Abstract
1. Exchangeable Ca in guinea-pig auricles and ventricular trabeculae
of sheep and calf hearts was labelled with (45)Ca and the loss of
radioactivity into inactive rinsing solutions of different ion composition
was measured for periods up to 6 hr. At no time did the decrease
of radioactivity in the muscle follow a single exponential course,
while the rate coefficient k (= fraction of (45)Ca lost per minute
from muscle into rinsing solution) decreased slightly with time.2.
On the basis of the temperature-sensitivity of Ca efflux from auricles
the activation energy has been calculated to have a value of 5900
cal/mole, corresponding to a Q(10) of 1.35. 2,4-Dinitrophenol (5.5
x 10(-5) - 5.5 x 10(-4)M) had either no effect on Ca efflux or increased
it slightly.3. Compared to control efflux in 1.8 mM Ca(o) Ca efflux
decreased to 70\% in Ca-free solution, to 20\% in Ca-free, Na-free
solution and to 65\% in Ca-containing, Na-free solution, NaCl being
replaced by either sucrose or LiCl. Quantitatively, Ca efflux from
auricles has been shown to depend to a large extent on the ratio
Ca$^2+$(o)/Na$^+$(o) (2). The affinity for Na of the
activation site for Ca efflux (carrier) is much less than for Ca.4.
The efflux from muscles soaked for about 2 hr in Ca-free solution
was not linearly related to Ca-concentration in the tissue but followed
a square law.5. While Ca content in auricles increased in Ca-containing,
Na-poor solution it decreased again when Tyrode solution was readmitted
indicating a Na-sensitive Ca net transport in cardiac muscle.6. The
results are interpreted in terms of a modified exchange diffusion
mechanism (Ussing, 1947) which is responsible for Ca extrusion from
mammalian cardiac muscle.
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