Statistical properties of spontaneous contractions of atrial muscle cells were examined and compared to those of ventricular Muscle cells, The cells derived from atria of neonatal rats exhibit spindle morphology, and they were found to express alpha-smooth muscle actin and hyperpolarization activited cation channel 4. both of which are known marker of neonatal atrial muscle cells. The short-term properties of spontaneous contractions of atrial cells, characterized by considerably large beat rate and absence Of bursts, Ire distinct from those of ventricular muscle cells. Despite of these differences, the long-term properties of the beat-rate fluctuations exhibit a remarkable similarity to those Of ventricular cells. In particular, the presence of power-law correlation characterized as 1/f(beta) noise (beta approximate to 1) was also confirmed for atrial cells for the First time. The observed similarity of the long-term characteristics of bear-rate fluctuation suggests the Presence of a general regulatory nnechanism of the cellular Function. (C) 2009 Elsevier Inc. All rights reserved.