Effects of diet on the function of sarcoplasmic reticulum.

We have examined the effect of diet on the phospholipid composition of the sarcoplasmic reticulum of rabbit muscle. Enriching the diet with corn or fish oil results in significant changes in the fatty acyl chain composition of the various phospholipid classes, with relatively little change in the relative contents of the phospholipids. These alterations in composition have no significant effect on the ATPase activity of vesicles of sarcoplasmic reticulum or on the pattern of Ca2+ uptake and release.     

Calcium-induced calcium release from fragmented sarcoplasmic reticulum.

A new method is introduced which allows the study of calcium-induced calcium release from fragmented sarcoplasmic reticulum. Results obtained with this method are in agreement with those obtained by previous investigators using skinned muscle fiber. It was also found that anesthetic drugs and alcohol increased the calcium- and caffeine-induced calcium release from the sarcoplasmic reticulum.     

Spin-labelling studies of fragmented sarcoplasmic reticulum

Vesicular fragments of sarcoplasmic reticulum (SR) were spin labelled with 2,2,6,6-tetramethyl, 4-isothiocyanate piperidine-1-oxyl (probe A) and 2,2,6,6-tetramethyl, 4-amino (N-iodoacetamide) piperidine-1-oxyl (probe B). Two to five moles of probe A or B were covalently bound to 10⁶g of membrane protein, with minimal loss of activity (ATPase, Ca²⁺, uptake). The EPR spectra of labelled SR were then studied in various experimental conditions.Strongly acid or alkaline pH, protein denaturation with ura, and membrane solubilization with deoxycholate produced marked alterations of the EPR spectra of spin-labelled SR, indicating changes in the local environment surrounding the probes, and the occurrence of conformational changes.A reversible modification of the EPR spectra of probe A and an accelerated efflux of accumulated Ca²⁺ were produced by increasing the temperature of SR suspensions from 30° to 40° C. Such a parallel behavior indicates that reversible structural transitions may control membrane permeability and Ca²⁺ efflux.ATP modifies the EPR spectra of probe B, suggesting that ATP binding to the membrane induces a structural change involving the local environment of certain sulfhydryl groups. The ATP concentration required for this effect is comparable to that requied for activation of ATPase. ADP and ITP are also effective, while pyrophosphate, AMP, and cyclic AMP are not. The effect of ATP is reversible.In other experiments, 2,2,6,6-tetramethylpiperidine-1-oxyl (probeC) was equilibrated with concentrated suspensions of SR. The EPR spectra obtained thereafter indicate that probe C binds to the membrane fragments, still maintaining a high degree of motional freedom. These spectra were markedly changed by deoxycholate solubilization of the membrane fragments, while they were little affected by protein denaturation with guanidine. These results confirm the hypothesis that the region of distribution of probe C into SR, is prevalently constituted by low-viscosity lipids.Supported by research grants from USPHS (HE 09878), the American Heart Association (66742), and the Muscular Distrophy Association of America.     

Effect of thyroxine on the function of rabbit skeletal muscle sarcoplasmic reticulum

Thyrotoxicosis in rabbits was induced by prolonged intraperitoneal injection of L-thyroxin. The development of thyroxicosis was assoiated with a decreased Ca2+ accumulation rate by sarcoplasmic reticulum (SR) fragments and a lowered Ca2+ dependent ATPase activity. As compared to the analogous parameters in normal animals. Ca2+ accumulation rate and ATPase activity of thyrotoxicosis animals decreased by 60 and 25%, respectively. The changes in the specific parameters of SR were also observed during incubation of normal SR samples in the medium containing thyroxin (10-5 M). The changes seen in SR functioning in thyrotoxicosis animals are likely to be related to structural rearrangements of lipoprotein surroundings of Ca-ATPase.     

Comparison between strontium and calcium uptake by the fragmented sarcoplasmic reticulum.

The ATP-supported uptake of strontium by the fragmented sarcoplasmic reticulum is monophasic and proceeds more rapidly than the fast uptake of calcium. Strontium uptake is not activated by Pi. The accumulation of strontium is nearly proportional to the external strontium concentration even in the millimolar range. Internal and external strontium quickly equilibrate. One mole of strontium is stored for every mole of ATP split by the Sr2+-activated ATPase. In the absence of oxalate most of the strontium is taken up with a transport ratio of one. On the opposite, the transport ratio of calcium decreases immediately, especially when ADP is not instantaneously phosphorylated to ATP. In this case, energy conversion is uncoupled more effectively by the simultaneous action of ADP and free internal calcium, resulting in the interruption of the fast uptake. After depletion of ATP most of the stored strontium is released and the remaining fraction appears to be not exchangeable. Strontium activates the slow uptake of calcium, but reduces the amplitude of the fast uptake. The calcium induced release of strontium, and vice versa, is partial and transient. The strontium activated ATPase does not transport calcium at low ionic calcium concentrations.     

Ca-releasing action of beta, gamma-methylene adenosine triphosphate on fragmented sarcoplasmic reticulum.

beta,gamma-Methylene adenosine triphosphate (AMPOPCP) has two effects on fragmented sarcoplasmic reticulum (FSR), i.e., inhibition of the rate of Ca uptake and the induction of Ca release from FSR filled with Ca. The Ca release brought about by AMPOPCP has many features in common with the mechanism of Ca-induced Ca release: i) it is inhibited by 10 mM procaine; ii) the amount of Ca release increases with increase in the extent of saturation of FSR with Ca; iii) increase of the Ca concentration in the extent of saturation of FSR with Ca; iii) increase of the Ca concentration in the medium facilitates the release of Ca. However, no facilitation of Ca release upon decrease of Mg concentration in the medium is observable. AMPOPCP and caffeine potentiate each other remarkably in their Ca-releasing action, irrespective of the kind of substrate. From the mode of action of AMPOPCP on the rate of Ca uptake, the amount of phosphorylated intermediate (EP), and the effect on Sr release, it is suggested that the state of the FSR-ATP complex is crucial for Ca-induced Ca release.     

Electron cytochemistry of calcium uptake in the fragmented sarcoplasmic reticulum

Summary Ultrastructural aspects and cytochemical localisation of Ca-uptake in the fragmented sarcoplasmic reticulum (FSR) have been studied with positive and various negative staining techniques.Effect of fixation, staining and other preparative procedures for electron microscopy on the result of the cytochemical reaction have been investigated.Calcium outflow from loaded vesicles occurs in various degree depending on the method used so that no quantitative estimation of Ca-uptake appears possible by ultrastructural studies alone.Ultrastructure and proportion of calcium containing vesicles also vary considerably and no certain correlations can be made between their size and shape and localisation of Ca-uptake. Certain informations can be obtained only by comparison of results with different methods, taking into account also the limits of each method applied.     

Effects of exercise of varying duration on sarcoplasmic reticulum function

Sarcoplasmic reticulum (SR) Ca2+ uptake and Ca2+-Mg2+-ATPase activity were examined in muscle homogenates and the purified SR fraction of the superficial and deep fibers of the gastrocnemius and vastus muscles of the rat after treadmill runs of 20 or 45 min or to exhaustion (avg time to exhaustion 140 min). Vesicle intactness and cross-contamination of isolated SR were estimated using a calcium ionophore and mitochondrial and sarcolemmal marker enzymes, respectively. Present findings confirm previously reported fiber-type specific depression in the initial rate and maximum capacity of Ca2+ uptake and altered ATPase activity after exercise. Depression of the Ca2+-stimulated ATPase activity of the enzyme was evident after greater than or equal to 20 min of exercise in SR isolated from the deep fibers of these muscles. The lowered ATPase activity was followed by a depression in the initial rate of Ca2+ uptake in both muscle homogenates and isolated SR fractions after greater than or equal to 45 min of exercise. Maximum Ca2+ uptake capacity was lower in isolated SR only after exhaustive exercise. Ca2+ uptake and Ca2+-sensitive ATPase activity were not affected at any duration of exercise in SR isolated from superficial fibers of these muscles; however, the Mg2+-dependent ATPase activity was increased after 45 min and exhaustive exercise bouts. The alterations in SR function could not be attributed to disrupted vesicles or differential contamination in the SR from exercise groups and were reinforced by similar changes in Ca2+ uptake in crude muscle homogenates.(ABSTRACT TRUNCATED AT 250 WORDS)     

Diaphragm disuse reduces Ca2+ uptake capacity of sarcoplasmic reticulum

Howell, Sandra, Wen-Zhi Zhan, and Gary C. Sieck.Diaphragm disuse reduces Ca²⁺uptake capacity of sarcoplasmic reticulum. J. Appl.Physiol. 82(1): 164-171, 1997.Chronic phrenictetrodotoxin (TTX) blockade and phrenic denervation (Dnv) of hamsterdiaphragm result in decreased maximum specific tension, prolongedcontraction time, and improved fatigue resistance (W. Z. Zhan and G. C. Sieck. J. Appl. Physiol. 72:1445-1453, 1992). An underlying increased relative contribution oftype I fibers to total muscle mass appears to be consistent with, butdoes not completely account for, changes in contractile and fatigueproperties. The present study was designed to evaluate a potential rolefor altered cellular Ca²⁺metabolism in the adaptive response of the diaphragm to chronic disuse.An analytic method based on simulation and modeling of long-term⁴⁵Ca²⁺efflux data was used to estimateCa²⁺ contents (nmolCa²⁺/g wet wt tissue) and exchangefluxes (nmolCa^{2+ ·} min^{1 ·} g¹)for extracellular and intracellular compartments in the in vitro hamster hemidiaphragm after prolonged disuse. Three groups were compared: control (Con, n = 5),phrenic TTX blockade (TTX, n = 5), andphrenic denervation (Dnv, n = 5).Experimental muscles were loaded with⁴⁵Ca²⁺for 1 h, and efflux data were collected for 8 h by using a flow-through tissue chamber. Compartmental analysis of efflux data estimated thatthe Ca²⁺ contents andCa²⁺ exchange fluxes of thelargest and slowest intracellular compartment (putative longitudinalreticulum) were reduced by ~50% in TTX and Dnv muscle groupscompared with Con. In addition, the kinetic model predicted significantdecreases in total intracellularCa²⁺ and total diaphragmCa²⁺ in TTX and Dnv muscles. Weconclude that the data support the hypothesis that the capac- ity ofthe sarcoplasmic reticulum for Ca²⁺ sequestration is reduced inchronic diaphragm disuse. The impact of this effect on diaphragmcontractile and fatigue properties is discussed.

     

Oxalate dependence of calcium uptake kinetics of rabbit skeletal muscle microsomes (fragmented sarcoplasmic reticulum)

The initial rate of oxalate-facilitated Ca²⁺ uptake by skeletal microsomes depends on both Ca²⁺ and oxalate concentrations in the medium. The apparent K_m for Ca²⁺ increases with increasing oxalate concentration, indicating that Ca²⁺ uptake can involve a carrier-mediated transport system.