Effect of nitroprusside and endothelium-derived products on slow-twitch skeletal muscle function in vitro

We tested the hypothesis that the products of endothelial cells alter the force developed by skeletal muscle. Since these products have a very short half-life and are produced in a low concentration, we developed a superfused muscle preparation in which the mouse soleus (SOL) was superfused at 10.5 mL/min with Krebs-Henseleit buffer (KH) (27 degrees C; pH 7.4), gassed with 95% O2-5% CO2. To evaluate this preparation, we compared the superfused muscles with muscles submerged in a bath. All muscles were stimulated at 50 Hz for 500 ms once every 30 s. Submerged SOL developed 275 +/- 15 mN/mm2, while the superfused muscles developed 271 +/- 15 mN/mm2. Both submerged and superfused SOL consistently increased rest tension to a 3-mL bolus of 25 mM caffeine and decreased developed force when exposed to a 3-mL bolus of 30 mM diprotonated phosphate (pH 6.4). We then exposed superfused SOL to 3 mL bolus injections of KH, 1 microM acetylcholine, 30 mM nitroprusside (a source of nitric oxide), and the supernatant from dishes of cultured endothelial cells from rabbit aorta challenged with acetylcholine. Nitroprusside and the supernatant significantly improved force maintenance, compared with KH and acetylcholine, respectively. Since the supernatant should contain products of endothelial cells, these products appear to have a positive effect on contractile function in slow-twitch skeletal muscle that is similar to the effect of nitric oxide.