nitroarginine and lacidipine

Experiments were performed on perfused human middle cerebral artery (MCA) precontracted with prostaglandin F2a. Nifedipine, isradipine and lacidipine induced dose-dependent relaxant effects on MCA segments. Effects of nifedipine were rather similar on MCA segments with and without endothelium; however, responses of endothelium-denuded vessel segments to isradipine and especially lacidipine were lower than those of vessels with intact endothelium. The vasodilator effects of these agents, especially isradipine and lacidipine, on vessel segments with intact endothelium in the presence of a NO-synthase inhibitor, N-nitro-L-arginine, were significantly attenuated whereas a cyclooxygenase inhibitor, indomethacin, did not significantly alter vasodilator responses of MCA segments to all calcium antagonists tested. It is suggested that the endothelium modulates vascular relaxation to dihydropyridines by an enhancement of calcium antagonist actions by basally released EDRF/NO at the level of vascular smooth muscles or by a dihydropyridine-induced increase in the release of EDRF/NO.

Topics: Adult; Arginine; Calcium Channel Blockers; Cerebral Arteries; Cyclooxygenase Inhibitors; Dihydropyridines; Dinoprost; Dose-Response Relationship, Drug; Endothelium, Vascular; Enzyme Inhibitors; Humans; In Vitro Techniques; Indomethacin; Isradipine; Muscle Relaxation; Muscle, Smooth, Vascular; Nifedipine; Nitric Oxide; Nitric Oxide Synthase; Nitroarginine; Vasodilator Agents

1. The present experiments were undertaken in order to characterize further the apparently irreversible inhibition of the contraction of depolarized rat aorta caused by lacidipine, a 1,4-dihydropyridine calcium antagonist. 2. We studied the effect of lacidipine on contraction evoked by 100 mM KCl solution in rat aorta, treated by N omega-nitro-L-arginine (0.1 mM), an inhibitor of nitric oxide (NO) synthesis. We compared the effect of prolonged depolarization on lacidipine and (+)-isradipine inhibition and the reversal of this inhibition after washout in the absence of dihydropyridines. Assuming that the onset of lacidipine-evoked inhibition was a pseudo-first order association kinetics, we estimated the dissociation rate constant (k-1 = 0.031 min-1), the association rate constant (k1 = 2.70 x 10(8) M-1 min-1) and the dissociation constant (KD = k-1/k1 = 115 pM) which was close to the IC50 value in steady-state conditions (160 pM). 3. The inhibitory effects of lacidipine and (+)-isradipine on rat aorta contraction were reversibly enhanced after preincubation with the drug in a 40 mM KCl-solution. Washout with drug-free 40 mM K(+)-depolarizing solution reversed inhibition in the (+)-isradipine-treated preparations, but not in the lacidipine-treated ones. 4. Radioligand binding studies were performed with [3H]-lacidipine and [3H]-isradipine in microsomes from rat aorta and rat ileum. Both ligands bound to a homogeneous population of binding sites (for[3H]-lacidipine: KD = 23 +/- 2.6 pM, Bmax = 380 +/- 21 fmol mg-1 protein in membranes from aorta; KD =23 +/- 3.1 pM, Bmax = 790 +/- 60 fmol mg-1 protein in membranes from ileum; for [3H]-isradipine:KD = 140 +/- 46 pM, Bmax = 350 +/- 64 fmol mg-1 protein in membrane from aorta; KD = 68 +/- 14 pM,Bmax = 760 +/- 75 fmol mg-1 protein in membranes from ileum). After isotopic dilution, [3H]-lacidipine and [3H]-isradipine dissociated according to a monoexponential kinetics. In membranes from ileum, the calculated dissociation rate constants (kappa_ 1) were 0.0257 min-1 and 0.0595 min-1, for [3H]-lacidipine and[3H]-isradipine, respectively.5. The non specific binding of [3H]-lacidipine and [3H]-isradipine, was measured in intact rat aorta preparations incubated under the conditions of the functional experiments, in the presence of nifedipine(1 microM). After incubation with [3H]-lacidipine 77.6 +/- 1.9 pM for 2 h the concentration of drug in the tissue was 15.15 +/- 1.18 fmol mg-1 w.wt. and still amounted to 7.24 +/- 0.

Topics: Animals; Arginine; Calcium Channel Blockers; Cell Membrane; Dihydropyridines; Electrophysiology; In Vitro Techniques; Isradipine; Kinetics; Male; Microsomes; Mitochondria, Muscle; Muscle, Smooth; Nitroarginine; Potassium Chloride; Radioligand Assay; Rats; Rats, Wistar