Different Ca2+ releasing action of caffeine and depolarisation in skeletalmuscle fibres of the rat

1. The relative abilities of caffeine and transverse tubular (T-) system de polarisation to induce Ca2+ release in mammalian skeletal muscle were compa red in mechanically skinned fibres of the rat, in order to determine whethe r normal excitation-contraction (E-C) coupling is achieved by up-regulating the Ca2+-induced Ca2+ release process, as caffeine is known to do. 2. Caffeine triggered Ca2+ release in soleus (slow-twitch) fibres at much l ower concentrations than in extensor digitorum longus (EDL) (fast-twitch) f ibres when the sarcoplasmic reticulum (SR) of each type was loaded with Ca2 + at close to endogenous levels. The difference in caffeine sensitivity res ulted at least in part from the SR being loaded endogenously at near maxima l capacity in soleus fibres but at less than half of maximal capacity in ED L fibres. The caffeine sensitivity could be reversed by reversing the relat ive level of SR loading. 3. The ability of caffeine to induce Ca2+ release was markedly reduced by l owering the level of SR loading or by raising the free [Mg2+] from 1 to 3 m M. Caffeine, even at 30 mM, triggered little or no Ca2+ release in EDL fibr es (a) at 1 mM (physiological) Mg2+ when the SR was loaded at two-thirds or less of the endogenous level, and (b) at 3 mM Mg2+ when the SR was loaded at close to the endogenous level. In contrast, depolarisation potently elic ited Ca2+ release under these conditions in the same fibres. 4. The inability of 30 mM caffeine to induce Ca2+ release under certain con ditions was not attributable to desensitisation or inactivation of the rele ase channels, because there was no response even upon initial exposure to c affeine and depolarisation always remained able to trigger Ca2+ release. It instead appeared that caffeine was a relatively ineffectual stimulus in ED L fibres except under conditions where (a) the SR was heavily loaded, (b) t he free [Mg2+] was low, or (c) a high [Cl-] was present. 5. These results show that the normal E-C coupling mechanism in mammalian s keletal muscle does not, involve just enhancing Ca2+-induced Ca2+ release, and evidently requires the removal or bypassing of the inhibitory effect of Mg2+ on the Ca2+ release channels.