NITRIC OXIDE-INDEPENDENT RESPONSE TO ACETYLCHOLINE BY TERMINAL ARTERIOLES IN RAT CREMASTER MUSCLE

The aim of the present study was to establish whether, in terminal arterioles from the rat cremaster, acetylcholine (ACh) elicits nitric oxide (NO)-independent dilation corresponding to the transient ACh-induced endothelium-dependent hyperpolarization described in arteries. For this purpose, the responses of terminal arterioles [mean dam 15.0 +/- 0.4 (SE) mu m] were studied by intravital microscopy in rat cremaster muscle. During 15 min of superfusion by 10(-5) M ACh, the response was characterized by an initial maximal dilation (peak time <3 min) followed by a more sustained dilation that slightly decreased with time. Inhibition of NO synthesis by 2 X 10(-4) M N omega-nitro-L-arginine (L-NNA) significantly reduced, but did not eliminate, both the peak and sustained responses. Simultaneous administration of 2 x 10(-4) M L-NNA and 2 X 10(-5) M mefenamic acid, an inhibitor of prostaglandin synthesis, did not induce a significantly different response from that observed with L-NNA alone. Procaine (10(-3) M), which is known to inhibit completely ACh-induced hyperpolarization in carotid artery, drastically reduced the initial part of the ACh-induced dilation but not the sustained response. Simultaneous administration of procaine and L-NNA almost completely inhibited the peak response to ACh. Similar results were obtained when L-NNA was combined with a superfusion bath containing 20 mM KCl, a concentration known to reduce hyperpolarization in arteries. Simultaneous administration of L-NNA and 10(-5) M ouabain, which inhibits Na+-K+-adenosinetriphosphatase; 3 X 10(-6) M glibenclamide, which inhibits ATP sensitive Kf channels; or 10(-8) M iberiotoxin, which inhibits Ca2+-dependent K+ channels, did not cause any additional inhibition to that resulting from L-NNA alone. We conclude that the ACh-induced dilation in terminal arterioles simultaneously involves NO-dependent and -independent mechanisms. The latter displayed characteristics very similar to those demonstrated for ACh-induced hyperpolarization in arteries. They were transient, chiefly occurred during the initial part of the response, and were abolished by procaine. In terminal arterioles, these NO-independent mechanisms were not mediated by either activation of the Na+-K+ adenosinetriphosphatase or ATP-sensitive or Ca2+-dependent K+ channels.