This should not be confused, however, with catecholamine-induced calcium uptake into the sarcoplasmic reticulum, which increases lusitropy.
Increased catecholamine levels promote positive lusitropy, enabling the heart to relax more rapidly.
Catecholamine-induced calcium influx into the sarcoplasmic reticulum increases both inotropy and lusitropy.
However, unlike the previously mentioned mechanism, a calcium uptake from the extracellular fluid into the cytosol without any catecholamine stimulation simply results in a sustained rise in calcium concentration in the cytosol.
This only serves to increase inotropy but doesn't allow total relaxation of the cardiac myocytes between contractions, decreasing lusitropy.