Cardiac physiology in tunas. II. Effect of ryanodine, calcium, and adrenaline on force–frequency relationships in atrial strips from skipjack tuna, Katsuwonus pelamis

Abstract

Force–frequency relationships and the dependence upon extracellular calcium as a source of activator calcium were investigated using atrial strips from the skipjack tuna (Katsuwonus pelamis). At 25 °C, active force generation increased over a stimulation range of 0.2–1.6 Hz and declined at higher stimulation frequencies. Ryanodine, a blocker of calcium release from the sarcoplasmic reticulum, decreased active force by ~30% but did not alter the shape of the force–frequency curve. Uniform contractions were maintained up to stimulation rates of 3.0 Hz following ryanodine application, compared with 3.4 Hz under control conditions. Active force increased with increasing extracellular calcium concentration in a concentration-dependent manner with greatest sensitivity over the range of 1.0–5.0 mM calcium. Activation duration decreased with increasing extracellular calcium concentrations up to 5.0 mM and with increasing contraction frequency up to 1.0 Hz. Relaxation duration also was reduced with increasing extracellular calcium over a range of 0.5–4.0 mM, and over a stimulation range of 0.2–0.6 Hz. Adrenaline increased active force threefold over the range of 10⁻⁸ to 10⁻⁵ M, with an EC₅₀ value of 4.90 ± 0.57 × 10⁻⁷ M. The EC₅₀ value was not dependent upon stimulation frequency. These results indicate that contraction in the skipjack tuna atrium is partially dependent upon intracellular calcium release from the sarcoplasmic reticulum. In this manner, the tuna heart more closely resembles the mammalian model of excitation–contraction coupling than that suggested for other fishes in which activator calcium is almost exclusively derived from extracellular influx.