Loss Of Sarcoplasmic Reticulum Ca2 Research Articles

Long-term administration of rosuvastatin prevents contractile and electrical remodelling of diabetic rat heart

In recent years, many findings have been presented about the potential benefit of statin therapy on diabetes-induced cardiovascular complications. Cardioprotective effects of statins were suggested to be mediated at least in part through inhibition of small GTPases, particularly those of the Rho family. The present study was designed to examine whether rosuvastatin can improve electrical remodeling and contractile dysfunction in type 1 diabetic rat heart via modulation of RhoA pathway. Type 1 diabetes was induced by single dose injection of STZ (50 mg/kg). One week after injection rosuvastatin (10 mg/kg/day) and sham treatment was given for 5 weeks in the diabetic rats, as well as in control groups. Shortening and Ca²⁺ transients were recorded in myocytes loaded with Fura2-AM. Membrane currents and Ca²⁺ transients were measured synchronously via whole-cell patch clamping. In untreated diabetic rats, relaxation of shortening and decay of the matched Ca²⁺ transients were prolonged. Fractional shortening and Ca²⁺ transients were also decreased. Rosuvastatin treatment reversed those changes. I(CaL) density did not change in either group but rosuvastatin recovered the loss of sarcoplasmic reticulum Ca²⁺ and Na⁺/Ca²⁺ exchange as evidenced from amplitude and decay of caffeine-induced Ca²⁺ transients, peak INCX and calculated sarcoplasmic reticulum Ca²⁺ content. Diabetes-induced attenuation of I(to) and I(sus) was also reversed, whilst I(K1) was unchanged in diabetes and unaffected by treatment. Rosuvastatin prevented the diabetes-induced increase in RhoA expression. Plasma cholesterol and triglyceride levels were higher in diabetic rats, but rosuvastatin reduced only the latter. In conclusion, HMG-CoA reductase inhibitor rosuvastatin can prevent diabetes-induced electrical and functional remodeling of heart due to inhibition of RhoA signalling rather than reduction of cholesterol level.

How to stop the fire? Control of Ca2+-induced Ca2+release in cardiac muscle

Cardiac excitation–contraction coupling in higher vertebrates is driven by the process of calcium-induced calcium release (CICR). According to the original hypothesis, an action potential initiates a small Ca2+ influx through plasmalemmal Ca2+ channels which indirectly activates contraction by triggering a much larger Ca2+ release from ryanodine receptors (RyR2s), Ca2+ channels on the sarcoplasmic reticulum (SR). It is believed that upon relaxation, SR Ca2+ release ceases via cytosolic Ca2+-dependent inactivation of RyR2s, which allows resequestration of Ca2+ to the SR by SR Ca2+-ATPase. Thus CICR, where Ca2+ serves as trigger, output and negative regulator, is an inherently unstable process. Accordingly, uncontrollable Ca2+ release is implicated in many hallmark features of cardiac disease; diastolic pro-arrhythmic aftercontractions and/or loss of SR Ca2+ resulting in reduced systolic Ca2+ transients and thereby impaired contractility.

Changes in intra-luminal calcium during spontaneous calcium waves following sensitization of ryanodine receptor channels

Cardiac contraction during systole is dependent on action potential-triggered Ca2+ release from the sarcoplasmic reticulum (SR) through ryanodine receptor (RyR) channels. SR Ca2+ release can also occur spontaneously during diastole, which causes a decrease in Ca2+ content within the SR and contributes to arrhythmogenesis. Here, we use measurements of cytosolic Ca2+ and intra-SR Ca2+ ([Ca2+]SR) to examine how RyR sensitization alters spontaneous SR Ca2+ release events in rabbit ventricular myocytes. RyR sensitization with caffeine (250 microM) increased the open probability of single RyR channels, increased the initial frequency and amplitude of local SR Ca2+ release events (Ca2+ sparks), and decreased the [Ca2+]SR level where Ca2+ sparks terminated. In intact myocytes, caffeine applied during rest after steady-state electrical stimulation increased the frequency of spontaneous Ca2+ waves and decreased the [Ca2+]SR level where waves terminated. These effects caused a marked loss of SR Ca2+ content. Therefore, increasing RyR activity has complex effects on cardiac function. Increased RyR activity during systole is beneficial as it increases SR Ca2+ release and contractile strength. However, increased RyR activity during diastole produces spontaneous, arrhythmogenic Ca2+ release events that lower SR Ca2+ content and subsequently decrease contractility.

FKBP12 Depletion Leads to Loss of Sarcoplasmic Reticulum Ca2+ Stores in Rat Vas Deferens

The immunophilin 12-kDa FK506 binding protein (FKBP12) stabilizes intracellular Ca2+ release channel (CRC) activity in different tissues. In this work, the presence of FKBP12 in rat vas deferens (RVD) and its possible contribution to RVD function was investigated. Treatment under appropriate pH, temperature, and ionic conditions was used to strip FKBP12 from CRC binding sites; Western blotting revealed FKBP12 in control but not in treated homogenates. Disruption of the FKBP12-CRC complex in RVD decreased the Ca2+ content of sarcoplasmic reticulum (SR) by increasing Ca2+ leakage through the ryanodine receptor (RyR3 isoform) but not through 1,4,5-iNOSitol trisphosphate receptors (IP3R1 and IP3R3 isoforms). The decrease of SR Ca2+ content was not related to inhibition of SERCA ATPase. It seems that dissociation of FKBP12-RyR leads to conformational changes in RyR that make it difficult for ryanodine to access its binding site. Rapamycin, which is commonly used as a pharmacological tool to disrupt the FKBP12-RyR complex, decreased phenylephrine-induced contractions in RVD epididymal halves. The data suggest that FKBP12 is expressed in RVD in a labile association with RyR3. Disruption of the FKBP12-RyR3 complex may lead to modifications of RVD physiology and in consequence may compromise male fertility.

Intracellular Ca2+ transients during rapid cooling contractures in guinea-pig ventricular myocytes.

1. We measured intracellular Ca2+ transients during rapid cooling contractures (RCCs) in guinea-pig ventricular myocytes using the fluorescent Ca2+ indicator, Indo-1. 2. Rapid cooling of myocytes from 22 to 0-1 degrees C induced a rapid increase in [Ca2+]i which preceded the peak of the contraction and was sometimes large enough to saturate Indo-1. This indicates that [Ca2+]i may reach greater than 10 microM during an RCC. 3. The [Ca2+]i during the RCC slowly declined from its peak value and most of this decline in [Ca2+]i can be attributed to slow reaccumulation of Ca2+ by the sarcoplasmic reticulum (SR) in the cold. RCCs induced in the absence of Cao2+, were not different from control, supporting previous conclusions that RCCs depend exclusively on intracellular Ca2+ stores. 4. RCCs are depressed by long rest periods (rest decay) or by exposure to ryanodine or caffeine, which supports conclusions that RCCs are due to Ca2+ release from the SR. The rest decay of RCCs can be almost completely prevented by applying Nao(+)-free solution during the rest period. This implies that the loss of SR Ca2+ during rest depends on the sarcolemmal Na(+)-Ca2+ exchange (and not the sarcolemmal Ca2(+)-ATPase pump). 5. Rapid rewarming during an RCC normally leads to an additional transient contraction (or rewarming spike), without any increase in [Ca2+]i. Thus, the rewarming spike might be attributable to an increase in myofilament Ca2+ sensitivity induced by rewarming. 6. A second RCC is used to assess the fraction of Ca2+ which is re-sequestered by the SR during relaxation from the first RCC. In control solution progressive RCCs decline in amplitude, but in Na(+)-free, Ca2(+)-free solution they are of constant amplitude. We conclude that the SR Ca2+ pump and Na(+)-Ca2+ exchange are responsible for relaxation and that the latter may account for 20-50% of relaxation. 7. These results support the use of RCCs as a useful means of assessing SR Ca2+ content in intact cardiac muscle cells.