LENGTH DEPENDENCE OF CA2-TENSION RELATIONSHIP IN AEQUORIN-INJECTED FERRET PAPILLARY-MUSCLES()

The possible contractile proteins, which are related to the length-dep endent change in the relationship between intracellular Ca2+ concentra tion ([Ca2+](i)) and tension, were investigated using aequorin-injecte d ferret papillary muscles. Tetanic contraction was produced by applyi ng repetitive stimulation to the ryanodine-treated preparations, and t he relationships between [Ca2+](i) and tension were measured. When the muscle length was decreased from maximal length (L-max), at which max imal tension is produced, to 95 and 90% L-max, the maximal tension was significantly decreased. [Ca2+](i) required for producing 50% of the maximal tension was significantly increased from 1.05 +/- 0.04 mu M L- max) to 1.17 +/- 0.04 mu M (95% L-max) and to 1.22 +/- 0.04 mu M (90% L-max). Isoproterenol (Iso) accentuated the length-dependent change in the [Ca2+](i)-tension relationship. The decrease in the Ca2+ sensitiv ity induced by Iso was larger at shorter muscle lengths compared with that at L-max. It is therefore, suggested that adenosine 3',5'-cyclic monophosphate-dependent phosphorylation of troponin I and/or C protein alters the length dependence of the [Ca2+](i)-tension relationship an d that troponin I and/or C protein might be involved in the length-ten sion-dependent change in the affinity of the contractile elements for Ca2+.