Activation and deactivation of sarcoplasmic reticulum calcium release channels: molecular dissection of mechanisms via novel semi-synthetic ryanoids

Ryanoids and related compounds — Isolation and characterization of 11 new minor ryanoids from the plantRyania SpeciosaVahl

In a search for minor ryanoids from the plant Ryania Speciosa Vahl, we recently characterized 11 new members of that family of natural compounds. Most of them represent ryanodine (1) and dehydroryanodine (2) with a modified stage of oxidation in ring C. A second member of the new 4-deoxy series has been identified.Key words: minor natural ryanoids, ryanodine, dehydroryanodine, deoxyryanoids.

Ryanodine receptor dysfunction in hearts of streptozotocin-induced diabetic rats

Studies have shown that evoked calcium release from sarcoplasmic reticulum is compromised in diabetic rat hearts. The present study was undertaken to determine whether this decrease might be ascribed to a reduction in expression and/or alteration in function of ryanodine receptor (RyR2) and whether changes could be minimized with insulin treatment. Hearts were isolated from 4- and 6-week streptozotocin (STZ)-induced diabetic, 4-week diabetic/2-week insulin-treated, and age-matched control rats. RyR2 mRNA and protein levels were determined using reverse transcription-polymerase chain reactions and polyacrylamide gel electrophoresis, respectively, whereas the functional integrity of RyR2 was assessed from their ability to bind [3H]ryanodine. RyR2 protein was unchanged with up to 6 weeks of untreated STZ-induced diabetes. Two weeks of insulin treatment initiated after 4 weeks of diabetes increased RyR2 mRNA levels by 42% and RyR2 protein levels by 45 to 61%. At equivalent amounts, RyR2 protein from 4-week STZ-induced diabetic rat hearts bound 9% less [3H]ryanodine than age-matched control rats (74.1 +/- 3.9 versus 67.4 +/- 3.4 fmol/microg RyR2), whereas that from 6-week STZ-diabetic rats bound 36% less than control rats (47.9 +/- 4.8 versus 74.2 +/- 4.5 fmol/microg RyR2, p < 0.05). RyR2 from insulin-treated animals bound significantly less [3H]ryanodine than control rats (65.2 +/- 4.9 fmol/microg RyR2, p < 0.05). Apparent affinity of ryanodine for RyR2 was similar among all groups (K(d) approximately 1.04 +/- 0.08 nM). Because expression did not change significantly but ryanodine binding decreased, these data suggest that the functional integrity of RyR2 is compromised in diabetic rat hearts, and these changes can be attenuated with 2 weeks of insulin treatment.

Rat arterial smooth muscle devoid of ryanodine receptor function: effects on cellular Ca(2+) handling

The roles of intracellular Ca(2+) stores and ryanodine (Ry) receptors for vascular Ca(2+) homeostasis and viability were investigated in rat tail arterial segments kept in organ culture with Ry (10 - 100 microM) for up to 4 days. Acute exposure to Ry or the non-deactivating ryanodine analogue C(10)-O(eq) glycyl ryanodine (10 microM) eliminated Ca(2+) release responses to caffeine (20 mM) and noradrenaline (NA, 10 microM), whereas responses to NA, but not caffeine, gradually returned to normal within 4 days of exposure to RY: Ry receptor protein was detected on Western blots in arteries cultured either with or without RY: Brief Ca(2+) release events (sparks) were absent after culture with Ry, whereas Ca(2+) waves still occurred. The propagation velocity of waves was equal ( approximately 19 microm s(-1)) in tissue cultured either with or without RY: Inhibition of Ca(2+) accumulation into the sarcoplasmic reticulum (SR) by culture with caffeine (5 mM), cyclopiazonic acid or thapsigargin (both 10 microM) decreased contractility due to Ca(2+)-induced cell damage. In contrast, culture with Ry did not affect contractility. Removal of Ca(2+) from the cytosol following a Ca(2+) load was retarded after Ry culture. Thapsigargin reduced the rate of Ca(2+) removal in control cultured rings, but had no effect after Ry culture. It is concluded that intracellular Ca(2+) stores recover during chronic Ry treatment, while Ry receptors remain non-functional. Ry receptor activity is required for Ca(2+) sparks and for SR-dependent recovery from a Ca(2+) load, but not for Ca(2+) waves or basal Ca(2+) homeostasis.

Structure-function relationships among ryanodine derivatives. Pyridyl ryanodine definitively separates activation potency from high affinity

Ryanodine derivatives are differentially effective on the two limbs of the ryanodine concentration-effect curve. This study comparing ryanodine, ryanodol, and pyridyl ryanodine and nine C10Oeq esters of them focuses on structure-function relations underlying their differential effectiveness. Ryanodol and pyridyl ryanodine had significantly lower affinities than ryanodine, but their EC50act values (concentration of ryanoid that induces one-half of full efficacy), potencies, and efficacies were not diminished in like fashion. Ryanodine and ryanodol were partial agonists, whereas pyridyl ryanodine was a full agonist, having a diminished deactivation potency. C10Oeq esterifications enhanced affinities and efficacies of the base ryanoids. The C10-Oeq ester derivatives of ryanodine and pyridyl ryanodine, but not those of ryanodol, lost their capacity to deactivate RyR1s. Thus, affinity differences among ryanoids clearly do not predicate functional differences as regards activation of Ca2+ release channels. The pyrrole carboxylate on the C3 of ryanodine is dispensable to ryanoid activation of Ca2+ release channels. Ryanodol lacks this ring, but it nevertheless effects substantial activation. Moreover, its C10-Oeq esters display full efficacy. The increased ability of all the C10-Oeq derivatives to release Ca2+ from the vesicles strengthens their role in directly impeding deactivation of RyR1, perhaps by interaction with some component within the transmembrane ionic flux pathway.