Increased catecholamine levels promote positive lusitropy, enabling the heart to relax more rapidly. This effect is mediated by the phosphorylation of
phospholamban and
troponin I via a
cAMP-dependent pathway. Catecholamine-induced calcium influx into the sarcoplasmic reticulum increases both inotropy and lusitropy. In other words, a quicker reduction in cytosolic calcium levels (because the calcium enters the sarcoplasmic reticulum) causes an increased rate of relaxation (a positive lusitropy), however, this also enables a greater degree of calcium efflux, back into the cytosol, when the next action potential arrives, thereby increasing inotropy as well. 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. ==Negative lusitropy==