Augmentation of SR Ca(2+) release by rapamycin and FK506 causes K(+)-channel activation and membrane hyperpolarization in bladder smooth muscle.
1. The immunosuppressants rapamycin and FK506 are known to relax smooth muscle despite facilitating Ca(2+) release through ryanodine-receptors of the sarcoplasmic reticulum (SR). The apparent contradiction was studied in isolated guinea-pig urinary bladder myocytes. 2. Modulation of spontaneous SR Ca(2+) release was monitored by means of spontaneous transient outward currents (or STOCs) in isolated smooth muscle cells voltage-clamped to -20 mV. Rapamycin (10 microM, n=18) significantly increased amplitude (50+/-12%, mean+/-s.d.), life time (77+/-19%), and time integral of STOCs (113+/-22%), and it reduced the interval between STOCs (20+/-7%). FK506 (20 microM, n=24) increased amplitude (15+/-7%), life time (50+/-7%), time integral (104+/-26%). Cyclosporin A (20 microM, n=18) had no significant effects on STOCs. 3. The basal cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) measured by Indo1-fluorescence was insensitive to rapamycin or FK506. Pretreatment with rapamycin (20 microM, 2 min) did not impair the SR Ca(2+) load as can be concluded from caffeine-induced Ca(2+)-transients. 4. As it was expected from the enhanced STOC activity, the non-clamped membrane was hyperpolarized by rapamycin (15+/-2 mV) or by FK506 (15+/-3 mV). 5. The data are consistent with the idea that rapamycin and FK506 augment spontaneous SR Ca(2+) release by removal of FK-binding proteins from the RyR-complex. Smooth muscle relaxation is interpreted as negative Ca(2+) feedback: augmented Ca(2+) activation of STOCs induces membrane hyperpolarization that reduces Ca(2+) influx through voltage gated channels.
Weidelt
T
T
Department of Physiology, University of Halle, D-06097 Halle, Germany.
Isenberg
G
G
D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D016572:Cyclosporine / Q000494:pharmacology; D004558:Electric Stimulation; D006168:Guinea Pigs; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects*; D009130:Muscle, Smooth / Q000187:drug effects* / Q000378:metabolism / Q000502:physiology; D015221:Potassium Channels / Q000187:drug effects* / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D020123:Sirolimus / Q000494:pharmacology*; D016559:Tacrolimus / Q000494:pharmacology*; D001743:Urinary Bladder / Q000187:drug effects / Q000378:metabolism / Q000502:physiology
D015221:Potassium Channels; D002110:Caffeine; D016572:Cyclosporine; D002118:Calcium; D020123:Sirolimus; D016559:Tacrolimus