Single smooth muscle cells from rat tail artery and ventricular myocytes from neonatal rat were isolated by repeated enzyme digestion. The change in cell area as determined photographically was used as an index of cell contraction. The photographic areas of single smooth muscle cells bathed in normal Tyrode solution were 403 +/- 22 (n = 13) square micra. Exposure of smooth muscle cells to a modified Tyrode solution containing 60 mM KCl induced cell contraction. This contraction was inhibited by bPTH-(1-34) at a concentration of 1 microM. The inhibitory effect of bPTH-(1-34) was time-dependent with maximum inhibition at 5 min after administration. The photographic areas of ventricular myocytes bathed in the culture medium without fetal calf serum were 516 +/- 47 (n = 29) square micra. At a concentration of 1 microM, bPTH-(1-34) produced a time-dependent contraction in ventricular myocytes as shown by the decrease in the photographic cell area (88 +/- 2% of the control value at 15 min, n = 9, p < 0.01). Furthermore, 1 microM nifedipine inhibited the effect of bPTH-(1-34) on the contraction of ventricular myocytes, indicating that bPTH-(1-34) might exert its action via a calcium channel related mechanism. In addition, bPTH-(1-34) increased the contraction frequency of single ventricular cells, which could also be inhibited by nifedipine. The present study suggests that bPTH-(1-34) directly relaxes precontracted single vascular smooth muscle cells and contracts single ventricular myocytes.