Effects of cyclopiazonic acid on Ca2+ regulation by the sarcoplasmic reticulum in saponin-permeabilized skeletal muscle fibres.
The effects of the sarcoplasmic reticulum (SR) Ca2+ pump inhibitor cyclopiazonic acid (CPA) were studied in saponin-permeabilized frog skeletal muscle fibres. Release of Ca2+ from the SR was triggered by brief (2 s) applications of 40 mM caffeine at 2-min intervals. Changes in [Ca2+] within the fibre were monitored continuously using Fura-2 fluorescence. At a bathing [Ca2+] of 100 nM, introduction of 20 microM CPA induced a slow release of Ca2+ from the SR. The following one to two caffeine-induced Ca2+ transients were markedly increased in amplitude and duration. Thereafter, the caffeine-induced Ca2+ transients decreased progressively and were barely detectable 6-7 min after introduction of CPA. However, increasing the bathing [Ca2+] or increasing the Ca2+ loading period resulted in a partial recovery of the caffeine-induced Ca2+ transients, suggesting that pump inhibition is incomplete, even in the presence of 100 microM CPA. The slow Ca2+ efflux induced by CPA was insensitive to ryanodine, but absent following abolition of SR Ca2+ pump activity by ATP withdrawal. These results suggest that the caffeine-induced Ca2+ transient reflects a balance between efflux via the SR Ca2+ channel and reuptake by the Ca pump. Ca2+ release upon addition of CPA may result from inhibition of SR Ca2+ uptake, which reveals a tonic Ca2+ efflux that is independent of the Ca2+ release channels.
Duke
A M
AM
Department of Biology, University of Leeds, Leeds LS2 9JT, UK.
Steele
D S
DS
D000255:Adenosine Triphosphate / Q000008:administration & dosage; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002463:Cell Membrane Permeability; D005456:Fluorescent Dyes; D016257:Fura-2; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology*; D007700:Kinetics; D018485:Muscle Fibers, Skeletal / Q000378:metabolism* / Q000648:ultrastructure; D018482:Muscle, Skeletal / Q000378:metabolism / Q000648:ultrastructure; D017830:Octoxynol / Q000494:pharmacology; D011896:Rana temporaria; D012503:Saponins; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
D005456:Fluorescent Dyes; D007211:Indoles; D012503:Saponins; D002110:Caffeine; D000255:Adenosine Triphosphate; D017830:Octoxynol; D000252:Calcium-Transporting ATPases; D002118:Calcium; D016257:Fura-2; C000543:cyclopiazonic acid