Effects of acidosis on Ca2+ sensitivity of contractile elements in intact ferret myocardium

We investigated the effects of acidosis on the intracellular Ca2+ concentration ([Ca2+](i)) and contractile properties of intact mammalian cardiac muscle during tetanic and twitch contractions. Aequorin was injected into ferret papillary muscles, and the [Ca2+](i) and tension were simultaneously measured. Acidosis was attained by increasing the CO2 concentration in the bicarbonate (20 mM)-buffered Tyrode solution from 5% (pH 7.35, control) to 15% (pH 6.89, acidosis). Tetanic contraction was produced by repetitive stimulation of the preparation following treatment with 5 mu M ryanodine. The relationship between [Ca2+](i) and tension was measured 6 s after the onset of the stimulation and was fitted using the Hill equation. Acidosis decreased the maximal tension to 81 +/- 2% of the control and shifted the [Ca2+](i)-tension relationship to the right by 0.18 +/- 0.01 pCa units. During twitch contraction, a quick shortening of muscle length from the length at which developed tension became maximal (L-max) to 92% L-max produced a transient change in the [Ca2+](i) (extra Ca2+). The magnitude of the extra Ca2+ was dependent on the [Ca2+](i) immediately before the length change, suggesting that the extra Ca2+ is related to the amount of troponin-Ca complex. Acidosis decreased the normalized extra Ca2+ to [Ca2+](i) immediately before the length change, which indicates that the amount of Ca2+ bound to troponin C is less when [Ca2+](i) is the same as in the control. The decrease in the Ca2+ binding to troponin C explains the decrease in tetanic and twitch contraction, and mechanical stress applied to the preparation induced less [Ca2+](i) change in acidosis.