The purpose of this study was to determine effects of long-term electrical stimulation of cardiocyte contraction on protein synthesis rates and total protein content. Adult feline cardiocytes were plated on laminin-coated culture trays and maintained in a serum-free medium consisting of M199 supplemented with ascorbate, bovine serum albumin, creatine, carnitine, taurine, and 10(-7) M recombinant insulin. Cardiocytes were electrically stimulated to contract with use of continuous electrical pulses of alternating polarity at a frequency of 1 Hz and pulse duration of 5 ms. Nonstimulated cardiocytes are normally quiescent and were used as the control group. In control quiescent cardiocytes, protein synthesis rate decreased by 14% between days 1 and 4 in culture and then remained stable through day 7. In electrically stimulated cardiocytes, protein synthesis rates increased by 19% between days 1 and 7. Protein synthesis rates were 18% higher on day 4 and 43% higher on day 7 in electrically stimulated than in quiescent cardiocytes. Protein content per cell was determined by measuring total fluorescence per cell by use of confocal microscopy of fluorescein isothiocyanate-stained cells. Electrical stimulation significantly increased cellular protein content by 52% after 7 days compared with controls. Quiescent and electrically stimulated cardiocytes remained rod shaped, retained their myofibrillar architecture, and were responsive to electrical stimulation over the 7-day period. These data demonstrated that electrically stimulated contraction of adult cardiocytes resulted in cell growth, as assessed by an increase in protein content per cell over 7 days in culture. This increase was due, at least in part, to an acceleration of steady-state protein synthesis rates.