Nonlinear summation of contractions in striated muscle. II. Potentiation of intracellular Ca2+ movements in single barnacle muscle fibres

Abstract

Nonlinear summation of contractions is studied in single barnacle (Balanus nubilus) muscle fibres, loaded with the photoprotein aequorin. The results indicate that nonlinear summation of aequorin transients is indeed present and for short interpulse intervals (25–250 ms), a more-than-linear summation of transients, which suggest an increase of the cytosolic Ca²⁺ concentration in the second response, is observed. This augmented Ca²⁺ concentration is not merely due to summation with the preceding conditioning transient, but to an enlargement of the second transient in its own right. Furthermore, the enlargement of the second Ca²⁺ response is not the result of prolonged release, or slowing of re-uptake by intracellular organelles. On the contrary, Ca²⁺ release is found to be enhanced and for short depolarizations (20 ms), its time to half re-uptake is reduced. The intensified Ca²⁺ release, triggered by the second standard depolarization, is related to the level of cytosolic Ca²⁺ concentration reached in the conditioning response and, for example, appears to be larger in the presence of Dantrolene-sodium, which is known to reduce Ca²⁺ movements in a single twitch. It is concluded that contractile potentiation observed during nonlinear summation of contractions, is associated with a potentiation of intracellular Ca²⁺ movements, which interact to regulate the cytosolic Ca²⁺ concentration during contraction.