Nitric oxide from vascular smooth muscle cells: regulation of platelet reactivity and smooth muscle cell guanylate cyclase

Incubation of smooth muscle cells (SMC) from bovine aorta for 3 min with human washed platelets treated with indomethacin (10 μm) promoted a cell number‐related inhibition of platelet aggregation induced by thrombin (40 mu ml−1). This inhibition was not attributable to products of the cyclooxygenase pathway for the SMC were also treated with indomethacin (10 μm). The inhibitory activity of the SMC on platelet aggregation was enhanced by incubating the SMC with E. coli lipopolysaccharide (LPS, 0.5 μg ml−1) for a period of 9 to 24 h. This effect was attenuated when cycloheximide (10 μg ml−1) was incubated together with LPS. Cycloheximide did not prevent the inhibitory activity of the non‐treated cells. The inhibition of platelet aggregation obtained with non‐treated or LPS‐treated SMC was potentiated by superoxide dismutase (SOD, 60 u ml−1) and ablated by oxyhaemoglobin (OxyHb, 10 μm). Preincubation of the SMC with NG‐monomethyl‐l‐arginine (l‐NMMA, 30–300 μm) for 60 min prevented their antiaggregatory activity. This effect was reversed by concurrent incubation with l‐arginine (l‐Arg, 100 μm) but not with d‐arginine (d‐Arg, 100 μm). Exposure of the non‐treated SMC (5 × 105 cells) to stirring (1000 r.p.m., 37°C) for 10 min led to a significant increase in their levels of guanosine 3′:5′‐cyclic monophosphate (cyclic GMP) but not adenosine 3′:5′‐cyclic monophosphate (cyclic AMP). l‐NMMA (300 μm) attenuated the increase in cyclic GMP induced by stirring but did not affect the basal levels of cyclic GMP in the cells. The inhibitory activity of l‐NMMA was reversed by co‐incubation with l‐Arg (100 μm) but not d‐Arg (100 μm). l‐Arg alone had no effect on the levels of cyclic GMP. In the absence of stirring, a 10 min stimulation of the non‐treated SMC with glyceryl trinitrate (GTN, 200 μm) or atrial natriuretic factor (ANF, 10−7 m) led to an increase in the levels of cyclic GMP but not cyclic AMP. The increase in cyclic GMP promoted by GTN or ANF was not affected by l‐NMMA. The levels of cyclic GMP were higher in the LPS (0.5 μg ml−1, 18 h)‐treated cells (5 × 10−5) and stirring was more effective in increasing the levels of cyclic GMP in these cells. These findings support the idea that non‐treated or LPS‐treated cultured SMC can produce an NO‐like factor. Production by the latter requires protein synthesis as evidenced by blockade with cycloheximide. This NO‐like factor may play a role in the auto‐regulation of smooth muscle cell reactivity through a cyclic GMP‐dependent mechanism. 1991 British Pharmacological Society