Sphingosylphosphocholine modulates the ryanodine receptor/calcium-release channel of cardiac sarcoplasmic reticulum membranes.

Sphingosylphosphocholine (SPC) modulates Ca2+ release from isolated cardiac sarcoplasmic reticulum membranes; 50 microM SPC induces the release of 70 80% of the accumulated calcium. SPC release calcium from cardiac sarcoplasmic reticulum through the ryanodine receptor, since the release is inhibited by the ryanodine receptor channel antagonists ryanodine. Ruthenium Red and sphingosine. In intact cardiac myocytes, even in the absence of extracellular calcium. SPC causes a rise in diastolic Ca2+, which is greatly reduced when the sarcoplasmic reticulum is depleted of Ca2+ by prior thapsigargin treatment. SPC action on the ryanodine receptor is Ca(2+)-dependent. SPC shifts to the left the Ca(2+)-dependence of [3H]ryanodine binding, but only at high pCa values, suggesting that SPC might increase the sensitivity to calcium of the Ca(2+)-induced Ca(2+)-release mechanism. At high calcium concentrations (pCa 4.0 or lower), where [3H]ryanodine binding is maximally stimulated, no effect of SPC is observed. We conclude that SPC releases calcium from cardiac sarcoplasmic reticulum membranes by activating the ryanodine receptor and possibly another intracellular Ca(2+)-release channel, the sphingolipid Ca(2+)-release-mediating protein of endoplasmic reticulum (SCaMPER) [Mao, Kim, Almenoff, Rudner, Kearney and Kindman (1996) Proc.Natl.Acad.Sci. U.S.A 93, 1993-1996], which we have identified for the first time in cardiac tissue.

Thyroid hormone effects on contractility and myosin composition of soleus muscle and single fibres from young and old rats.

1. Young (3-6 months) and old (20-24 months) male Wistar rat soleus muscles were examined for myosin isoform composition, fibre type, contractility and sarcoplasmic reticulum (SR) Ca2+ release properties either in control rats or in rats treated with thyroid hormone (3,5,3'-triiodothyronine, T3) for 4 weeks. 2. T3 treatment had a strong impact on myosin heavy chain (MyHC) and light chain (MyLC) isoform composition in both young and old rats. That is, all single fibres co-expressed type I and IIA (type I/IIA fibres) or type I, IIA and IIX MyHCs (type I/IIAX fibres) after treatment. Slow and fast MyLC isoforms, i.e. MyLC1s, MyLC1f, MyLC2s, MyLC2f and MyLC3, co-existed in each of the type I/IIA and I/IIAX fibres in variable proportions. 3. In old rats the maximum velocity of unloaded shortening (V0) was related to MyHC isoform composition: V0 for type I fibres was less than that for type I/IIA fibres which was less than that for type I/IIAX fibres. In young rats, on the other hand, V0 did not differ between pure type I fibres from controls and those co-expressing type I and type II MyHC isoforms from T3-treated rats. 4. Contraction and half-relaxation times of the isometric twitch were significantly longer in old than in young controls. This was paralleled by an age-related decrease in the caffeine threshold of the SR. Four weeks of T3 treatment eliminated the age-related differences in both speed of twitch contraction and caffeine thresholds. V0, on the other hand, was slower in old than in young animals, both control and T3-treated, when cells with a similar MyHC composition were compared. 5. In conclusion, thyroid hormone can substantially reverse at least some of the changes that occur in ageing muscle. Further, the age-related decline in V0 in soleus fibres from control and hyperthyroid rats suggests that: (1) the identification of beta/slow myosin isoforms is incomplete; or (2) the molecular characteristics of MyHC differ between young and old age; or (3) MyHC is not the only determinant of V0.

Effects of thyroid hormone on fast- and slow-twitch skeletal muscles in young and old rats.

1. The effects of 4 weeks of thyroid hormone treatment on contractile, enzyme-histochemical and morphometric properties and on the myosin isoform composition were compared in the slow-twitch soleus and the fast-twitch extensor digitorum longus (EDL) muscle in young (3-6 months) and old (20-24 months) male rats. 2. In the soleus of untreated controls, contraction and half-relaxation times of the isometric twitch increased by 19-32% with age. The change in contractile properties was paralleled by an age-related increase in the proportions of type I fibres and type I myosin heavy chain (MHC) and slow myosin light chain (MLC) isoforms. 3. In the EDL of controls, contraction and half-relaxation times were significantly prolonged (21-38%) in the post-tetanus twitch in the old animals. No significant age-related changes were observed in enzyme-histochemical fibre-type proportions, although the number of fibres expressing both type IIA and IIB MHCs and of fibres expressing slow MLC isoforms was increased in the old animals. 4. Serum 3,5,3',5'-tetraiodothyronine (T4) levels were lower (34%) in the old animals, but the primary byproduct of T4, 3,5,3'-triiodothyronine (T3), did not differ between young and old animals. 5. The effects of 4 weeks of thyroid hormone treatment were highly muscle specific, and were more pronounced in soleus than in EDL, irrespective of animal age. In the soleus, this treatment shortened the contraction and half-relaxation times by 35-57% and decreased the number of type I fibres by 66-77% in both young and old animals. In EDL, thyroid hormone treatment significantly shortened the contraction time by 24%, but the change was restricted to the old animals. 6. In conclusion, the ability of skeletal muscle to respond to thyroid hormone treatment was not impaired in old age and the age-related changes in speed of contraction and enzyme-histochemical properties and myosin isoform compositions were diminished after thyroid hormone treatment in both the soleus and EDL.

Interaction of dystrophin with cytoskeletal proteins: binding to talin and actin.

Dystrophin, the protein product of the Duchenne gene, is thought to be a member of muscle membrane cytoskeleton. In this work we studied the interactions of purified dystrophin from rabbit skeletal muscle sarcolemma membranes with other cytoskeletal proteins. The interaction of dystrophin with purified talin from chicken gizzard was tested by solid phase immunoassay. Under these conditions dystrophin bound talin with high affinity (Kd 3.5 nM). Vinculin purified from chicken gizzard did not bind dystrophin, but it inhibited the binding of dystrophin to talin. Furthermore, co-sedimentation and solid phase immunoassay experiments both demonstrated that native dystrophin binds purified actin from rabbit skeletal muscle. In conclusion, our results show that dystrophin can interact in vitro with proteins that are members of muscle membrane cytoskeleton. These proteins may represent additional sites for anchoring dystrophin to sarcolemma.

The effects of sphingosine on sarcoplasmic reticulum membrane calcium release.

In this study, we report that sphingosine is a potent inhibitor of sarcoplasmic reticulum (SR) calcium release. Evidence is presented demonstrating a direct effect of sphingosine on the SR ryanodine receptor. Calcium release from "skinned" rabbit skeletal muscle fibers and isolated junctional SR derived from the terminal cisternae (TC) was measured in response to caffeine, doxorubicin, 5'-adenylyl-beta,gamma-imidodiphosphate or calcium. Sphingosine inhibited caffeine-induced release in a dose-dependent manner with an IC50 of 0.1 microM for the single muscle fibers and 0.5 microM for the isolated TC vesicles. Near complete blockage of TC calcium release rate was observed with 3 microM sphingosine. Neither sphingomyelin nor sphingosylphosphorylcholine had any effect at the 3 microM level, suggesting that the sphingosine effect was specific. Doxorubicin-induced calcium release and spontaneous calcium release were also blocked by sphingosine. Sphingosine was also capable of stimulating calcium transport in the isolated TC vesicles without an effect on Ca-ATPase activity. Ruthenium red was not capable of substantial additional stimulation of calcium transport nor inhibition of calcium release beyond the action of sphingosine. Sphingosine's blockage of calcium release was not reversed by the protein kinase inhibitor, 1-(5-isoquinolinesulfonyl)-2- methylpiperazine dihydrochloride, suggesting that the action of sphingosine on calcium release was not dependent on ryanodine receptor phosphorylation. Sphingosine significantly increased (8-fold) the Kd for specific [3H]ryanodine binding to TC membranes and decreased the Bmax with a dose dependence similar to the inhibition of calcium release, but sphingosine did not affect the pCa tension relationship of skinned skeletal muscle fibers. These data are consistent with a direct effect of submicromolar sphingosine on the ryanodine receptor. Substantially higher concentrations of sphingosine (30-50 microM) or sphingosylphosphorylcholine (10-20 microM) were capable of inducing calcium release by themselves. Preliminary data indicate that the transverse tubule and not the SR contain substantial sphingomyelinase activity consistent with a transverse tubule source of sphingosine production. Considering that sphingosine is found in micromolar concentrations in some cells, our data indicate that sphingosine generated by the transverse tubule membranes may be a physiologically relevant mechanism for modulating SR calcium release.

Congenital lung malformations: a unified concept and a case report.

A left upper lobectomy in a 5-month-old infant revealed a specimen whose tissue, both grossly and microscopically, was characteristic of intr- and extrapulmonary swquestration, bronchial atresia, and adenomatoid cystic malformation. This case may be considered the missing link between the various congenital lung malformations, for it illustrates their interrelationship or their common origin. Atresia or malformation of the main bronchus seems to be the defect shared by all these anomalies. A similar bronchial abnormality probably is responsible for congenital lobar emphysema and isolated congenital lung cysts.