Effects of endogenous calcium transport inhibitor from

Effects of endogenous calcium transport inhibitor from heart muscle on the active calcium uptake and passive calcium release properties of sarcoplasmic reticulum

Can. J. Physiol. Pharmacol. Downloaded from www.nrcresearchpress.com by Depository Services Program on 06/06/13 For personal use only.

N J A N ~NARAYANAN,' R PHILIBBEBARD,AND TRXLSGHAN L$. WARAHGH Department of Physiokoa, Health Sciences Center, The e/P8iversiQ of Western Omtario, London, Ont., C a d a N6A 5CP Received December 6, 1988 NARAYANAN, N., BEDARD,P., and WAWAICM, T. S. 1989. Effects of endogenous calcium transport inhibitor from beart muscle on active calcium uptake and passive calcium release properties of swcoplasmic reticulum. Can. J. Physiol. Phmmcol. 6'7:999- 1 W . In the present study, the effects of the cytosolic ca2+transport inhibitor on ATPdepeadent ca2+uptake by, and unidirectional passive ca2+ release from, sarcoplassmic reticulum enriched membrane vesicles were exmined in parallel experiments to determine whether inhibitor-mediated enhmcement in ca2+ efflux contributes to inhibition of net 81a2+uptake. When assays were performed at pH 6.8 in the presence of oxalate, low concentrations ( e l 0 0 pg1m.L) of the inhibitor caused substmt.id inhibition of ca2+uptake by SR (28-5Q%). At this pH, low concentrations of the inhibitor did not cause enhmcement of passive ca2+release from actively ~ a ~ + - l o a dsarcoplassnic ed reticulum. Under these conditions, high concentrations(>100 pg1n-L) of the inhibitor caused stimulationof passive Ca2+release but to a much lesser extent when corn wed with the extent of inhibition of when ca2+ uptake and active ca2+ uptake (i.e., twofold greater inhibition of ca2+ uptake than stimulation of CaE relea~e)~ release assays were carried out at pH 7.4, the ca2+release promoting action of the inhibitor became more pronounced, such that the magnitude of enhmcement in ca2+ release at varying concentrations of the inhibitor (20-280 pg1n-L) was not m a r k d y different from the magnitude of inhibition of ca2+uptake. In the absence of oxalate in the assay medium, inhibition of Ca2+ uptake was observed at dkdine but not acidic pH. These findings imply that the inhibition of ca2+uptake observed at pH 6.8 is mainly due to decrease in the rate of active Ca2+ transport into the membrane vesicles rather than stimulation of passive ca2+ efflux; at alkaline pH (pH 7.4,enhanced ca2+efflux contributes substantially,if not exclusively, to the decrease in ca2+uptake observed in the presence of the inhibitor. It is suggested that if the cytosolic inhibitor has actions similar to those observed in vitro in intact cardiac muscIe, acid-base status of the intracellular fluid would be a ma'or factor influencing the nature of its effects (inhibition of ca2+ uptake or stimulation of Ca2+ release) on transmembrane C d t fluxes across the sarcoplasmic reticulum. Key words: sarcoplasmic reticulum, Ca2+uptake, ca2+release, endogenous inhibitor, heart muscle. NARAYANAN, N., BEDARD, P., et WARAICH, T. S. 1989. Effects of endogenous calcium transport inhibitor from heart muscle on active calcium uptake and passive calcium release properties of sarcoplasmie reticulum. Can. J. Physiol. Pharmacol. 67 : 999- E 006. Dms la prksente Ctude, on a exmink les effets de l'inhibiteur de transport du Ca2+ cytosolique sur la capture de c a 2 + dependante de 1 ' A P par des vtsicules membrarnaires ewichies du r6ticulum sarcoplasmique et sur sa liberation passive unidirectionnellepar celles-ci. Ceci visait !determiner si la stimulation de I'efflux de ca2+mediCe par l'inhibiteur contribue B I'inhibition de la capture new de ca2+. A un pH 6,8 en presence d'oxalate, de faibles concentrations (ilW~ g l m l de ) l'inhibiteur ont provoqd une inhibition importante de la capture de ca2+par 1er6ticulum sarcoplasmique (28-504). A ce pH, de faibles concentrations de l'inhibiteur n'ont pas pmvoqu6 de stimulation de la liwration passive de Ca2+ du reticulum sarcsplasfnique charge activement de a2+. Bans ces conditions, de fortes concentrations de l'inhibiteur (>100 p g l d ) ont provoque m e stimulation de Ha liberation passive de ca2+,mais B un degrt? beaucoup plus faible que celui de E'inhibition de la capture active de ca2+ (c.-A-d. inhibition de la capture de Ca2+d'un facteur deux plus forte que la stimulation de la liMration de ca2+).A un pH 7,4, l9actionde I'inhibiteur favorisantla libdration de ca2+est devenue plus prononcke, de sorte que 19mplitude de la stimulationde la liberation de ca2', A diverses concentrations de l'inhibiteur (20-200 p g l d ) , n'a pas differ6 nettement de celle de l'inhibition de la capture de ca2+.En I'absence d'oxalate dans le milieu de dosage, on a observe une inhibition de la capture de Ca2' A un pH alcalin m i s Ron Bun pH acide. Ces resultats impliquent que H'inhibition de la capture de c a 2 + ,obsm6e h un pH 6,8, est Wncipalement due B une diminution du tam de transport actif du ca2+dans les v6sicules membrainajresplutdt qu'a une stimulation de 19effluxpassif de ca2+;B un H alcalh (pH 7,4), un efflux accm de ca2+est un factew impoftant, sinon le seul facteur de diminution de la capture de Ca+ ! observee en presence de I'inhibiteur. On sugghre que si l'inhibiteur cytosolique a des actions shilakes B celles observees in vitro dans le muscle cardiaque intact, B'Ctat acido-basique du flrmide intracellulaire serait un factem majeur influensant la nature de ses effets (inhibition de la capture de ca2+ ou stimulation de la liberation de ca2+)sur les efflux de ca2+ transmembranaires via le r6ticulum sarcoplasmique. [Traduit gar la revue]

Imtroduction In the mammalian heart, Ca2+ fluxes across the sarcoplasmic reticulum (SR) and sarcolemma (SL) play a central role in excitation-contraction coupling. Muscle contraction is triggered by elevation of intracellular free c a 2 + , which occurs as a result of excitation-induced c a 2 + influx into the myocardial cell (Langer 1982; Tsien 8983) and calcium-induced c a 2 + release from the SR (Fabiato and Fabiato 1979). Subsequent muscle relaxation is achieved by lowering of the sarcoplasmic free c a 2 + ' ~ u t h o rto whom correspondence skoud be addressed.

concentration by mechi~rmisrnsthat include sequestration of Cazi by ATP-driven ca2+pumps (Mg2+ ca2+ ATPase) located in the SR (Tada e t al. 1984) and S L (Suldche and St. Louis 1979; Caroni and Carafoli 1980) and transport of c a 2 + out of the cell via the sarcolemmal ~ a + - C a 2 +exchanger (Reeves and S u k o 1979; Langer 1982; Mullins 1984). The best known ph siologicd mechanism for the regulation of bansmembrane CaJ+ fluxes (influx and efflux, and therefore cardiac contraction and relaxation) involves phosphsrylation of specific membrane proteins by cyclic AMP--dependent and ca2+cdmodulin-dependent protein kinases, and dephosphorylation

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by phosphoprotein phosphatase (Tada and Katz 1982; Sperelakis 1984; \.Adatmabe and L i n d e m m 198%).Original observations previously reported from our laboratory have suggested a novel type of regulation of the ATP-dependent c a 2 + transport activity of SR by cytosolic proteins present in heart muscle (Narayman et all. 1982,1983). Thus, it was demonstrated that a cytosolic protein fraction from rabbit heart caused strong inbibition of ATP-supported c a 2 + uptake by rabbit heart SR; this inhibition could be antagonized by another cytosolic fraction devoid of the inhibitor. These observations, coupled with the finding that the cytosolic protein inhibitor of SR Ca2+ transport is selectively distributed in slow-@ontractingmuscle types such as the heart and slow skeletal muscle but not fast skeletal muscle, prompted us to suggest a potential role for the inhibitor (and its antagonist) in the in vivs regulation of the SR ca2+ pump in slow- contracting muscle fibers (Narayanan et al. 1983). Ira subsequent studies, we have found that the inhibitor and its antagonist are present in cardiac muscle of various other mammalian species such as the rat, guinea pig, and dog (N. Narayanan and M . Newland, unpublished observations), and the ATP-supported CaZ+ uptake activity of isolated cardiac sarcolermmal vesicles can also be modulated by these cytosolic proteins (Nafayman et a1. B 989). Recent studies by Chiesi a d co-workers (Chiesi and Guerini 198'7; Chiesi and Schwaller 1987), using cytosolic fractions from guinea pig he& and SR from dog heart and rabbit skeletal muscle, have confirmed the inhibitory and antagonist actions of the cytosolic proteins on SR Ca2+ transport described by us. Further, in these studies, the cytosolic Ca2+ transport inhibitor and its antagonist were isolated and characterized as two distinct proteins; the former with an apparent molecular weight of 43 OQB showed similarities to actin isofoms (p- and (or) ?-type, distinct from the a-type actin of myofibrils) while the latter with an apparent molecular weight of 68 000 resembled B-actinin (muscle albumin). In their study, Chiesi and Schawller (1987) observed that the cytosolic ca2' transport inhibitor caused potentiation of EGTA-induced passive c a 2 release from SR when the Ca2+ uptake reaction was carried out in the absence of oxalate (a c a 2 + precipitating anion), and they concluded that the mechanism of inhibition involved activation of a ca2+ release pathway in the SR membrane. However, they did not observe such a c a 2 + release promoting action of the inhibitor when active c a 2 + loading of the SR vesicles was carried out in the presence of oxalate in the Ca2+ uptake assay medium (Chiesi m d Schwaller 1987), which is in accordance with the results of our previous studies (Narayman et al. 1982, 1983). In this report, we describe the results of a detailed quantitative analysis of the effects of the cytosolic c a 2 + transport inhibitor on ATP-dependent c a w uptake by, and unidirectional passive ca2+ release from, SR-enriched membrane vesicles. Our results indicate that the inhibition of Ca2+ uptake at low concentrations of the inhibitor (I80 pg/snaL), and especially at alkaline pH (pH 7 4 , the inhibitor might cause marlred activation of Ca" release.

Materials and methods Chemicals 4 5 ~ a ~(31.61 1 2 mCi/mg; 1Ci = 37 GBq) was purchased from New England Nuclear, Montreal, Canada. All other chemicals were of highest purity available from Sigma Chemical Co., St. Louis, MO, U.S.A., or BDH Chemicals, Toronto, Canada.

Preparation of cytosolie ~ a " +ansport inhibitor The cytosralic protein fraction enriched in ca2+transport inhibitor was prepared h m rabbit heart ventricles as desckkd previously (Natayanan et d. 1982, 1983). Briefly, cardiac tissue was rinsed, minced md homogenized in 110 volumes of ice-cold homogenizing buffer (10 nd4 Tris, 0.25 M sucrose, and 8.2 mM dithiserytheitol at pH 7) using a Bolytron IT-20 homogenizer. Soluble cytosolic proteins were obtained after centrifugation at 10 000 X g for 20 min and at 105 886 x g for 2 h. Solid (Nt-LJ2S04 was added to the supernatant (cytosol) to produce 364% saturation and the protein grecipitatd was collectd by centrifbgation at 10 808 x g for 20 f i n . This protein fraction containing the inhibitor was dissolved in the homogenizing buffer, dialysed against the same buffer and was stored at -20°C. Isoliztion of 3R-enriched membrane vesicles and determimutionof Cd+ uptake SR-enriched membrane vesicles from rabbit heart (ventricles) md white skeletal muscle were prepared according to the procedure of Harigaya and Schwrn (1%9) with ratinor modifications as described previously (Sulakhe and Narayanm 1979). An-dependent ca2+ uptake by SR vesicles was determined using a Millipore filtration technique as detailed elsewhere (Narayanm 1981). The standard incubation medium (totalvolume 1 d) contained 58 rnEM Tris-maleate (pH 6.$), 5 anrM MgClz, 2.5 & ATP, 120 d v l KCI, 2.5 mha potassium oxalate, 5 R M I NaN3,0.1 rdvl EGTA, membrane vesicles (30 kg protein), and 0.1 rnld 4 5 ~ a ~(115 2000 - 20 000 c p d m o l ) to gj,vefree ca2+ concentration of 1B .9 M (Narayanan 1981). The assays were performed at 37°C; the Cay; uptake reaction was initiated by the addition of ATP &er prehcubation of the rest of the assay components for 3 min. The ca2+uptake assays were also performed in the absence of oxalate in the assay medium and at varying pH; deviations from standard assay conditions are specified in the figure captions. All studies were carried out using fieskly prepared membrane vesicles. Measurement of unidirectioml Ca2+ eBux Unidirectional passive ca2+ efflux from actively ca2+-loadedSR vesicles was determined as follows. The membrane vesicles (I mg protein) were incubated at room temperature (23°C) for 20 rnin in the standard caZf u take assay medium (total volume 10 d) containing 11.9 phi free C$+. Following this, the membranes were chilled on ice and were collected by centrifugation at 464 004) X g for 30 min. The vesicles were resuspended in 10 ndv! Tms-maleate (pH 4.8) containing 100 nmRl W l . To initiate ca2+ release, aliquots of the ca2' -loaded vesicles (180 pg protein) were diluted 10-fold into a ca2+ release medium (50 .mh4 TPis-maleate, pH 6.8, containing 100 Hlnif KC1 and 3 mh/i EGTA) that was preincubated for 3 min at 37°C. Subsequently, aliquots of the incubation mixture were filtered though Millipore filters at 30-s intervals for a period of 4.5 min. The filters were washed with 3 d of ice-cold 10 Tris-maleate buffer (pH 6.8), dried at @"C, md the radioactivity was determined by liquid scintillation spectrometry. Initial ca2' content (zero time control) of the ca2+loaded vesicles was determined by the Millipre filtration technique after 10-folddilution of the vesicles in 10 nd4 Tris-maleatebuffer (pH 6.8) containing 100 nd4 KC1. In experiments where ca2' efflux rates were determined at pH 7.4, the pH of the buffer (Tris-maleate) solutions used was adjusted to 7.4 instead of 6.8. Determimution of protein Rotein was determined by the method of Lowry et ak. (1951) using bovine serum albumin as the standard. Data analysis Each of the experiments described here was repeated at least thee times using separate SR preparations and the results obtained were similar. Data from representative experiments are illustrated in %he figures.

Results The results presented in Fig. 1 show the rates of unidirectional passive Ca2+ efflux from actively @+-loaded SR vesicles in the absence (control) and presence of the ca2+ transport


FIG. 1. Effects of Ca2+transport inhibitor on the rates of passive ca2+release from actively Ca2+loaded SR vesicles. Active ca2+loading (at pH 6.8) of membrane vesicles and subsequent ca2+ release assays (at pH 6.8) were performed as described under Materials md methods. The experiments were carrid out using skeletal muscle SR (A) and cardiac SR (B) in the absence of caZ+transport inhibitor in the release medium (a), and in the presence of 80 (@), 120 (a), or 200 (m) p.g/mL inhibitor in the release assay medium.

inhibitor in the release medium. With SR from skeletal muscle (Fig. 1A) and h e m (Fig. 1B), approximately 40-5096 of the initial Ca2+load was depleted in 4.5 min when ca2+release was measured in the absence of the inhibitor in the release medium. Inclusion of a low concentration of the inhibitor (80 pg/mL) in the release medium did not result in any appreciable alteration in the rates of Ca2+ =lease from SR.However, when the release medium was supplemented with high concentrations of the inhibitor ( 120 or 200 pg/mL), the rates of ca2+release from the membrane vesicles were enhanced substantially (ca. 15-1496 and 32-48% increase over control, respectively, with 120 and 200 pg/mL inhibitor). The effects of the inhibitor on ATP-dependent, oxalatefacilitated Ca2+ uptake by SR are shown in Fig. 2. The rates of ca2+ uptake by the membrane vesicles were decreased markedly in the presence of the inhibitor (80 or 128 ~ g / m Lin ) the Ca2+ uptake assay medium. It is noteworthy that substantial (ca. 35-58%) inhibition in the rates of ca2+ uptake could be observed with a low concentration of the inhibitor such as 80 pg,/mL; as noted already, at this concentration, the inhibitor did not enhance passive ca2+ release from Ca2+ preloaded membrane vesicles (see Fig. 1). Further, as reported in our previous study (Narayanan et al. 1983), inhibition of ca2+ uptake under these conditions (PH 4.8, oxalate present in the assay) is accompanied by inhibition of Ca2+-stimulatedATPase activity of SR. The findings described above suggested that at low concentrations the inhibitor may cause selective inhibition of active Ca2+ uptake by the membrane vesicles, whereas at high concentrations its effects might involve activation of ca2+ release. In order to evaluate the quantitative relationship between the inhibition of ca2+ uptake and stimulation of Ca2+ release by the inhibitor, we examined the effects of varying concentrations of the inhibitor on ATP-dependent ca2+ uptake by the membrane vesicles and unidirectional passive Ca2+ release from actively ca2+-loaded vesicles. The ca2+ uptake and release experiments were pefformed using same membrane

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FIG. 2. Effects of ~ a ' +transport inhibitor on the rates of ATBdependent ca2+uptake by skeletal muscle SR (A) and cardiac SR (B). The Ca2+uptake assays were performed (at pH 6.8) as described under Materials and methods in the absence of the inhibitor (0)and in the presence of 80 (e),or 120 (n) p g / d inhibitor.

preparations. The results of one such experiment are summarized in Fig. 3. It can be seen that at concentrationsbelow 120 ~ g / m L , the inhibitor did not produce passive ca2+ release, whereas active ca2+ uptake was inhibited as much as 28% in skeletal muscle SR (Fig. 3A) and 58% in cardiac SR (Fig. 3B). Further, even at higher concentrations of the inhibitor (120 or 200 pg,/mL), the extent of inhibition of active Ca2+ uptake was


lklH!EIlTOR FRACTION (p.gB FIG.^. Comparison of the effects sf Ca2+trans rt inhibitor on active ca2+uptake by, and passive Ca2+release from, membrane vesicles of skeletal muscle SR (A) md cardiac SW (B). For Ca4"'uptake experiments, incubations were carried out for 3 rnin in the standard Q'+uptake assay medium (pH 6.8) in the absence of the inhibitor and in the presence of v y p g concentrations of the inhibitor. For ca2' release experiments, actively Ca2+-loadedmembrane vesicles were incubated for 3 m i w in the Ca release medium (pH 6.8) in the absence of the inhibitor and in the presence of varying concentrationsof the inhibitor. Both ca2+uptake md release experiments were performed using same membrane pre mations. Other details were as described under Materials and methods. The percent inhibition of ca2+uptake (@) and the percent stimulation of C$+ release (s),as a function of inhibitor concentration ( ~ g 1 n - Lin) the assay medium, are illustrated here. The Ca2+uptake (nmol ca2+/mgprotein per 3 min) measured in the absence of the inhibitor was 34863 for skeletal muscle SR and 132 for cardiac SW;Ca2+release (mol ca2+/rngprotein per 3 min) measured in the absence of the inhibitor was 301 for skeletal muscle SR (initid ca2+conknt, 970 nmol ~a'+/rngprotein) and 32 for cardiac SW (initial Ca2+ content, 95 nmol protein).

found to be about twofold greater than stimulation of Ca2+ release. In our previous studies (Na~ayanmet al. 1982, 1983) we did not observe any effect of the endogenous Ca2+ transport inhibitor on passive Ca2+ release from SR even at a relatively high concentration such as 120 pg1m.L. In these earlier studies, the effect of the inhibitor on Ca2+ release was examined by adding the inhibitor directly into the ca2+ uptake assay medium at a time when ATP-dependent sxdate-facilitated ca2' uptake had reached a steady state. In a recent study, Chiesi and SchwdBer (1987) noted %$atwhen SR vesicles (from skeletal muscle) were preloaded with Ca2+ in the absence of oxdate, subsequent addition of the inhibitor together with EGTA into the assay medium resulted in greater rates of ca2+ release than that observed by the addition of EGTA done. On the other h a d , in conformity with our previous finding (Nxayanan et d. 1982, 1983), no such CaZf release-promoting action of the inhibitor was observed in similar experiments where ca2+ loading was carried out in the presence of oxalate (Chiesi and SehwdBer 1987). In view of these observations, we also examined the effects of the inhibitor on ca2+ uptake in the absence of oxalate in the assay medium. Surprisingly, when the Ca" uptake assays were performed in the absence of axalate and at pH 6.8, no inhibition of Ca2+uptake could be observed at a wide range of inhibitor concentrations (20-200 pg/mL) in the assay; in fact, a modest stimulation (up to 25%) of ca2+uptake was consistently observed under these conditions (Fig. 4). On the other hand, when the ca2+ uptake assays were performed in the absence of oxalate, and at pH 7.4, the inhibitor caused concentration-dependentinhibition of Ca2+ uptake by SR (Fig. 4) in a manner malogsus to that seen in studies of oxalatefacilitated ca2+ uptake (Nxayman et al. 1982, 1383). In

d d i t i m d experiments, the effects of the inhibitor on ca2+ uptake by SW in the absence of oxdate was examined at varying pH. As shown in Fig. 4 (inset), inhibition of Ca2+ uptake was observed only at above neutral pH. While the data presented in Fig. 4 were obtainedusing skeletal muscle SW, essentially similar observations were also made in experiments using S R from cardiac muscle as well (not shown). Figure 5 shows the results of experiments in which the effects sf the inhibitor was examined at varying pH in the presence of oxalate in the assay medium. Under these conditions, with SR from heart. (Fig. 5A) and skeletal muscle (Fig. 5B), inhibition of Ci?+ uptake was observed at acidic as well as alkaline pH; the magnitude of inhibition was more pronounced at the pH range 6-7 5 . Since high concentrations (>I00 eg/mL) of the inhibitor were found to enhance passive c a w release from SW at pH 6.8 (Figs. 1 and 31, in subsequent studies we examined whether this coam@en@ation-dependenteffect of the inhibitor on the ca2+ release property sf S W was also influenced by changes in pH of the release assay medium. It was also deemed important to assess the quantitative relationship between the effects of the inhibitor on ca2+ uptake and ca2+ release at rn alkaline pH, as opposed to pH 6.8, routinely used for most ~f the studies. Thus, experiments were carried out in which ATP-dependent (sxalatefacilitated) ca2+ uptake by SW and unidirectional passive ca2+ release from actively Ca2+-loaded SIP were measured at pH 7.4 in the absence and presence of varying concentrations of the inhibitor. Figure 6 shows the results sf such an experiment using SR from skeletal muscle (A) and h e m (B). It can $e seen that the Ca2+ release-promoting action of the inhibitor is substantially greater at pH '7.4 (Fig. 6) than that seen at pH 6.8 (Fig. 3). Further, the difference between the magnitude 0% stimulation of


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FIG. 4. Concentration- and H-dependence of the ca2+ transport inhibitor on ATP-dependent C$+ uptake by skeletal muscle SR in the absence of oxalate. Ca2+ uptake was determined under standard assay conditions (see Materids and methods) except that oxdate was omitted from the assay medium and the incubations were carried ~ u(for t I min) in the absence and presence of the inhibitor, at pH 6.8 and 7.4 or at varying pH (inset). The results are presented as percent of control; the control (180%) ca2+uptake activities (mmo1 CaD/rng protein per 1 min) measured at pH 6.8 (E) and 7.4 (0) were 15 1 md 58, respectively. In the inset, denotes Ca2+ uptake in the absence of the inhibitor, and E denotes ca2+ uptake in the presence of 80 p g / d inhibitor.

ca2+ release and inhibition of Ca2+ uptake at varying concentrations of the inhibitor is less marked when the Ca2+release and uptake assays are performed at pH 7.4; this is in contrast to the more pronounced effect of the inhibitor on active Caa+ uptake than passive ca2' release seen when the assays are performed at pH 6.8 (compare data in Figs. 3 and 6).

Discussion Our previous studies (Narayanan et al. 1982, 1983) documented the strong inhibitory effect of the endogenous cytosolic protein from heart muscle on ATP-dependent ca2* uptake by cardiac and skeletal muscle SR. In these earlier studies, no effect of the inhibitor on passive ca2+ release from SR was observed when the inhibitor was added directly to the Ca2+ uptake assay medium at a time when active ca2+ sequestration by the membrane vesicles had reached a steady state. We therefore attributed the inhibitory action of the cytosolic protein on ea2' uptake by SW to a decrease in the turnover rates of the membrane c a w pump (Narayanan et al. 1982, 1983). In recent reports, Chiesi and co-workers (Chiesi and Guerini 1987;Chiesi and Schwaller 1987) confirmed several characteristics of the inhibitor originally described by us (Narayanan et d. 1982, 1983);however, they observed potentiation, by the inhibitor, of EGTA-induced ca2+ release from SR preloaded with ca2+ in the absence of oxalate (but not in the presence of oxalate), and concluded that the mechanism of inhibition of ca2+ uptake involved activation of a Ca2+ release pathway in the SR membrane (Chiesi and Schwaller 1987). The major focus of the present study was to determine whether or not an inhibitor-

FIG.5. Effects of the Ca" transport inhibitor on ATP-dependent ca2+uptake by cardiac (A) md skeletal muscle (B) SR at varying pH. Ca2+ uptake was determined under standard assay conditions (see Materials and methods) except that the pH of the incubation medium was varied as shown and the assays were carried out in the absence (0) of the inhibitor and in the presence (E) of 100 p+g/rnLinhibitor.

mediated activation of passive Ca2' release can account for the decrease in ATP-dependent ca2+ uptake observed in the psence of the inhibitor. "he results presented here demonstrate that depending on the experimental conditions employed, the decrease in ATB-dependent ca2+ uptake by membrane vesicles observed in the presence of the endogenous Ca2+ transport inhibitor can result from inhibition of active ca2+ sequestration or stimulation of passive ca2+ release. Thus, when assays are conducted at pH 6.8, low concentrations of the inhibitor (