Contribution of the ryanodine-sensitive fraction to capabilities of cardiac SR

Ryanodine-sensitive, thapsigargin-insensitive calcium uptake in rat ventricle homogenates

Thapsigargin is a natural product that specifically inhibits all known SERCA calcium pumps with high affinity. We investigated the effects of thapsigargin on cardiac sarcoplasmic reticulum (SR) by measuring the oxalate-supported calcium uptake rate in the unfractionated homogenate and in the isolated SR fraction. The uptake rate in both the isolated SR and unfractionated homogenate are stimulated about two-fold by preincubation with high concentrations of ryanodine, which closes the SR efflux channel. Thapsigargin stoichiometrically and completely inhibited the calcium uptake rate in the isolated SR, both in the presence and absence of SR channel blockade. In contrast, thapsigargin nearly completely inhibited the homogenate calcium uptake only in the absence of SR channel blockade; in the presence of blockade, about 20% of the uptake activity was insensitive to thapsigargin. This result unmasks a thapsigargin-insensitive, ryanodine-sensitive component of calcium uptake in the heart. This activity is in an oxalate-permeable pool and is inhibited by cyclopiazonic acid, another inhibitor of the SERCA calcium pumps. There was no TG-insensitive activity in the rat EDL muscle homogenate. The absence of thapsigargin-insensitive uptake activity in the isolated SR can be attributed to its inactivation during the isolation of the SR. The oxalate permeability and ryanodine sensitivity suggest that the TG-insensitive calcium uptake activity is closely related to the classical SR. The different thapsigargin sensitivities suggests the existence of two kinds of intracellular calcium pumps in the heart.

Comparison of sarcoplasmic reticulum capabilities in toadfish (Opsanus tau) sonic muscle and rat fast twitch muscle

The sonic muscle of the oyster toadfish, Opsanus tau, can produce unfused contractions at 300 Hz. Electron microscopy shows a great abundance of the Sarcoplasmic reticulum (SR) in this muscle, but no functional characterization of the capabilities of the SR has been reported. We measured the oxalate-supported Ca2+ uptake rate and capacities of homogenates of toadfish sonic muscle and rat extensor digitorum longus (EDL) muscle, and estimated the number of pump units by titration with thapsigargin, a high-affinity, specific inhibitor of the SR Ca-ATPase. The Ca2+ uptake rate averaged 70.9 +/- 9.5 mumol min -1 per g tissue for the toad fish sonic muscle, and 73.5 +/- 3.7 mumol min -1 g-1 for rat EDL. The capacity for Ca2+ -oxalate uptake was 161 +/- 20 mumol g -1 and 33 +/- 2 mumol g -1 for toadfish sonic muscle and rat EDL, respectively. Thus, the rates of Ca2+ uptake were similar in the two muscles, but the toadfish sonic muscle had about five times the capacity of the rat EDL. The number of pumps as estimated by thapsigargin titration was 68 +/- 4 nmol of Ca-ATPase per g tissue in the toadfish, and 42 +/- 5 nmol Ca-ATPase per g tissue in the rat EDL. The turnover number, defined as the Ca2+ uptake divided by the number of pumps, was 1065 +/- 150 min -1 for toadfish and 1786 +/- 230 min -1 for rat EDL (p < 0.05) at 37 degrees C. The Ca2+ uptake rate of toadfish sonic muscle at 22 degree C, a typical temperature for calling toadfish, averaged 42 +/- 1% of its rate at 37 degree C. At these operating temperatures, the toadfish SR is likely to be slower than the rat fast-twitch SR, yet the toadfish sonic muscle supports more rapid contractions. One explanation for this is that the voluminous SR provides activator Ca2+ for contraction, but the abundant parvalbumin plays a major role in relaxation.

The effects of ryanodine on calcium uptake by the sarcoplasmic reticulum of ischemic and reperfused rat myocardium

The effects of ischemia and reperfusion on sarcoplasmic reticulum (SR) calcium uptake were measured in crude heart homogenates of rats and were compared to published results for rabbit hearts. Isolated rat hearts (n = 5 in each group) were Langendorff-perfused at 37 degrees C and were either kept normally perfused (control group), or submitted to 15 min normothermic ischemia (ischemic group), or reperfused for 10 min after 15 min ischemia (reperfused group). Mechanical function recovered to 50-60% of control after 10 min reperfusion following ischemia. Ca uptake (control Vmax: 23.0 +/- 2.20 nmol.min.1.mg of protein-1) decreased during ischemia (Vmax: 15.7 +/- 1.60 nmol.min-1.mg-1) but recovered to control level on reperfusion (Vmax: 20.8 +/- 2.02 nmol.min-1.mg-1). An increased Ca uptake was obtained when the measurements were carried out in the presence of ryanodine (430 microM) to block Ca leakage through SR Ca-release channels. The relative magnitude of ryanodine effect in the ischemic myocardium (increase: 77.2 +/- 18.20%) was more marked than in control (32.0 +/- 8.22%) or reperfused myocardium (39.0 +/- 10.66%). This result is different from that of rabbit myocardium where similar ryanodine effect is present in all groups (56.7 +/- 13.76%, 50.0 +/- 13.56% and 54.2 +/- 6.88% in control, ischemic and reperfused hearts, respectively) and suggests that a component of cytosolic Ca overload via SR Ca-release channels is present during ischemia in rat, but not in rabbit myocardium.

Effects of simulated ischemia and reperfusion on the sarcoplasmic reticulum of digitonin-lysed cardiomyocytes

ATP-dependent, inorganic phosphate-supported 45Ca2+ uptake by digitonin-lysed adult rat ventricular cardiomyocytes was used to evaluate the effects of simulated ischemia and reperfusion on the physically intact sarcoplasmic reticulum. Mitochondrial reactions were inhibited with rotenone and oligomycin. 45Ca2+ accumulation in the presence of the calcium efflux inhibitors, procaine (10 mM) and ruthenium red (30 microM), was used to characterize unidirectional uptake kinetics. A decrease in pH from 7.2 to 6.6 increased the [Ca2+] K0.5 from 0.5 to 2.0 microM and reduced the apparent Vmax by 28%. In the absence of procaine and ruthenium red, at a free [Mg2+] of 0.5 mM, maximum net uptake occurred at pCa 6.2 when pH was 7.2 and at pCa 6.0 when pH was 6.6. At lower pCa, net Ca2+ accumulation declined. Increasing free [Mg2+] from 0.5 to 1 mM at pH 6.6 or to 2.5 mM at pH 7.2 increased net 45Ca2+ accumulation in the absence of procaine and ruthenium and shifted maximum uptake to pCa 5.6 and 6.0, respectively. Increases in cytosolic free [Mg2+] thought to occur during myocardial ischemia are therefore capable of inhibiting calcium efflux from the sarcoplasmic reticulum. Reducing [ATP] from 10 to 1 mM reduced maximum net 45Ca2+ uptake by 30% both in the presence and absence of efflux inhibitors. Preincubation of intact myocytes under conditions designed to simulate ischemia and reperfusion decreased 45Ca2+ uptake greater than or equal to 50%. The data indicate that myocardial ischemia and reperfusion can alter both Ca2+ accumulation and calcium release by the sarcoplasmic reticulum.

Kinetics of restitution of left ventricular relaxation

Although the kinetics of cardiac systolic force restitution have been well described, the restitution kinetics of left ventricular relaxation have not been examined. To define relaxation restitution behavior, we studied seven dogs chronically instrumented with left ventricular high-fidelity micromanometers and piezoelectric dimension crystals. After a priming period at a basic cycle length of 375 msec, test extrastimuli were introduced after a range of extrasystolic intervals (ESIs). Relaxation behavior of control and extrasystolic beats was characterized by the time constant of isovolumic relaxation, tau. Relaxation restitution can be described by two concatenated monoexponential curves, an early phase described by a rapid time constant and a late phase described by a slower time constant (TC1, 36.21 +/- 7.90 msec; TC2, 75.94 +/- 10.65 msec; p less than 0.05). The first phase of relaxation restitution parallels systolic force restitution over the same range and displays faster recovery (TCs, 58.93 +/- 10.01 msec, p less than 0.05). Postextrasystolic restitution of test pulses after beats at fixed ESIs depends on the initial ESI. Relaxation recovery of postextrasystolic beats proceeds faster with smaller initial ESIs (TC1 for ESI of 300 msec, 13.27 +/- 4.05 msec; TC1 for ESI of 450 msec, 72.85 +/- 21.72 msec; p less than 0.0001). The monoexponential pattern of restitution was seen with model-independent descriptors of relaxation as well as with tau.

Reversibility of the effects of normothermic global ischemia on the ryanodine-sensitive and ryanodine-insensitive calcium uptake of cardiac sarcoplasmic reticulum

The effect of normothermic ischemia and ischemia/reperfusion on the function of cardiac sarcoplasmic reticulum (CSR) was investigated using a modified Langendorff perfusion of isolated rat hearts. The function of the CSR was assessed by the oxalate-supported Ca2+ uptake rate of ventricular homogenates. The contribution of the ryanodine-sensitive portion of the CSR was determined by using 20 microM ruthenium red or 625 microM ryanodine to close the CSR Ca2+ release channel. The Ca2+ uptake rate of the CSR decreased progressively with increasing duration of ischemia, but this depression was much less when uptake was assayed in the presence of ryanodine. The depression in CSR Ca2+ uptake preceded ischemic contracture. Ryanodine and ruthenium red stimulated uptake almost equally in control hearts, but ruthenium red was much less effective than ryanodine after ischemia. This difference could not be overcome by increasing the ruthenium red concentration. These results confirm the suggestion that the Ca2+ release channel is inappropriately opened after ischemia. The CSR uptake rates were almost completely restored at 15 minutes of reperfusion after 5 and 10 minutes of ischemia but were only partially restored after 15 minutes of ischemia. At reperfusion, mechanical function (end-diastolic pressure and peak systolic developed pressure) was markedly depressed after only 15 minutes of ischemia. The degree of "stunning" correlated well with the depression of CSR function in individual hearts. The decreased Ca2+ uptake of the CSR was not due to a buildup of ADP in the homogenates.(ABSTRACT TRUNCATED AT 250 WORDS)

Seasonal variations in the rate and capacity of cardiac SR calcium accumulation in a hibernating species

The rate of calcium uptake and the level of calcium accumulation was measured in cardiac muscle SR from hibernating and nonhibernating Richardson's ground squirrels. In whole heart homogenates, the rate of calcium uptake was higher (P less than 0.05) in hibernating animals than it was in active animals. Further purification of homogenates into sacroplasmic reticulum (SR) preparations showed that the hibernating animals had the highest rate of calcium uptake and the greatest level of calcium accumulation. These results could not be explained by variations in non-SR membrane contaminants nor by changes in the maximal activity or total amount of a SR marker enzyme, the Ca(2+)-ATPase. The addition of ryanodine to the calcium uptake medium increased the level of calcium accumulation in all groups by a similar amount. It is concluded that the high rate of calcium uptake by isolated cardiac SR vesicles from hibernating ground squirrels reflects the activity of the organelle in vivo, and that the ability of the ryanodine-insensitive population of SR vesicles to accumulate calcium is affected by hibernation.

Stabilization of rat cardiac sacroplasmic reticulum Ca2+ uptake activity and isolation of vesicles with improved calcium uptake activity

The Ca2+ uptake activity of rat cardiac sacroplasmic reticulum (CSR) in ventricular homogenates is highly unstable, and this instability probably accounts for the low specific activity of Ca2+ uptake in previously reported fractions of isolated rat CSR. The instability was observed at either 0 degrees or 37 degrees, but the Ca2+ uptake activity was relatively stable at 25 degrees. The decay of Ca2+ uptake activity at 0 degrees could not be prevented by either PMSF or leupeptin, but dithiothreitol exerted some protective effects. Sodium metabisulfite prevented decay of the Ca2+ uptake activity of homogenates kept on ice but not of homogenates kept at 37 degrees. We also found that release of the CSR from the cellular debris required homogenization in high KCl. This distinguishes rat CSR from canine CSR. Isolated CSR was produced by a combination of differential centrifugation and discontinuous sucrous gradient centrifugation. The average rate of the sustained oxalate-supported calcium uptake in the resulting CSR fraction was 0.36 mumol/min-mg in the absence of CSR calcium channel blockers and 0.67 mumol/min/mg in the presence of 10 microM ruthenium red. Thus, this preparation has the advantage of containing both the releasing and non-releasing fractions of the CSR. The Ca2(+)-ATPase rates averaged 1.07 mumol/min/mg and 0.88 mumol/min-mg in the absence and presence of ruthenium red, respectively. Although these rates are higher than previously reported rates, this CSR preparation should still be considered a 'crude' preparation. A major distinction between the rat CSR and dog CSR was the lower content of Ca2(+)-ATPase in rat CSR, as judged by SDS-PAGE. Preparations of CSR isolated by this method may be useful in evaluating alterations in CSR function.

Ca-ATPase isozyme expression in sarcoplasmic reticulum is altered by chronic stimulation of skeletal muscle

Chronic stimulation of a predominantly fast skeletal muscle enhanced the expression of type I (slow muscle) Ca-ATPase and suppressed the expression of the type II (fast muscle) Ca-ATPase. Monoclonal antibodies IID8 and IIH11 against type I (slow) and type II (fast) isozymes respectively, were used to type the Ca-ATPases of the isolated SR (sarcoplasmic reticulum) by Western blots, and the Ca-ATPases of the muscle fibers by immunohistochemistry. Of the fibers from control muscles 80% stained for the type II isozyme and 20% for the type I isozyme. Following chronic stimulation all fibers stained for type I isozyme and none stained for type II isozyme. Ca-ATPase isozyme distribution in isolated SR confirmed this effect of chronic stimulation. The calcium uptake activities of homogenates of stimulated muscles were 22% of the control muscles. The Ca-ATPase and calcium-uptake activities of the isolated SR from stimulated muscles were, respectively, 32 and 45% of the control muscles.

Differential effect of global ischemia on the ryanodine-sensitive and ryanodine-insensitive calcium uptake of cardiac sarcoplasmic reticulum

The effect of ischemia on the function of cardiac sarcoplasmic reticulum (SR) was assessed by the calcium uptake rate of rat whole-heart homogenates in the presence of 10 mM oxalate. Previous studies have shown that this uptake is restricted to the SR. The contribution of the ryanodine-sensitive fractions of the SR to the total homogenate uptake was assessed by using 20 microM ruthenium red and 625 microM ryanodine to close the SR calcium release channel under previously established optimal conditions. Global ischemia of 10, 15, 30, and 60 minutes depressed homogenate calcium uptake rate 19 +/- 2%, 50 +/- 6%, 65 +/- 3%, and 81 +/- 5%, respectively. This decrease was not observed when the uptake rates were measured after closure of the calcium channel with ryanodine or ruthenium red. Similar results were obtained with a Langendorff in vitro perfusion preparation, in which calcium uptake was decreased 35 +/- 5%, 37 +/- 8%, 58 +/- 7%, and 64 +/- 4% after 10, 15, 30, and 60 minutes of ischemia, but no significant decrease was observed when homogenate uptake rates were measured in the presence of ryanodine. Thus, ischemia caused a depression in the calcium uptake rate of cardiac SR only when this activity was measured in the absence of SR calcium channel blockers. Reperfusion of ischemic hearts in a Langendorff preparation resulted in recovery of homogenate calcium uptake activity that correlated well with the return to sinus rhythm of the reperfused hearts. These reperfused hearts showed no change in the calcium uptake rate measured in the presence of ryanodine. These results suggest that the decrease in homogenate calcium uptake caused by ischemia is not due to a defect in calcium pumping capabilities but is due to an increased efflux through the ryanodine-sensitive calcium release channel of cardiac SR.