K. Komukai et al., EFFECTS OF ACIDOSIS ON CA2+ SENSITIVITY OF CONTRACTILE ELEMENTS IN INTACT FERRET MYOCARDIUM, American journal of physiology. Heart and circulatory physiology, 43(1), 1998, pp. 147-154
We investigated the effects of acidosis on the intracellular Ca2+ conc
entration ([Ca2+](i)) and contractile properties of intact mammalian c
ardiac muscle during tetanic and twitch contractions. Aequorin was inj
ected into ferret papillary muscles, and the [Ca2+](i) and tension wer
e simultaneously measured. Acidosis was attained by increasing the CO2
concentration in the bicarbonate (20 mM)-buffered Tyrode solution fro
m 5% (pH 7.35, control) to 15% (pH 6.89, acidosis). Tetanic contractio
n was produced by repetitive stimulation of the preparation following
treatment with 5 mu M ryanodine. The relationship between [Ca2+](i) an
d 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 relatio
nship to the right by 0.18 +/- 0.01 pCa units. During twitch contracti
on, a quick shortening of muscle length from the length at which devel
oped tension became maximal (L-max) to 92% L-max produced a transient
change in the [Ca2+](i) (extra Ca2+). The magnitude of the extra Ca2was dependent on the [Ca2+](i) immediately before the length change, s
uggesting that the extra Ca2+ is related to the amount of troponin-Ca
complex. Acidosis decreased the normalized extra Ca2+ to [Ca2+](i) imm
ediately 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 d
ecrease in tetanic and twitch contraction, and mechanical stress appli
ed to the preparation induced less [Ca2+](i) change in acidosis.