CONTROL OF CALCIUM RELEASE AND THE EFFECT OF RYANODINE IN SKINNED MUSCLE-FIBERS 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) © 1990 The Physiological Society