Inducible nitric oxide synthase (iNOS) mediates late preconditioning (PC) induced by ischemia and pharmacological agents. Since alpha -adrenoceptor (alpha -AR) stimulation is one of the key triggers of PC, we hypothesized that activation of this receptor may induce delayed cardioprotective effect via iNOS-sensitive mechanisms. Adult male ICR mice were treated i.p. with either vehicle/inhibitors or phenylephrine (10 mg/kg) and subjected to 30 min of global ischemia and 30 min reperfusion in Langendorff mode 24 h later. 5-Methyl-urapidil (3 mg/kg) and chloroethylclonidine (3 mg/kg) were injected 15 min prior to phenylephrine to block alpha -AR(1A) and alpha -AR(1B) receptors respectively. S-Methylisothiourea (3 mg/kg), an iNOS inhibitor, was given 60 min prior to ischemia in phenylephrine-pretreated mice. Preischemic NO(x) was measured using a chemoluminescence reaction. Phenylephrine treatment reduced infarct size from 31.10 +/- 0.79% (vehicle) to 14.24 +/- 0.84% (P<0.001). Chloroethylclonidine blocked the effect of phenylephrine (infarct size 31.31 +/- 1.69%) but 5-methyl-urapidil (17.72 +/- 1.25%) did not. Phenylephrine-induced delayed cardioprotection was abolished by S-methylisothiourea and absent in iNOS knockout mice. Baseline NO(x) content was significantly increased in phenylephrine and 5-methyl-urapidil+phenylephrine treated hearts, but remained at baseline levels in hearts treated with chloroethylclonidine, 5-methyl-urapidil or S-methylisothiourea. Western blot analysis revealed a 1.8-fold increase in iNOS with phenylephrine, which was inhibited by chloroethylclonidine but not by 5-methyl-urapidil. We conclude that phenylephrine-induced delayed PC is mediated by selective activation of alpha-AR(1B). Enhanced iNOS expression concomitant with increased NO synthesis, as well as pharmacological blockade and absence of cardioprotection in iNOS knockout mice suggests an essential role of NO in phenylephrine triggered late PC.
Copyright 2002 Academic Press.