Abstract： Objective To explore the effect of neuropeptide Y (NPY) on calcium/calmodulindependent protein kinase Ⅱ (CaMK Ⅱ ) in rat cardiomyocytes.Methods The cardiomyocytes from neonatal Sprague-Dawley (SD) rats were primarily cultured and divided into the control and NPY groups (n=6 each).The NPY group was added with 100 nmol/L NPY (final concentration), while the control group was left untreated.After 15 min, the activity of CaMK Ⅱ was detected by isotope 32P incorporation assay, and the expression of phospho - CaMK Ⅱ (p - CaMK Ⅱ ) protein by Western blotting.After 24 h, intranuclear concentration of Ca2+ in resting cardiomyocytes was recorded under laser scanning confocal microscope (LSCM).In addition, real-time PCR was used to detect the expressions of P-myosin heavy chain (β-MHC)mRNA and atrial natriuretic factor (ANF) mRNA in cardiomyocytes.Results After NPY stimulation for 15 min, the NPY group showed significantly elevated activity of CaMK Ⅱ (336 094.80±0 744.61 vs 273 301.50±±16 737..99, P＜0.05) and increased expression of p-CaMK Ⅱ protein in cardiomyocytes, compared with the control group.After 24 h, the NYP group showed a significant increase in intranuclear Ca2+ concentration compared wiith the control group (226.87±17.37 vs 84.04±6.50,P＜0.01).Moreover, real-time PCR also showed signficant increase in expressions of β- MHC and ANF mRNA.Conclusion The CaMK Ⅱ is activated with the increase of intranuclear Ca2+ concentration by NPY, which may play a major role in NYP-induced cardiomyocyte hypertrophy.