Frog myocardium depends almost entirely on calcium entry from extracellular
spaces for its beat-to-beat activation. Atrial myocardium additionally
shows internal calcium release under certain conditions, but internal
release in the ventricle is absent or very low. We have examined
the content and distribution of the sarcoplasmic reticulum (SR) calcium
release channels (ryanodine receptors, RyRs) and the surface membrane
calcium channels (dihydropyridine receptors, DHPRs) in myocardium
from the two atria and the ventricle of the frog heart using binding
of radioactive ryanodine, immunolabeling of RyR and DHPR, and thin
section and freeze-fracture electron microscopy. In cells from both
types of chambers, the SR forms peripheral couplings and in both
chambers peripheral couplings colocalize with clusters of DHPRs.
However, although a low level of high affinity binding of ryanodine
is detectable and RyRs are present in peripheral couplings of the
atrium, the ventricle shows essentially no ryanodine binding and
RyRs are not detectable either by electron microscopy or immunolabeling.
The results are consistent with the lack of internal calcium release
in the ventricle, and raise questions regarding the significance
of DHPR at peripheral couplings in the absence of RyR. Interestingly,
the free SR membrane in both heart chambers shows a low but equal
density of intramembrane particles representing the Ca$^2+$ ATPase.