Control of calcium release and the effect of ryanodine in skinned muscle fibres of the toad.
1. Skinned muscle fibres from the toad were used to investigate the roles of T-system membrane potential and Ca2+ in controlling the calcium release channels of the sarcoplasmic reticulum (SR). 2. Replacement of K+ in the bathing solution with Na+ produced a large contraction which could last for 30 s or more under certain circumstances. This prolonged contraction could be quickly and completely terminated by repolarizing the fibre in the K+ solution and then immediately re-initiated by returning to the Na+ solution. These data indicate that the membrane potential tightly controlled the substantial and prolonged release of calcium. 3. T-system depolarization in the presence of 10 mM-free EGTA (pCa greater than 9) markedly depleted the SR of Ca2+. This implies that depolarization of the T-system can still trigger substantial release of Ca2+ from the SR even when the myoplasmic [Ca2+] is very low and very heavily buffered by EGTA. 4. When the SR was heavily loaded with Ca2+, substitution of a weakly buffered high [Ca2+] solution (pCa 5.4, 50 microM-EGTA) could produce a small to moderate, transient contraction taking between 3 and 12 s to reach a peak and lasting 30 s or more. 5. This contraction may be produced at least partly by 'calcium-induced calcium release' as ruthenium red (2 microM) completely blocked the responses. Moreover, repeated substitutions produced successively smaller responses in parallel with the 'run-down' of the depolarization-induced contractions. 6. Depolarization could always produce an additional large and fast response at any stage during a 'Ca2(+)-induced' response. 7. In the presence of 25 microM-ryanodine, the rapid contraction produced by T-system depolarization was prolonged and could not be stopped by repolarization. During and after this contraction no depolarizing stimulus could induce a further contraction, even though in some fibres addition of 30 mM-caffeine produced a maximum response which indicated that there was still a substantial amount of calcium in the SR. 8. At pCa 6.4, 25 microM-ryanodine could itself induce a substantial slow contracture in a normally polarized fibre within 30-60 s, after which little or no response could be induced by T-system depolarization. At higher concentrations (25 microM) ryanodine produced a near-maximum contraction in only a few seconds.(ABSTRACT TRUNCATED AT 400 WORDS)
Lamb
G D
GD
Department of Zoology, La Trobe University, Bundoora, Australia.
Stephenson
D G
DG
D000470:Alkaloids / Q000494:pharmacology*; D000818:Animals; D002024:Bufo marinus; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism; D004533:Egtazic Acid / Q000494:pharmacology; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology; D009132:Muscles / Q000187:drug effects / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D012433:Ryanodine / Q000494:pharmacology*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D012964:Sodium / Q000494:pharmacology
D000470:Alkaloids; D015220:Calcium Channels; D012433:Ryanodine; D002110:Caffeine; D004533:Egtazic Acid; D012964:Sodium; D011188:Potassium; D002118:Calcium