ISCHEMIC TOLERANCE IN SKELETAL-MUSCLE - ROLE OF NITRIC-OXIDE

We tested the hypothesis that ischemic preconditioning (PC) of skeleta l muscle provided tolerance to a subsequent ischemic event 24 h later, and that such protection was due to nitric oxide (NO). Male Wistar ra ts, anesthetized with halothane, were randomly assigned to groups: isc hemic (no PC; n = 11), PC (n. = 11), PC + N-nitro-L-arginine methyl es ter (L-NAME; 100 mu mol/l; n = 5), PC + N-nitro-D-arginine methyl este r (100 mu mol/l; n = 4), PC + aminoguanidine (AMG; 100 mu mol/l; n = 4 ), ischemic + L-NAME (n = 4), or ischemic + AMG (n. = 4). PC consisted of 5 x 10 min of ischemia and reperfusion, and, 24 h later, 2 h of is chemia were induced by a tourniquet applied to the limb. With the use of intravital microscopy, the number of perfused capillaries (N-pc) in the extensor digitorum longus (EDL) muscle was measured over a 90-min reperfusion period. The ratio of ethidium bromide- to bisbenzimide-la beled nuclei was used to estimate tissue injury. PC preserved N-pc (23 .6 +/- 2.5) following 2 h of ischemia compared with sham muscles (11.5 +/- 5.1), significantly elevating inducible NO synthase (iNOS) activi ty (81% increase), but did not afford protection to the parenchyma. L- NAME and AMG prevented ischemia-reperfusion-induced reduction in N-pc in muscles without PC. However, after 90 min of reperfusion, L-NAME (N -pc = 15.0 +/- 1.7), but not AMG (N-pc = 22.8 +/- 3.1), significantly reduced the microvascular protection afforded by PC. We conclude that PC of the EDL muscle resulted, 24 h later, in protection to microvascu lar perfusion only, and that such protection was due to NO from source s other than iNOS.