The properties of two types of calcium current (I-Ca(T) and I-Ca(L) an
d their relationship with contraction were studied in isolated frog at
rial trabeculae using the double mannitol gap technique. In Ringer sol
ution containing TTX the calcium currents observed are similar to the
I-Ca(T) and I-Ca(L) found in single frog atrial cells. The results obt
ained by using inorganic calcium channel blockers have shown that the
phasic contraction appears only when I-Ca(T) and I-Ca(L) are simultane
ously present. It is proposed that I-Ca(T) loads the sarcoplasmic reti
culum (SR) while I-Ca(L)) triggers Ca2+ release from the SR.