- The effect of adenosine and nitroglycerine on single smooth muscle cells isolated from the right coronary artery of the pig is reported. Trypan Blue exclusion and electron microscopy revealed that the cells used in this study were structurally intact. These cells were 79.2 +/- 1.6 microns long. Within 1 min of exposure to solutions containing norepinephrine, carbachol and high KCl, the cells contracted by 20-30%. The concentrations required for half maximal contraction for norepinephrine, carbachol and KCl were 0.1 to 1 nM, 0.01 to 0.1 nM and 15 to 25 mM, respectively. Adenosine and nitroglycerine inhibited the contractions caused by high (30 mM) K+ and also relaxed the cells previously contracted by high K+. Thus adenosine and nitroglycerine relax by acting directly on the smooth muscle cells.