Effects of heparin on excitation-contraction coupling in skeletal muscle toad and rat.
1. Intracellularly applied heparin was found to cause a novel, use-dependent block of excitation-contraction (E-C) coupling in skinned skeletal muscle fibres of the toad. After one to four depolarizations in the presence of 100 micrograms ml-1 heparin, no further depolarization-induced responses could be elicited, even though addition of caffeine or lowering [Mg2+] could still induce massive Ca2+ release. This effect could not be reversed by extensive wash-out of the heparin (> 15 min). 2. Heparin (100 micrograms ml-1) did not abolish subsequent depolarization-induced responses if applied while the voltage sensors were in either their resting or inactivated states, that is (a) while a fibre remained fully polarized, (b) when a fibre was already chronically depolarized or (c) after a fibre had been depolarized in the presence of D600 (gallopamil) and then repolarized. 3. When a toad fibre was depolarized in heparin, with the associated Ca2+ release blocked by the presence of 10 mM intracellular Mg2+, subsequent E-C coupling was abolished. Heparin did not interrupt E-C coupling when Ca2+ release was triggered in the absence of any depolarization, by either caffeine or low [Mg2+]. Thus, the opening of the Ca2+ release channels was neither necessary nor sufficient for heparin to abolish E-C coupling. 4. Heparin had direct effects on the contractile apparatus in toad fibres, increasing the Ca2+ sensitivity and decreasing the maximum Ca(2+)-activated force. These effects could only be partly reversed by extensive wash-out of heparin. 5. At 100 micrograms ml-1, both low molecular weight heparin and pentosanpolysulphate, another highly sulphated polysaccharide, were less effective than heparin in blocking the depolarization-induced response and in changing the properties of the contractile apparatus, and these effects could be substantially reversed by wash-out. Two other polyanions, de-N-sulphated heparin (100 micrograms ml-1), which lacked N-sulphate groups, and polyglutamate (500 micrograms ml-1), had no measurable effect on either E-C coupling or the contractile apparatus. 6. In skinned fibres of the extensor digitorum longus muscle of the rat, 100 micrograms ml-1 heparin had little or no effect on E-C coupling and on the Ca2+ sensitivity of the contractile apparatus, but caused a larger reduction of the maximum Ca(2+)-activated force than in skinned fibres of the toad. 7. These results indicate that heparin blocks E-C coupling in toad muscle if, and only if, it is present when the voltage sensors are activated by depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)
Lamb
G D
GD
Department of Zoology, La Trobe University, Bundoora, Victoria, Australia.
Posterino
G S
GS
Stephenson
D G
DG
D000818:Animals; D002024:Bufo marinus; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000502:physiology; D004594:Electrophysiology; D005711:Gallopamil / Q000494:pharmacology; D006493:Heparin / Q000494:pharmacology*; D006495:Heparin, Low-Molecular-Weight / Q000494:pharmacology; D066298:In Vitro Techniques; D008274:Magnesium / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects; D009132:Muscles / Q000187:drug effects*; D009467:Neuromuscular Depolarizing Agents / Q000494:pharmacology; D010426:Pentosan Sulfuric Polyester / Q000494:pharmacology; D051381:Rats; D013045:Species Specificity
D006495:Heparin, Low-Molecular-Weight; D009467:Neuromuscular Depolarizing Agents; D010426:Pentosan Sulfuric Polyester; D005711:Gallopamil; D002110:Caffeine; D006493:Heparin; D008274:Magnesium; D002118:Calcium