Abstract
Ryanodine at concentrations of 0.01-10 microM increased, while greater concentrations of 10-300 microM decreased the calcium permeability of both rabbit fast twitch skeletal muscle junctional and canine cardiac sarcoplasmic reticulum membranes. Ryanodine did not alter calcium binding by either sarcoplasmic reticulum membranes or the calcium binding protein, calsequestrin. Therefore, the effects by this agent appear to involve only changes in membrane permeability, and the characteristics of the calcium permeability pathway affected by ryanodine were those of the calcium release channel. Consistent with this, the actions by ryanodine were localized to junctional sarcoplasmic reticulum membranes and were not observed with either longitudinal sarcoplasmic reticulum or transverse tubular membranes. In addition, passage of the junctional sarcoplasmic reticulum membranes through a French press did not diminish the effects of ryanodine indicating that intact triads were not required. Under the conditions used for the permeability studies, the binding of [3H]ryanodine to skeletal junctional sarcoplasmic reticulum membranes was specific and saturable, and Scatchard analyses indicated the presence of a single binding site with a Kd of 150-200 nM and a maximum capacity of 10.1-18.9 pmol/mg protein. [3H]ryanodine binding to this site and the increase in membrane calcium permeability caused by low concentrations of ryanodine had similar characteristics suggesting that actions at this site produce this effect. Depending on the assay conditions used, ryanodine (100-300 microM) could either increase or decrease ATP-dependent calcium accumulation by skeletal muscle junctional sarcoplasmic reticulum membranes indicating that the alterations of sarcoplasmic reticulum membrane calcium permeability caused by this agent can be determined in part by the experimental environment.
Highlights
From the Departments of $Physiobgy and §Inter& Medicine, The University of Texas Health ScienceCenter, Dallas, Texm
Consistent with this, the actions by ryanodine were localized to junctional sarcoplasmic reticulum membranes andwere not observed with either longitudinal sarcoplasmic reticulum or transverse tubular membranes
Aithough the actions by ryanodine on both intact muscle cells and isolated SR membranes can be explained by alterations in SR membrane calcium permeability, whether this is the case is uncertain for the following reasons
Summary
Branes by effecting the movement of calcium through the calcium release channel. As can be seen in Fig. 2, under. Under the assay conditions usedin the present experiments, thecalcium channel present in the cardiac membranes may be predominantly open in the absence mycin (1 PM) whichwas added to ensure that an equilibrium had of ryanodine. This would diminish the apparenteffectiveness been reached. High concentrations of ryanodine (100-300 PM)were ineffective in altering either the influx or efflux of calcium from these membranes (data not shown) These results are consistent with a similar localization of [3H]ryanodinebinding to HSR membranes described below. Similar results were obtained when the dialysis was conducted for 3 h a t 37 "C
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