Calycophoran siphonophore muscle fibres without any sarcoplasmic reticulumbut with tubular invaginations morphologically analogous to a T-system

Different marine invertebrates have different tubular or vesicular systems within their locomotor muscle fibres. The siphonophores Chelophyes, Abylops is and Muggeia have invaginated tubules which are the morphological equival ent of the vertebrate invaginated tubular system, but lack a sarcoplasmic r eticulum. In Chelophyes the previous suggestion that Ca2+ channels in the e xtensive invaginated tubule system allow ingress of Ca2+ is shown to be inc orrect. Contraction of the swimming muscles in Chelophyes is not blocked by 20 mu M ryanodine, nor is it induced by 10 mM caffeine, hence intracellula r Ca2+ stores appear absent. Contraction is, however, maintained by replace ment of the greater part of the usual external Na+ by Li+ or by or N-methyl -D-glucamine, although action potentials can still be evoked. Hence we conc lude that following contraction, internal Ca2+ is reduced by a Na/Ca2+ exch ange mechanism.