Effects of ryanodine on skinned myocardial fibers of the rabbit

Structural and functional insight into a new emerging target IP3R in cancer

Calcium signaling has been identified as an important phenomenon in a plethora of cellular processes. Inositol 1,4,5-trisphosphate receptors (IP3Rs) are ER-residing intracellular calcium (Ca2+) release channels responsible for cell bioenergetics by transferring calcium from the ER to the mitochondria. The recent availability of full-length IP3R channel structure has enabled the researchers to design the IP3 competitive ligands and reveal the channel gating mechanism by elucidating the conformational changes induced by ligands. However, limited knowledge is available for IP3R antagonists and the exact mechanism of action of these antagonists within a tumorigenic environment of a cell. Here in this review a summarized information about the role of IP3R in cell proliferation and apoptosis has been discussed. Moreover, structure and gating mechanism of IP3R in the presence of antagonists have been provided in this review. Additionally, compelling information about ligand-based studies (both agonists and antagonists) has been discussed. The shortcomings of these studies and the challenges toward the design of potent IP3R modulators have also been provided in this review. However, the conformational changes induced by antagonists for channel gating mechanism still display some major drawbacks that need to be addressed. However, the design, synthesis and availability of isoform-specific antagonists is a rather challenging one due to intra-structural similarity within the binding domain of each isoform. HighlightsThe intricate complexity of IP3R's in cellular processes declares them an important target whereby, the recently solved structure depicts the receptor's potential involvement in a complex network of processes spanning from cell proliferation to cell death.Pharmacological inhibition of IP3R attenuates the proliferation or invasiveness of cancers, thus inducing necrotic cell death.Despite significant advancements, there is a tremendous need to design new potential hits to target IP3R, based upon 3D structural features and pharmacophoric patterns.Communicated by Ramaswamy H. Sarma.

Doxorubicin-induced late cardiotoxicity: delayed impairment of Ca2+-handling mechanisms in the sarcoplasmic reticulum in the rat

Doxorubicin treatment causes delayed development of cardiotoxicity. Whether the doxorubicin-induced impairment of cardiac functions reverses or progresses with time after the cessation of the treatment was examined. The rats were injected with doxorubicin (2.5 mg/kg, i.v., once a week for 3 weeks) and sacrificed at 1 (1W), 13 (13W), or 18 (18W) weeks after the final doxorubicin administration. The time to peak of twitch contraction observed at 2-Hz stimulation was not altered in left atrial or ventricular muscle preparations isolated from 1W rats, but it was prolonged in those from 13W and 18W rats. The reduction of the magnitude of postrest contraction and the alteration of force-frequency relationships in left atrial muscle preparations in 1W rats were not significant, but were intensified in the 13W and 18W groups. Alterations in the postrest contraction and the force-frequency relationships in ventricular muscle preparations isolated from doxorubicin-treated rat hearts were weaker, but the pattern of alteration was similar to that observed in left atrial muscle preparations. Caffeine-induced contraction observed in skinned fibers that were isolated from the 1W rats was not altered, but it was reduced in the 18W rats. The Ca2+ sensitivity of contractile proteins was not altered in doxorubicin-treated rat hearts in any of the groups. The Kd values estimated from a [3H]ryanodine binding study were not altered, but the Bmax values were significantly lower in the 13W and 18W groups than those observed in control rats. These results suggest that the dysfunction of the sarcoplasmic reticulum progresses after the completion of doxorubicin treatment and contributes to the doxorubicin-induced late cardiotoxicity.Key words: doxorubicin, late cardiotoxicity, rat heart, sarcoplasmic reticulum.

Modulation of the ryanodine receptor sarcoplasmic reticular Ca2+ channel in skinned fibers of fast- and slow-twitch skeletal muscles from rabbits

This study was performed to compare skinned fibers from rabbit adductor magnus (AM) and soleus (SL) muscles with regard to the influence of caffeine, Ca2+ and Mg2+ on the depressive effects of ryanodine (RYA) on the caffeine-induced tension transients. Single skinned fibers were immersed in solutions to load Ca2+ into, and release Ca2+ from the SR (a load-release cycle). Three cycles were sequentially performed in each skinned fiber: (1) a control (no RYA), (2) a conditioning period in which activation was carried out in the presence of ryanodine plus various concentrations of the modulators, i.e. caffeine, Ca2+ or Mg2+, and (3) a test (no RYA) which monitored the release activity retained after the conditioning cycle. The depressive effect of RYA was found to be a function of [ryanodine], [caffeine], or [Ca2+], and an inverse function of [Mg2+], where [] denotes concentration. The half-maximal effects of RYA in AM (5 microM RYA) and SL (10 microM RYA), respectively, occurred at a pCa50 of 5.32 versus 5.43 without caffeine, or pCa50 of 7.24 versus 6.88 and pMg50 of 3.29 versus 3.61 with 25 mM caffeine, at a [caffeine] of 4.96 versus 7.29 mM, and at a [ryanodine] of 31.0 versus 101.6 microM. Thus, the RYA depression in skinned muscle fibers is modulated by caffeine, Ca2+, and Mg2+ in both muscle types, and AM is at least two- to fourfold more sensitive than SL.

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.

Modulation of sarcoplasmic reticulum Ca(2+)-release channels by caffeine, Ca2+, and Mg2+ in skinned myocardial fibers of fetal and adult rats

Ryanodine causes depression of the caffeine-induced tension transient (ryanodine depression) in skinned muscle fibers, because it blocks the sarcoplasmic reticulum (SR) Ca(2+)-release channels [Su, J. Y. (1988) Pflügers Arch 411:132-136, 371-377; (1992) Pflügers Arch 421:1-6]. This study was performed to examine the sensitivity of SR Ca(2+)-release channels to ryanodine in fetal compared to adult myocardium and to investigate the influence of Ca2+, caffeine, and Mg2+ on ryanodine depression in skinned fibers. Ryanodine (0.3 nM-1 microM) caused a dose-dependent depression in skinned myocardial fibers of the rat, and the fetal fibers (IC50 approximately 74 nM) were 26-fold less sensitive than those of the adult (IC50 approximately 2.9 nM). The depression induced by 0.1 microM or 1 microM ryanodine was a function of [caffeine], or [Ca2+] (pCa < 6.0), which was potentiated by caffeine, and an inverse function of [Mg2+]. At pCa > 8.0 plus 25 mM caffeine, a 20% ryanodine depression was observed in both the fetal and adult fibers, indicating independence from Ca2+. Ryanodine depression in skinned fibers of the fetus was less affected than that seen in the adult by pCai, [caffeine]i, or 25 mM caffeine plus pCai or plus pMgi (IC50 approximately pCa 4.5 versus 5.1; caffeine 12.7 mM versus 2 mM; pCa 6.7 versus 7.3; and pMg 3.9 versus 3.3 respectively). The results show that the SR Ca(2+)-release channel in both fetal and adult myocardium is modulated by Ca2+, caffeine, and Mg2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of caffeine, Ca2+, and Mg2+ on ryanodine depression of the tension transient in skinned myocardial fibers of the rabbit

Ryanodine, a blocker for Ca(2+)-release channels of the sarcoplasmic reticulum (SR Ca(2+)-release channels), induces depression of myocardial contraction in isolated intact muscle, which is consistent with depression of the caffeine-induced tension transient in skinned muscle fibers. In isolated SR, ryanodine binds to a specific receptor with high affinity, and this binding is enhanced by caffeine and increasing Ca2+ and decreased by increasing Mg2+. The aim of this study was to test the hypothesis that depression of myocardial contraction is mediated by changes in ryanodine-receptor binding properties. Accordingly, factors (caffeine, Ca2+, and Mg2+) affecting ryanodine-receptor binding properties in the isolated SR membrane were studied in skinned myocardial fibers from adult rabbits. The depression of the caffeine-induced tension transient by ryanodine (ryanodine depression) influenced by these three factors was measured. In a dose-dependent manner, increasing caffeine or Ca2+ concentrations enhanced the ryanodine depression. The concentrations for 50% ryanodine depression (IC50) approximated 7 mM for caffeine, and pCa 5.25 for Ca2+. When 1 microM ryanodine and 25 mM caffeine were combined, ryanodine depression was independent of Ca2+ at low Ca2+ concentrations (20%-30% at pCa greater than 8 and 7.5) and was a direct function of Ca2+ at higher concentrations (pCa 7.5-6.0 with IC50 approx. pCa 6.75). In contrast, increasing Mg2+ reduced the ryanodine depression with IC50 approximately equal to pMg 3.3. In conclusion, the caffeine- or Ca(2+)-enhanced, and Mg(2+)-reduced ryanodine depression observed in this study is consistent with known ryanodine-receptor binding properties.

Mechanisms of ryanodine-induced depression of caffeine-induced tension transients in skinned striated rabbit muscle fibers

Evidence suggests that ryanodine affects ligand-gated calcium channels in the sarcoplasmic reticulum (SR) resulting in depressed muscle contraction. In skinned fibers from striated muscle the effects of ryanodine were examined (1) on Ca2+ uptake and on Ca2+ release to differentiate whether the effects are on the pump or channel, and (2) during the tension transient, with ryanodine exposure at various times either simultaneous with or directly after exposure to caffeine. Of total calcium content in the SR, 25 mM caffeine released greater than 90% in papillary muscle (PM), approximately equal to 25% in soleus (SL), and approximately equal to 20% in adductor magnus (AM). Ryanodine (100 microM for 1-3 s for AM and SL; 1 microM for 7-10 s for PM), in the initial loading phase, did not significantly change, and in the initial release phase, markedly depressed the subsequent control caffeine-induced tension transients (C2) in all three muscle types. The depression increased with increasing time of exposure to ryanodine (10 microM) in the order of PM greater than AM greater than SL. Upon introduction of ryanodine after caffeine-induced tension transients, maximal depression was observed at half-maximum rise of the tension transient, followed by recovery of depression to completion in SL, and only partially in AM and PM at steady state of relaxation. The extent of recovery was in the order of SL greater than AM greater than PM. The data suggest that ryanodine affects Ca2+ releasing channel as a result of its binding to open channels.