would increase the Ca2+ influx associated with action potentials. However, mechanical and optical experiments performed in Ca(2+) -free solutions and in the presence of Ca2+ channel blockers revealed that twitch and tetanic potentiation occur even in the absence of Ca(2+) -influx.

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.

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.

G protein effects on Ca2+ release and excitation-contraction coupling in skeletal muscle fibers

Skeletal muscle excitation-contraction (EC) coupling may involve secondary mechanisms, such as those involving G proteins. The aim of this study was to identify possible G protein effects on sarcoplasmic reticulum (SR) Ca2+ release, in general, and on voltage-dependent EC coupling, in particular. Effects of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) were studied using a single peeled rabbit skeletal muscle fiber preparation that is capable of releasing SR Ca2+ in response to transverse tubule (TT) depolarization. Because of possible nonspecific and residual effects of 200 microM GTP gamma S, a lower concentration of 50 microM GTP gamma S was used to stimulate G proteins in the peeled fiber system. Under conditions for steady-state (resting) polarization of TT, GTP gamma S rarely elicited SR Ca2+ release. When the TTs are in steady-state (resting) depolarization, 50 microM GTP gamma S or GTP elicited SR Ca2+ release and associated tension transients in only 69% of fibers tested. In contrast GTP gamma S always augmented Ca2+ release during TT depolarization-induced EC coupling. These results indicate the presence of at least two excitatory G proteins for SR Ca2+ release, only one of which is a modulator, albeit nonessential, of peeled fiber EC coupling.

Lack of prophylactic or therapeutic efficacy of 5-HT2A receptor antagonists in halothane-induced porcine malignant hyperthermia

During halothane-induced malignant hyperthermia (MH), plasma levels of serotonin (5-hydroxytryptamine, 5-HT) increase in pigs. Administration of 5-HT agonists which stimulate the 5-HT2A subreceptor triggers MH in susceptible pigs. A possible link between MH induced by 5-HT2A receptor agonists and halothane could be an increase of second messengers such as phosphoinositides (inositol polyphosphates), which have recently been implicated in the abnormal regulation of skeletal muscle calcium release in MH. If so, antagonists of 5-HT2A receptors which are linked to phosphoinositide turnover should be capable of preventing, retarding or attenuating halothane-induced MH. This possibility was investigated in the present study in MH susceptible pigs, using dantrolene for comparison, Development of MH triggered by a halothane challenge (inhalation of 3% halothane for 15 min) was completely prevented by dantrolene, 3.5 mg/i.v., whereas the 5-HT2A receptor antagonists ritanserin (0.5-10 mg/kg i.v.) or ketanserin (0.5-10 mg/kg i.v.) exerted no prophylactic effect. In pigs in which dantrolene, ritanserin or ketanserin where given in combination with hyperventilation after development of MH, dantrolene exerted therapeutic efficacy, whereas neither ritanserin nor ketanserin were effective treatments. The data indicate that 5-HT is not critically involved in the mechanisms of halothane-induced MH, at least under the conditions of the present experimental study.

The metabolism of D-myo-inositol 1,4,5-trisphosphate and D-myo-inositol 1,3,4,5-tetrakisphosphate by porcine skeletal muscle

In soluble and particulate extracts from muscle D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and D-myo-inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] are metabolised stepwise to inositol. Ins(1,4,5)P3 is rapidly dephosphorylated to D-myo-inositol 1,4-bisphosphate then to D-myo-inositol 4-phosphate and finally inositol. In soluble extracts Ins(1,3,4,5)P4 is dephosphorylated to D-myo-inositol 1,3,4-trisphosphate then sequentially to D-myo-inositol 3,4-bisphosphate, D-myo-inositol 3-phosphate and inositol, while in particulate extracts D-myo-inositol 1,3-bisphosphate is the predominant inositol bisphosphate formed. Dephosphorylation of these inositol polyphosphates is Mg2+ dependent and inhibited by D-2,3-bisphosphoglyceric acid. Ins(1,4,5)P3 is also phosphorylated to form Ins(1,3,4,5)P4 in soluble extracts by Ins(1,4,5)P3 3-kinase. Ins(1,4,5)P3 3-kinase activity is Mg2+ and ATP dependent and is stimulated by Ca2+ and calmodulin. Particulate (sarcotubular) inositol polyphosphate 5-phosphatase (5-phosphatase) is found in membranes which are intimately involved in excitation-contraction coupling and the generation of the primary Ca2+ signal of muscle cells. Particulate 5-phosphatase had the highest specific activity in the transverse-tubule membrane, when compared to the terminal cisternae and longitudinal-tubule membranes of the sarcoplasmic reticulum. Particulate Ins(1,3,4,5)P4-3-phosphatase activity was also detected after fractionation of solubilised sarcotubular membranes by DEAE-Sephacel. Particulate 5-phosphatase activity was purified 25,600-fold to a specific activity of 25.6 mumol Ins(1,4,5)P3 hydrolysed.min-1.mg protein-1, after DEAE-Sephacel and novel affinity chromatography using D-2,3-bisphosphoglycerate/agarose and Sepharose-4B-immobilised Ins(1,4,5)P3-analog matrices. Purified particulate 5-phosphatase had apparent Km of 46.3 microM and 1.9 microM and Vmax of 115 and 0.046 mumol substrate hydrolysed.min-1.mg protein-1, for Ins(1,4,5)P3 and Ins(1,3,4,5)P4, respectively. In contrast, purified soluble type I 5-phosphatase had apparent Km of 8.9 microM and 1.1 microM and Vmax of 3.55 and 0.13 mumol substrate hydrolysed.min-1.mg protein-1, for Ins(1,4,5P3 and Ins(1,3,4,5)P4, respectively. As in other cells, muscle 5-phosphatases have a lower affinity, but a higher capacity to metabolise Ins(1,4,5)P3 than Ins(1,3,4,5)P4. Soluble type I 5-phosphatase may have a functional role in the metabolism of both inositol polyphosphates, while particulate 5-phosphatase may primarily metabolise Ins(1,4,5)P3. Purified Ins(1,4,5)P3 3-kinase had an apparent Km of 0.42 microM and a Vmax of 4.12 nmol Ins(1,4,5)P3 phosphorylated.min-1.mg protein-1. The profile of inositol polyphosphate metabolism in muscle is similar to that reported in other tissues.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

Role of ryanodine receptors

Recent findings on the ryanodine receptor of vertebrates, a Ca-release channel protein for the caffeine- and ryanodine-sensitive Ca pools, are reviewed in this article. Three distinct genes, i.e., ryr1, ryr2, and ryr3, express different isoforms in specific locations: Ryr1 in skeletal muscle and Purkinje cells of cerebellum; Ryr2 in cardiac muscle and brain, especially cerebellum; Ryr3 in skeletal muscle of nonmammalian vertebrates, the corpus striatum, and limbic cortex of brain, smooth muscles, and the other cells in vertebrates. While only one isoform (Ryr1) is expressed in mammalian skeletal muscles, two isoforms (alpha- and beta-isoforms expressed by ryr1 and ryr3, respectively) are found in nonmammalian vertebrate skeletal muscles. Although the coexistence of two isoforms may merely be related to differentiation and specialization, the biological significance remains to be clarified. Ryanodine receptors in vertebrate skeletal muscles are believed to mediate two different modes of Ca release: Ca(2+)-induced Ca release and action potential-induced Ca release. All results obtained so far with any isoform of ryanodine receptor are related to Ca(2+)-induced Ca release and show very similar characteristics. Ca(2+)-induced Ca release, however, cannot be the underlying mechanism of Ca release on skeletal muscle activation. Susceptibility of the ryanodine receptor's ryanodine-binding activity to modification by physical factors, such as osmolality of the medium, might be related to action potential-induced Ca release. A hypothesis of molecular interaction in view of the plunger model of action potential-induced Ca release is discussed, suggesting that the model could be compatible with Ryr1 and Ryr3, but incompatible with Ryr2. The functional relevance of ryanodine receptor isoforms, especially Ryr3, in brain also remains to be clarified. Among ryr1 gene-related diseases, malignant hyperthermia was the first to be identified; however, there is still the possibility of involvement of the other genes. Central core disease has been added to the list recently. A molecular approach for the diagnosis and treatment of diseases is now in progress.

Effects of the novel potassium channel opener, UR-8225, on contractile responses in rat isolated smooth muscle

1. The effects of UR-8225 [(1,2-dihydro-4-(1,2-dihydro-2-oxo-1-pyridyl)-2,2-dimethyl-1-oxonapht halen-6- carbonitrile)] and levcromakalim were studied on the electrical and contractile responses induced by noradrenaline and KCl and on 86Rb+ efflux in rat aortic rings and on spontaneous mechanical activity in rat portal vein segments. 2. UR-8225 and levcromakalim, 10(-9) M-10(-5) M, relaxed the contractile responses induced by noradrenaline (IC50 = 2.7 +/- 0.4 x 10(-6) M and 6.6 +/- 1.3 x 10(-7) M, respectively) or 30 mM KCl (IC50 = 1.4 +/- 0.2 x 10(-7) M and 9.4 +/- 1.3 x 10(-8) M, respectively) more effectively than those induced by 80 mM KCl. The relaxant effect on noradrenaline-induced contractions was independent of the presence or absence of functional endothelium. 3. The vasorelaxant effect of UR-8225 and levcromakalim can be competitively antagonized by glibenclamide, an ATP-sensitive K+ channel blocker. There were no differences in the calculated pA2 values for glibenclamide to inhibit UR-8225- and levcromakalim-induced relaxations (7.61 +/- 0.08 and 7.69 +/- 0.10, respectively). The slope of the Schild plot yielded values not significantly different from unity (0.95 +/- 0.06 and 0.96 +/- 0.05, respectively). 4. UR-8225 (10(-5) M) hyperpolarized the resting aortic membrane potential from -50.7 +/- 0.7 mV to -66.0 +/- 2.0 mV and stimulated 86Rb+ efflux. 5. UR-8225 and levcromakalim inhibited the contractions induced by Ca2+ in aortae incubated in Ca(2+)-free PSS containing methoxyverapamil in the presence of noradrenaline. 6. Both drugs inhibited the amplitude of spontaneous activity in portal veins (IC50 = 5.1 +/- 1.4 x 10-8 M and 1.5 +/- 0.7 x 10-8 M, respectively), this effect being competitively antagonized by glibenclamide.7. These results indicated that UR-8225 exhibited qualitatively similar, but slightly less potent,vasorelaxant effects than those exerted by levcromakalim, which suggests that they can be related to its ability to activate ATP-sensitive K+ channels in vascular smooth muscle cells.

Membrane potential modulates inositol 1,4,5-trisphosphate-mediated Ca2+ transients in guinea-pig coronary myocytes

1. Vascular smooth muscle cells were isolated from the coronary artery of the guinea-pig. At 2.5 mM [Ca2+]o and 36 degrees C, whole cell membrane currents were recorded under voltage-clamp and the concentration of ionized calcium in the cytoplasm ([Ca2+]i) was monitored by indo-1 fluorescence. 2. At -60 mV, [Ca2+]i was 143 +/- 36 mM (mean +/- S.D.) and was insensitive to clamp steps to +100 mV. During 1 min application of acetylcholine (ACh, 10 microM) [Ca2+]i increased within approximately 2 s to 1480 +/- 250 nM. During the subsequent slow decay, [Ca2+]i was transiently increased by depolarizing clamp steps and decreased during hyperpolarizing steps. [Ca2+]i transients in response to caffeine (10 mM) could not be modulated by voltage steps. The results suggest that modulation of [Ca2+]i by membrane potential involves inositol 1,4,5-trisphosphate (Ins(1,4,5)P3)-induced Ca2+ release (IICR). 3. Modulation of IICR by membrane potential did not depend on sarcolemmal Ca2+ fluxes; it persisted after block of sarcolemmal Ca2+ fluxes with 3 mM lanthanum or after a change to nominally Ca(2+)-free bathing solutions. 4. Modulation of [Ca2+]i by membrane potential was recorded during cell dialysis of 50 microM GTP-gamma-S in the absence of ACh. Cell dialysis of exogenous Ins(1,4,5)P3 (50 or 100 microM) did not mimic the effects. The sensitivity of [Ca2+]i to depolarizing clamp steps was also induced by cell dialysis of lithium ions which, presumably, inhibited the breakdown of Ins(1,4,5)P3. The results are compatible with the idea that the membrane potential modulates the liberation of Ins(1,4,5)P3. 5. Modulation of IICR by membrane potential is discussed as a new mechanism that contributes to the regulation of activator calcium and to the modulation of contraction in vascular smooth muscle cells.

Effects of local anesthetics on single channel behavior of skeletal muscle calcium release channel

The effects of the two local anesthetics tetracaine and procaine and a quaternary amine derivative of lidocaine, QX314, on sarcoplasmic reticulum (SR) Ca2+ release have been examined by incorporating the purified rabbit skeletal muscle Ca2+ release channel complex into planar lipid bilayers. Recordings of potassium ion currents through single channels showed that Ca(2+)- and ATP-gated channel activity was reduced by the addition of the tertiary amines tetracaine and procaine to the cis (cytoplasmic side of SR membrane) or trans (SR lumenal) side of the bilayer. Channel open probability was lowered twofold at tetracaine and procaine concentrations of approximately 150 microM and 4 mM, respectively. Hill coefficients of 2.0 and greater indicated that the two drugs inhibited channel activity by binding to two or more cooperatively interacting sites. Unitary conductance of the K(+)-conducting channel was not changed by 1 mM tetracaine in the cis and trans chambers. In contrast, cis millimolar concentrations of the quaternary amine QX314 induced a fast blocking effect at positive holding potentials without an apparent change in channel open probability. A voltage-dependent block was observed at high concentrations (millimolar) of tetracaine, procaine, and QX314 in the presence of 2 microM ryanodine which induced the formation of a long open subconductance. Vesicle-45Ca2+ ion flux measurements also indicated an inhibition of the SR Ca2+ release channel by tetracaine and procaine. These results indicate that local anesthetics bind to two or more cooperatively interacting high-affinity regulatory sites of the Ca2+ release channel in or close to the SR membrane. Voltage-dependent blockade of the channel by QX314 in the absence of ryanodine, and by QX314, procaine and tetracaine in the presence of ryanodine, indicated one low-affinity site within the conduction pathway of the channel. Our results further suggest that tetracaine and procaine may primarily inhibit excitation-contraction coupling in skeletal muscle by binding to the high-affinity, regulatory sites of the SR Ca2+ release channel.

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.

Lack of effect on the sodium efflux of the microinjection of D-Ins(1,4,5)P3 into ouabain-poisoned barnacle muscle-fibers

A study has been carried out using relatively intact mature muscle fibers from the barnacle Balanus nubilus to see whether D-Ins(1,4,5)P3 stimulates the ouabain-insensitive Na efflux following its microinjection and whether this is accompanied by a contraction of the fiber. Part of the impetus for a study of this type came from the on-going debate between Vergara, Rojas and co-workers and Lea and co-workers, the former group holding the view that skinned barnacle fibers and skeletal fibers in general are responsive to this isomer. The evidence brought forward indicates that the injection of D-Ins(1,4,5)P3 in concentrations in the range of 10(-2) M to 10(-6) M into cannulated unskinned fibers pretreated with ouabain fails to increase the residual efflux, and additionally fails to elicit a contraction. A similar picture emerges with the use of non-hydrolyzable DL-Ins(1,4,5)P3[S]3, analog following its injection in a concentration of 0.5 microM. Higher concentrations of the analog were unavailable for use. This work also involved verification of the idea that an effect might be obtainable in depolarized fibers. However, doubling or tripling K0+ and injection of the isomer or the analog simultaneously failed to support this idea. Since nothing is known as to whether D-Ins(1,4,5)P3 influences the behavior of the Na(+)-Ca2+ exchanger when operating in the reverse mode, experiments were done to check this possibility. ATPNa2 which is though to activate Na(+)-Ca2+ exchange was injected prior to the isomer or the analog but no significant results were obtained. A similar line of reasoning was followed, that of activating the L-type Ca2+ channel by injecting GTPNa2 which in addition is known to activate adenylate cyclase. Again, neither the isomer nor the analog were effective. Thus, the only conclusion possible is that in relatively intact, mature barnacle fibers there is no coupling between the phosphoinositide signalling pathway and two other key systems, viz. the Na(+)-Ca2+ exchanger when operating in the reverse mode and the L-type Ca2+ channel. Equally clear is that for some unknown reason the ouabain-insensitive Na efflux and the contractile apparatus are insensitive to D-Ins(1,4,5)P3[S]3.

The metabolism of microinjected inositol trisphosphate in Xenopus oocytes

Microinjection of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) into Xenopus oocytes evokes a complex physiological response composed of a transient and a slow depolarizing chloride current. We investigated the relationship between intracellular levels of Ins(1,4,5)P3 and the kinetics of the physiological response. Microinjected Ins(1,4,5)P3 was slowly degraded following first order kinetics of disappearance (t1/2 = 10 min). The degradation products were inositol bisphosphate (InsP2), inositol monophosphate (InsP) and inositol, as well as inositol tetrakisphosphate (InsP4). The rate of degradation of injected 3[H]-Ins(1,4)P2 was much greater (t1/2 = 3 min), indicating that the conversion of InsP3 to InsP2 may be the rate-limiting step in the degradation process. The slow degradation of 3[H]-Ins(1,4,5)P3 was not a result of its conversion to Ins(1,3,4)P3 since no accumulation of InsP3 was observed within 10 min of microinjection of 3[H]-Ins(1,3,4,5)P4. Activation of protein kinase C (PK-C) with a phorbol ester transiently increased the rate of conversion of 3[H]-Ins(1,4,5)P3 to InsP2. This, however, did not significantly affect the overall kinetics of 3[H]-Ins(1,4,5)P3 disappearance. Our results indicate that the kinetics of Ins(1,4,5)P3 degradation do not correlate well with the termination of both the rapid and the slow components of the physiological response. The termination of the slow component of the response, however, may be related to the decay of Ins(1,4,5)P3-induced 45Ca efflux, which lasted about 10 min.

Actions of perchlorate ions on rat soleus muscle fibres

1. The effects of perchlorate (ClO4-) on contraction have been studied in rat soleus muscle fibres using (i) potassium (K+) contracture and (ii) two-microelectrode-point voltage clamp techniques. 2. Membrane potentials (Vm) at all external [K+] were 3-5 mV more negative in ClO4-. The hyperpolarization could not be attributed to a change in Na+, K+, or Cl- permeability, or to an effect on the Na(+)-K+ pump. 3. ClO4- shifts the voltage dependence of tension activation, and contraction threshold, to more negative membrane potentials without altering maximum tension. Consequently, twitches and submaximal K+ contractures were potentiated, whereas tetanic contractions and 200 mM-K+ contractures were unaltered. 4. The decay of K+ contractures during steady depolarization with ClO4- developed a slow exponential phase with an average time constant of 6.05 +/- 0.76 min at -38 mV, and 1.68 +/- 0.15 min at -19 mV. This slow component was (a) under the rapid control of the surface Vm and (b) did not depend on external Ca2+. 5. Inactivation of E-C coupling was measured with a test 200 mM-K+ depolarization following 3-10 min depolarizations in conditioning solutions containing 20-120 mM-K+. ClO4- induced a negative shift in the curve-relating test K+ contracture amplitude to conditioning Vm but did not alter the rate of repriming of tension upon repolarization. 6. The results suggest that ClO4- increases the amount of activator produced during depolarization and thus allows the slow inactivation step in excitation-contraction (E-C) coupling to be reflected in the decay of K+ contracture tension. 7. A 'perchlorate contracture', which did not depend on the activation of E-C coupling, was observed. The contracture depended on external Ca2+, but not on voltage-dependent Ca2+ channels or Na(+)-Ca2+ exchange.

The mechanical hypothesis of excitation-contraction (EC) coupling in skeletal muscle

The mechanism of transmission in skeletal muscle EC coupling is still an open question. There is some indirect evidence in favour of the mechanical coupling hypothesis, deriving mostly from consideration of the structure of the Ca2+ release channel protein. A new functional approach is proposed, that consists in comparing the properties of the complete system--EC coupling in a skeletal muscle fibre--with those of the EC coupling molecules in bilayers. In this approach, those properties of the whole system that are not traceable to its constitutive molecules, are ascribed to the physiological interaction, and are expected to yield new information on the nature of this interaction.

GTP gamma S causes contraction of skinned frog skeletal muscle via the DHP-sensitive Ca2+ channels of sealed T-tubules

We have investigated the involvement of G-proteins in excitation-contraction coupling of fast-twitch skeletal muscle, using a fibre preparation designed to retain intact T-tubules and sarcoplasmic reticulum. The nonhydrolysable analogue of guanosine triphosphate, GTP gamma S (50-500 microM) caused a strong, transient isometric contraction in this preparation. Reduction of ethylene-bis(oxonitrilo)tetraacete (EGTA) in the sealed T-tubules from 5 mM to 0.1 mM lowered the threshold to GTP gamma S and removal of sodium reversibly raised it. The dihydropyridine (DHP) calcium channel antagonists nicardipine and nifedipine allowed a first contraction and then blocked subsequent GTP gamma S action. The phenylalkylamine methoxyverapamil (D-600) did likewise, reversibly, at 10 degrees C. The guanosine diphosphate analogue, GDP beta S, and procaine reversibly blocked the action of GTP gamma S; pertussis toxin also blocked it. Photolytic release of 40-100 microM GTP gamma S within 0.1 s from S-caged GTP gamma S caused contraction after a latent period of 0.3-20 s. We conclude that GTP gamma S can activate contraction in frog skeletal muscle via a route requiring both the integrity of the T-tubular DHP-sensitive calcium channel (DHPr) and the presence of sodium in the sealed T-tubules. We propose that in this preparation GTP gamma S activates a G-protein, which in turn activates the DHPr as a calcium channel and releases stored calcium from within the sealed T-tubule. Implications of these results for the excitation-contraction coupling mechanism in skeletal muscle are discussed.

Inositol trisphosphate enhances Ca2+ oscillations but not Ca(2+)-induced Ca2+ release from cardiac sarcoplasmic reticulum

The role of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] in excitation-contraction coupling in cardiac muscle is still unclear, although many laboratories are beginning to assume a critical role for this putative second messenger. Earlier studies from this laboratory [Nosek et al. (1986) Am J Physiol 250:C807] found that Ins(1,4,5)P3 enhanced spontaneous Ca2+ release and the caffeine sensitivity of Ca2+ release from myocardial sarcoplasmic reticulum (SR) and proposed an increase in the Ca2+ sensitivity of the release as a possible mechanism. In order to clarify the physiological relevance of these actions of Ins(1,4,5)P3 and specifically to test the effect of Ins(1,4,5)P3 on the Ca2+ sensitivity of Ca2+ release, we compared the effects of Ins(1,4,5)P3 on Ca2+ oscillations and on Ca(2+)-induced Ca2+ release (CICR) from the SR in saponin-skinned rat papillary muscle. We found that: (a) 30 microM Ins(1,4,5)P3 enhanced the Ca2+ oscillations (measured by tension oscillations) from the rat cardiac SR, consistent with the previous report on guinea pig tissue; (b) both GTP and GTP[S] enhanced Ca2+ oscillations. The effect was not additive to that of Ins(1,4,5)P3 indicating that two different Ca(2+)-release pools do not exist in cardiac SR; (c) 30 microM Ins(1,4,5)P3 had no effect on the Ca2+ sensitivity of CICR; (d) Ins(1,4,5)P3 (up to 30 microM) had no effect on SR Ca2+ loading. The studies were performed in the presence of Cd2+ or 2,3-bisphosphoglycerate, agents that inhibit Ins(1,4,5)P3 hydrolysis.(ABSTRACT TRUNCATED AT 250 WORDS)

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).

Calcium release modulated by inositol trisphosphate in ruptured fibers from frog skeletal muscle

To investigate the effect of inositol 1,4,5-trisphosphate on calcium release, we used fiber bundles of frog sartorius muscle mechanically permeabilized by a scratching procedure, and we detected increments in calcium concentration by measuring aqueorin light signals. Submicromolar concentrations of inositol 1,4,5-trisphosphate induced fast calcium-release signals, with a half time to peak of 60 ms or less. Similar responses were elicited by caffeine. The calcium-release signal induced by inositol 1,4,5-trisphosphate occurred at pCa values of 7 or lower, and the dose-response curve depended on the ionic composition of the incubation solution. Lower inositol 1,4,5-trisphosphate concentrations were needed to induce release when incubation solutions of ionic composition expected to depolarize the transverse tubule membrane were used. Inositol 1,4,5-trisphosphate was more effective than inositol 1,3,4-trisphosphate, inositol 1,4,5,6-tetrakisphosphate, and inositol 1,4-bisphosphate. The effect of inositol 1,4,5-trisphosphate was synergistic with that of caffeine, and was not inhibited by heparin. These results, by showing directly that at resting calcium levels inositol 1,4,5-trisphosphate elicited calcium release, are consistent with a role for inositol 1,4,5-trisphosphate as a chemical modulator in excitation/contraction coupling in skeletal muscle.

Functional expression of the calcium release channel from skeletal muscle ryanodine receptor cDNA

Combined patch-clamp and fura-2 measurements were performed to study the calcium release properties of Chinese hamster ovary (CHO) cells transfected with the rabbit skeletal muscle ryanodine receptor cDNA carried by an expression vector. Both caffeine (1-50 mM) and ryanodine (100 microM) induced release of calcium from intracellular stores of transformed CHO cells but not from control (non-transfected) CHO cells. The calcium responses to caffeine and ryanodine closely resembled those commonly observed in skeletal muscle. Repetitive applications of caffeine produced characteristic all-or-none rises in intracellular calcium. Inositol 1,4,5-trisphosphate (IP3) neither activated the ryanodine receptor channel nor interfered with the caffeine-elicited calcium release. These results indicate that functional calcium release channels are formed by expression of the ryanodine receptor cDNA.

Isolation of a Ca2(+)-releasing factor from caffeine-treated skeletal muscle fibres and its effect on Ca2+ release from sarcoplasmic reticulum

In the presence of 2 mM caffeine, skeletal muscles of the frog exert small irregular oscillations of single sarcomeres. A factor, released from these oscillating muscles, was partially purified, and its activity was tested on skinned fibres and isolated vesicles of the sarcoplasmic reticulum (SR). Purification was achieved in three steps by gel filtration and reversed phase chromatography, and the active compound of the released material was shown probably to be a small peptide. In skinned fibres, the purified factor evoked repetitive contractions and subthreshold sarcomeric oscillations. In 'heavy' SR vesicles passively loaded with 45Ca2+, the factor induced a small but significant increase in the 45Ca2+ efflux rate. At the single channel level, the open probability of the SR Ca2+ release channel increased when the factor was added to the cytoplasmic side of the channel. The results reveal that the released factor potentiates Ca2+ release from the SR by increasing the open time of the Ca2+ release channel.

Further observations on the behaviour of ouabain-insensitive sodium efflux towards proctolin in barnacle muscle fibres

1. A further study has been made of the stimulatory action of proctolin on the ouabain-insensitive Na+ efflux in single muscle fibres from the barnacle, Balanus nubilus. 2. (i) Strontium (Sr2+) behaves as a substitute for external Ca2+. In this case, however, the response to proctolin fails to decay. (ii) Injection of Sr2+ stimulates the ouabain-insensitive Na+ efflux. This effect is mimicked by injecting Ca2+. 3. Depolarization of the fibre membrane with 30 and 100 mM-external K+ augments the response to proctolin. 4. Pre-injection of GTP or Gpp(NH)p (sodium 5-guanylylimidodiphosphate) prevents the response to proctolin from completely decaying. 5. Pre-injection of guanine nucleotides in conjunction with membrane depolarization stops the response to proctolin from decaying. 6. Measurements of Em before and during treatment with proctolin indicate a prompt but small and reversible fall in the membrane potential. 7. (i) The aequorin response of fibres pre-treated with ouabain to proctolin is monophasic or multiphasic, and concentration dependent, the minimal effective concentration being in the nanomolar range. (ii) The duration of these signals is usually less than 5 min; this is about half the time it takes for the stimulated Na+ efflux to reach a maximum. (iii) The aequorin response to proctolin occurs quite often in fibres suspended in nominally Ca2(+)-free artificial sea water. (iv) Sudden graded elevations in external K+ following complete decay of the aequorin response to proctolin are rapidly followed by stepwise transitory increments in light emission. (v) The aequorin response to 100 mM-external K+ is frequently a triplet. 8. (i) Together, these results are in line with the view that the action of proctolin on the ouabain-insensitive Na+ efflux is the result of a temporary fall in internal pCa and that its point of action is the Ca2+ channel, where a putative G protein in the presence of GTP or Gpp(NH)p is able to maintain constancy of the hormonal effect. (ii). They strengthen the argument that the barnacle muscle fibre as a preparation is especially suitable for studies of this kind.

Does muscle activation occur by direct mechanical coupling of transverse tubules to sarcoplasmic reticulum?

Our knowledge of the physiological and biochemical constituents of skeletal muscle excitation has increased greatly during the last few years but this has not led to a consensus of the physiological mode of muscle activation. Three hypotheses of transmission, involving either transmitter-receptor interaction or direct mechanical coupling, are still under active consideration. The hypothesis of direct mechanical coupling currently being evaluated proposes that the dihydropyridine receptor in the transverse tubules serves as a voltage sensor that communicates directly with the junctional foot protein/Ca2+ channel of sarcoplasmic reticulum to initiate opening of the channel.

Inositol trisphosphate and excitation-contraction coupling in skeletal muscle

The role of inositol trisphosphate as a chemical messenger in excitation-contraction coupling is discussed, both in terms of positive and negative results. The evidence presented includes experiments on the effect of inositol trisphosphate in intact and skinned fibers, in calcium release from isolated sarcoplasmic reticulum vesicles, in activation of single calcium release channels incorporated in planar bilayers, and biochemical experiments that have established the presence of all the intermediate steps involved in the metabolism of phosphoinositides, both in intact muscle and in isolated membranes. From these results, it is clear that a role for inositol triphosphate in skeletal muscle function is highly likely; whether this molecule is the physiological messenger in excitation-contraction coupling remains to be established.

Intracellular localization of inositol-phospholipid-metabolizing enzymes in rabbit fast-twitch skeletal muscle. Can D-myo-inositol 1,4,5-trisphosphate play a role in excitation-contraction coupling?

Rabbit fast-twitch skeletal muscle microsomes have been separated by isopycnic centrifugation on a linear sucrose gradient into triads and light sarcoplasmic reticulum. In both fractions phosphatidylinositol-kinase activity is found [Varsányi et al. (1986) Biochem. Biophys. Res. Commun. 138, 1395]. In contrast, phosphatidylinositol-4-phosphate kinase is nearly exclusively associated with triads. The phosphatidylinositol-4,5-bisphosphate-phosphodiesterase activity shows a biphasic distribution: approximately 50% of the activity is associated with triads and 50% appears in the overlay. Triads have been broken mechanically into transverse tubules and terminal cisternae, then separated by isopycnic sucrose-gradient centrifugation. Both fractions exhibit phosphatidylinositol-kinase activity; the activities of phosphatidylinositol-4-phosphate kinase and phosphatidylinositol-4,5-bisphosphate phosphodiesterase are associated mainly with the transverse tubules. Consequently, in rabbit fast-twitch skeletal muscle all necessary enzymes for production of D-myo-inositol 1,4,5-trisphosphate are associated with transverse tubules. Phosphatidylinositol-4,5-bisphosphate phosphodiesterase associated with triads shows a pH optimum at 6.8. The enzyme is maximally active between pCa 5 and pCa 4. Mg2+ inhibits the enzyme activity half-maximally at about 1 mM. Guanine-nucleotide-binding proteins seem not to be involved in the regulation of enzyme activity; guanosine 5'-[gamma-thio]triphosphate does not influence phosphatidylinositol-4,5-bisphosphate phosphodiesterase activity. It correlates well with the observation that neither alpha 1-adrenergic nor muscarinic receptors have been found in fast-twitch rabbit skeletal muscle. On basis of the respective enzyme activities estimations on maximal phosphatidylinositol turnover were made and a possible involvement of this signal pathway in excitation-contraction coupling has been discussed. Furthermore, calculations show that during a single twitch D-myo-inositol 1,4,5-trisphosphate concentration does not reach more than 2 nM. However, during a 4-s tetanus D-myo-inositol 1,4,5-trisphosphate can accumulate to a level which could effect force generation [Thieleczek and Heilmeyer (1986) Biochem. Biophys. Res. Commun. 135, 662] and aldolase distribution (Thieleczek et al., unpublished results).

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

In many non-muscle cells, D-inositol 1,4,5-trisphosphate (InsP3) has been shown to release Ca2+ from intracellular stores, presumably from the endoplasmic reticulum. It is thought to be a ubiquitous second messenger that is produced in, and released from, the plasma membrane in response to extracellular receptor stimulation. By analogy, InsP3 in muscle cells has been postulated to open calcium channels in the sarcoplasmic reticulum (SR) membrane, which is the intracellular Ca2+ store that releases Ca2+ during muscle contraction. We report here that InsP3 may have a second site of action. We show that InsP3 opens dihydropyridine-sensitive Ca2+ channels in a vesicular preparation of rabbit skeletal muscle transverse tubules. InsP3-activated channels and channels activated by a dihydropyridine agonist in the same preparation have similar slope conductance and extrapolated reversal potential and are blocked by a dihydropyridine antagonist. This suggests that in skeletal muscle, InsP3 can modulate Ca2+ channels of transverse tubules from plasma membrane, in contrast to the previous suggestion that the functional locus of InsP3 is exclusively in the sarcoplasmic reticulum membrane.