Abstract
Striking correlations are found between cardiac contractility and
Ca2+ binding to isolated cardiac sarcolemma in rabbit, neonatal rat,
and frog ventricular tissue. Deviations from this correlation are
seen in the adult rat ventricle and rabbit atrium. The observation
of this correlation in the three former tissues and under various
ionic conditions suggests that this correlation is not coincidental
and that Ca2+ bound to the cardiac sarcolemma is of major importance
in the control of myocardial contractility. The data are consistent
with a functional Ca2+-induced Ca2+ release system in the sarcoplasmic
reticulum (SR) of all the tissues (which is controlled by Ca2+ entry
from sarcolemmal sites), with the adult rat ventricular and rabbit
atrial SR Ca2+ release being much more sensitive to CA2+. It is suggested
that the frog, neonatal rat, and rabbit ventricles depend more directly
on the entry of Ca2+ from sarcolemmal sites for the control of tension
development, whereas the adult rat ventricle and rabbit atrium depend
to a greater extent on CA2+ released from the SR.
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