Bastadins potently interact with the FK-506-binding protein of 12 kDa (FKBP12)-ryanodine receptor (Ry1R) complex in skeletal muscle to enhance a high-affinity ryanodine binding conformation (M. M. Mack, T. F. Molinski, E. D. Buck, and I. N. Pessah. J. Biol. Chem. 269: 23236-23249, 1994). Bastadins are used to examine the relationship between ryanodine-sensitive and ryanodine-insensitive Ca2+ efflux pathways that coexist in junctional sarcoplasmic reticulum (SR) vesicles from rabbit skeletal muscle and differentiated BC3H1 cells. Complete block of caffeine-sensitive Ca2+ channels with micromolar ryanodine or ruthenium red does not alter the steady-state loading capacity of SR. Inhibition of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) pumps with thapsigargin unmasks a ryanodine- and ruthenium red-insensitive Ca2+ efflux pathway. Bastadin 5 alone does not inhibit Ca2+ efflux unmasked by inhibition of SERCA pumps, but, in combination with blocking concentrations of ryanodine or ruthenium red, it eliminates the ryanodine-insensitive Ca2+ "leak" and enhances steady-state loading capacity of SR vesicles approximately 2.5-fold. These actions of bastadins occur in the same concentration range that enhances the number of high-affinity binding sites for [3H]ryanodine (50% effective concentration of approximately 2 microM). Similar effects on SR Ca2+ transport are found with FK-506 and ryanodine in combination. Block of Ry1R in intact BC3H1 cells with ryanodine does not eliminate the prominent Ca2+ leak unmasked by thapsigargin. A membrane-permeant mixture of bastadins in combination with ryanodine nearly eliminates the Ca2+ leak unmasked by thapsigargin, even though the Ca2+ stores are replete. The requirement of both a known Ry1R blocker and bastadins in combination provides a pharmacological link between ryanodine-sensitive Ca2+ channels and ryanodine-insensitive leak pathways in isolated junctional SR and BC3H1 cells. Together, these results strongly suggest that bastadins, through their modulatory actions on the FKBP12-Ry1R complex, convert ryanodine-insensitive leak states into ryanodine-sensitive channels that recognize [3H]ryanodine with high affinity.