Inotropic effects on mammalian skeletal muscle change with contraction frequency.

Over the last decade, we have attempted to determine if mammalian skeletal muscle's steady-level force development as established by mechanical and stimulation parameters can be increased or decreased by physiological signals. In these experiments, nitric oxide (NO), endothelin-1 (ET-1), adenosine (Ado), and beta-adrenergic agonists (beta) modified force production in the soleus and (or) the extensor digitorum longus (EDL) of the mouse. NO and beta increased the force produced by 0.5-s tetanic contractions at 0.6 contractions/min in both muscles. While EDL did not respond to either Ado or ET-1, the developed force of the soleus was amplified by Ado but attenuated by ET-1. Increased cAMP analogue concentrations amplified developed force in both muscles, but a cGMP analogue had no effect on either muscle. Following an increase in the contraction frequency of the soleus, the increased force in response to NO disappeared, as did the decreased force to ET-1. The increase in force due to a cAMP analogue disappeared during fatigue but reappeared quickly during recovery. Thus, steady-level developed force can be modified by a number of substances that can be released from locations in the body or muscle. The response to a given compound is determined by a complex interaction of metabolic and intracellular signals on the force-generating cascade.

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.

Nitric oxide synthase inhibitors do not alter functional hyperemia in canine skeletal muscle.

To test the hypothesis that endothelium-derived products contribute to functional hyperemia in skeletal muscle, we infused nitric oxide synthase inhibitors, either 200 microM N omega-nitro-L-arginine (NNA) (N = 4) or 1 mM N gamma-monomethyl-L-arginine (NMMA) (N = 4), before and during 6 min of 4 Hz stimulation of canine gastrocnemius in situ. We infused saline (N = 4) as a control. NNA significantly decreased steady-level resting flow by 3.8 +/- 0.4 mL.kg-1.s-1. The increase in flow from rest to 5 min of stimulation was not changed by the nitric oxide synthase inhibitors. We also stimulated muscles for 60 min either with saline infusion (N = 4) or with the infusion of saline during the first 15 min and NNA for the remaining 45 min (n = 4). There was no difference in the flow during contractions. To clarify the effect of these inhibitors on canine vessels, we challenged rings of canine femoral artery with and without endothelium with acetylcholine and bradykinin (both 1 microM) before and after the addition of NNA and NMMA (both 10 microM). The nitric oxide synthase inhibitors decreased the relaxation accompanying acetylcholine. Both inhibitors caused only endothelium-intact rings to contract. Thus, the presence of a nitric oxide synthase inhibitor identified an endothelium-dependent contribution to the regulation of blood flow to skeletal muscle at rest but had no effect on functional hyperemia.