Involvement of neurohumoral factors in the pressor mechanism of NG-nitro-L-arginine

Effects of DX-9065a, an inhibitor of factor Xa, on ellagic acid-induced plantar skin thrombosis assessed in tetrodotoxin- and N(omega)-nitro-L-arginine-treated rats

Effects of DX-9065a ((+)-2S-2-[4-[[(3S)-1-acetimidoyl-3-pyrrolidinyl]oxy]phenyl]-3-[7-amidino-2-naphthyl]propanoic acid hydrochloride pentahydrate), a dibasic propanoic acid and an inhibitor of factor Xa, were compared with those of argatroban, a low molecular weight thrombin inhibitor, on the ellagic acid-induced plantar skin thrombosis in the rats treated with tetrodotoxin and N(omega)-nitro-L-arginine. Plantar skin blood flow was measured with laser Doppler flow meters, and skin temperature of the hindlimb was monitored simultaneously. In order to induce thrombus in plantar skin vasculature, ellagic acid (300 microg, i.a.) was injected into a branch of femoral artery. The formation of thrombus in femoral and plantar vessels was assessed by light microscopy. Ellagic acid decreased plantar skin blood flow and skin temperature. Intravenous injections of DX-9065a (3 mg/kg) and argatroban (1-3 mg/kg) significantly inhibited the ellagic acid-induced disturbance of plantar skin blood flow and lowering skin temperature without affecting bleeding time. The oral administration of DX-9065a (30-100 mg/kg) significantly prevented the decrease in skin temperature induced by ellagic acid, but it partially inhibited the disturbance of plantar skin blood flow. DX-9065a and argatroban also prolonged prothrombin time in a dose-dependent manner. These results suggest that DX-9065a effectively prevented thrombosis produced by ellagic acid in the skin circulation without a risk of bleeding.

Non-contribution of renin-angiotensin system to pressor response to N(G)-nitro-L-arginine in dogs

Acute systemic blockade of nitric oxide (NO) production by nonselective inhibitors of NO synthase (NOS) isoforms, including N(G)-nitro-L-arginine methyl ester (L-NAME) and N(G)-nitro-L-arginine (L-NNA), has been shown to produce a long-lasting pressor response in conscious and anaesthetised animals. The present study was undertaken to clarify whether the renin-angiotensin system contributes to the development of this pressor response to L-NNA. Systemic blood pressure and heart rate were continuously monitored in dogs anaesthetised with pentobarbital. Plasma renin activity in the blood obtained from a femoral artery and a renal vein was measured by use of radioimmunoassay. The acute pressor response produced by the intravenous administration of L-NNA was accompanied by reduced renin activity in both systemic and renal vascular beds. Captopril, an angiotensin converting enzyme inhibitor, counteracted the pressor response to L-NNA, whereas candesartan, an angiotensin AT1-receptor antagonist, had no apparent effect on it. The counteraction by captopril of the L-NNA-induced pressor response was likely to be attributable to enhancement by captopril of depressor responses to bradykinin, as HOE-140, a bradykinin B2 receptor antagonist, neutralised the effect of captopril. These results suggest that the pressor response acutely produced by the intravenous injection of a NOS inhibitor is not mediated by the renin-angiotensin system in anaesthetised dogs.

Specific augmentation of plantar skin blood flow by lipo-PGE1 assessed in tetrodotoxin- and N(G)-nitro-L-arginine-treated rats

1Vasodilating effects of prostaglandin E1 incorporated in lipid microspheres (lipo-PGE1) were compared with those of prostaglandin E1 (PGE1) or its cyclodextrin clathrated preparation (PGE1-CD) on plantar skin blood flow in rats treated with tetrodotoxin and N(G)-nitro-L-arginine (L-NNA). Tetrodotoxin (50 microg/kg, i.v.) could totally inhibit the pressor response to electrical stimulation of the spinal cord, and the reflex tachycardia due to the depressor response to acetylcholine. Furthermore, L-NNA (30 mg/kg, i.v.) was used to counteract the lowering of the systemic blood pressure and peripheral vascular tone by elimination of sympathetic nerve activity, and to maintain the arterial blood pressure at the control level. Lipo-PGE1 increased plantar skin blood flow 4 to 6 times more potently than PGE1-CD or PGE1 in the treated rats. Furthermore, lipo-PGE1 increased plantar skin blood flow about 3 times more selectively than PGE1-CD. We also assessed several vasodilators, including terbutaline, nitroprusside, nicardipine, and papaverine in tetrodotoxin- and L-NNA-treated rats. However, none of them could selectively increase plantar blood flow despite the prominent depressor responses achieved. These results suggest that PGE1 preparations, especially lipo-PGE1 could potently and selectively increase plantar skin blood flow in rats treated with tetrodotoxin and L-NNA.

Inhibitory mechanism of N(G)-nitro-L-arginine on acetylcholine-induced depressor responses in dogs

The significance of the blood pressure elevation caused by N(G)-nitro-L-arginine (L-NNA) to inhibitory mechanism of the drug on depressor responses to acetylcholine in anesthetized dogs was investigated. L-NNA (50 mg kg(-1), i.v.) elevated blood pressure to a plateau of 30-50 mm Hg above baseline level and shifted the dose-response curve for acetylcholine-induced responses to the right by about 70-fold. Prevention by hydralazine (1 mg kg(-1), i.v.) of the blood pressure elevation over baseline level caused by L-NNA attenuated the inhibitory effect of L-NNA on the responses to acetylcholine. Intravenous neostigmine (30 microg kg(-1) bolus followed by 15 microg kg(-1) min(-1)) attenuated the inhibitory effect of L-NNA. The magnitude of the rightward shift in the dose-response curve for carbachol-induced depressor responses was only 3-fold. These results suggest that the accelerated acetylcholine metabolism by blood pressure elevation contributes to a considerable degree to the inhibitory mechanism of L-NNA.

Role of nitric oxide in the control of renin secretion

Because of the significant constitutive expression of NO synthases in the juxtaglomerular apparatus, nitric oxide (NO) is considered as a likely modulator of renin secretion. In most instances, NO appears as a tonic enhancer of renin secretion, acting via inhibition of cAMP degradation through the action of cGMP. Depending on as yet unknown factors, the stimulatory effect of NO on renin secretion may also switch to an inhibitory one that is compatible with the inhibition of renin secretion by cGMP-dependent protein kinase activity. Whether NO plays a direct regulatory role or a more permissive role in the control of renin secretion remains to be answered.

Possible participation of spinal nitric oxide in the control of the blood pressure in anesthetized rats

In pentobarbital-anesthetized rats the intrathecal (i.t.) injection of the nitric oxide (NO) precursor, L-arginine (10 and 20 micromol), elicited a decrease in the mean blood pressure (MBP) whereas the inhibitor of the NO synthase (NOS) N(G)-nitro-L-arginine methyl ester (L-NAME; 0.1-10 micromol) produced a dose-dependent pressor effect. The pressor response to L-NAME was prevented by pretreatment with L-arginine. Neither D-arginine nor D-NAME modified the MBP. The NO donor sodium nitroprusside (SNP; 0.125 and 0.25 micromol, i.t.) induced a hypotensive response followed by a pressor effect. The dual response to SNP as well as the hypotensive effect of L-arginine were abolished by the guanylate cyclase inhibitor Methylene blue (0.3 micromol, i.t.). Nicotinic ganglionic blockade by hexamethonium (10 mg/kg, i.v.) reduced the hypotensive effects of both L-arginine and SNP and prevented almost completely the pressor effects of either L-NAME or SNP. The pressor effect of L-NAME was abolished by 2-amino-5-phosphonovaleric acid (APV; 30 nmol, i.t.), a selective antagonist of glutamate receptors of the NMDA subtype. These results suggest that in the spinal cord of pentobarbital-anesthetized rats NO exerts both inhibitory and excitatory effects on the preganglionic sympathetic nerve activity related to the control of the BP. The synthesis of NO appears to be tonically activated through the stimulation of spinal glutamate receptors of the NMDA subtype.