Antihypertensive effects of nisoldipine and reference drugs in certain types of hypertensive rats

The hypotensive effects of nisoldipine (Bay K 5552), compared with those of nifedipine, nicardipine and hydralazine, in normotensive rats (NR), spontaneously hypertensive rats (SHR), DOCA-NaCl hypertensive rats (DNR) and renal hypertensive rats (RHR), were studied. The changes in plasma renin activity (PRA) after treatment with nisoldipine and these reference drugs were also studied. The results of this study revealed that: 1. Nisoldipine caused more potent antihypertensive effects in SHR, DNR and RHR than in NR. 2. The antihypertensive effects of nisoldipine in SHR were almost equipotent to those of nifedipine, nicardipine and hydralazine. However, in NR, DNR and RHR, the effects of nisoldipine were weaker than those of the reference drugs. 3. The positive chronotropic effects of nisoldipine were less remarkable than those elicited by nifedipine, nicardipine and hydralazine in all types of hypertensive rats, except SHR. 4. As did nifedipine and nicardipine, nisoldipine caused an increase of the plasma renin activity in NR and SHR, though its potency was weaker than those of nifedipine and nicardipine.     

Chronic antihypertensive effects of captopril (SQ 14,225), an orally active angiotensin I-converting enzyme inhibitor, in conscious 2-kidney renal hypertensive rats.

Indirect systolic blood pressure (SBP) was monitored in 9 groups of 15 male conscious 2-kidney renal hypertensive rats (RHR) for over 6 months. Daily oral dosing with captopril (SQ 14,225, D-3-mercapto-2-methylpropanoyl-L-proline, 30 mg/kg), an orally active angiotensin I-converting enzyme inhibitor, lowered SBP 30--50 MM Hg during this period. Withdrawal of captopril for 5 days at 1, 3 and 6 months resulted in gradual return of SBP to control levels without overshoot. Resumption of dosage with captopril again decreased SBP. Daily oral dosing with hydrochlorothiazide (HCTZ, 6 mg/kg/day) alone for 6 months had little or no effect on SBP, but increased the antihypertensive effect of captopril. Daily oral dosing with hydralazine (6 mg/kg) caused an initial marked antihypertensive effect greater than that of captopril but almost complete tolerance developed within 4 weeks of dosing. Highest survival rates occurred in RHR treated with captopril plus HCTZ. In four other similarly treated groups of RHR and normotensive rats (NR), least cardiac hypertrophy and highest plasma renin activity occurred in captopril-treated animals compared with vehicle-treated controls. Plasma renin activity was about 2 to 4 fold higher in the rats dosed with captopril compared with vehicle-treated rats. Heart weight/body weight ratios, initially higher in the two RHR groups compared to NR, decreased only in the captopril treated group to or near those of the NR groups. These results indicate that chronic treatment with captopril decreased SBP and cardiac weights of RHR, and that HCTZ, or possibly other diuretics, can augment the antihypertensive effect of captopril while having little or no effect by themselves.     

Effects of YM358, an angiotensin II type 1 (AT1) receptor antagonist, and enalapril on blood pressure and vasoconstriction in two renal hypertension models

The effects of the angiotensin II type 1 receptor antagonist YM358 on blood pressure were compared to those of the angiotensin converting enzyme inhibitor enalapril in one-kidney, one-clip renal hypertensive rats (1K1C RHR), two-kidney, one-clip renal hypertensive rats (2K1C RHR) and normotensive rats (NTR). Additionally, the local drug actions in peripheral tissues were investigated using isolated mesenteric arteries from these rats. In 2K1C RHR, YM358 and enalapril produced a long-lasting hypotensive effect in a dose-dependent manner. In 1K1C RHR, YM358 (30 mg/kg) also produced an antihypertensive effect, whereas enalapril (30 mg/kg) had no effect. Administration of YM358, but not enalapril, to 1K1C RHR, 2K1C RHR and NTR did not affect heart rate. In isolated mesenteric arteries from 1K1C RHR and 2K1C RHR, angiotensin II (Ang II), angiotensin I (Ang I) and tetradecapeptide (TDP), a physiologically active renin substrate, produced concentration-dependent vasoconstriction. YM358 (10(-7) M) inhibited the vasoconstricting responses to Ang II, Ang I and TDP in isolated mesenteric arteries. In contrast, enalaprilat (10(-7) M), an active metabolite of enalapril, did not completely inhibit the response to Ang I and TDP. These results indicate that YM358 has higher efficacy than enalapril for the treatment of hypertension.     

Differences in the response to periarterial nerve stimulation or exogenous noradrenaline infusion in the mesenteric vascular bed with the intestinal tract harvested from commonly used rat models of hypertension

Many hypertensive animal models have been developed and used to elucidate the pathophysiology of hypertension and to develop antihypertensive drugs. Among them, the spontaneous hypertensive rat (SHR), deoxycorticosterone acetate (DOCA)‐treated and high salt intake rat (DOCA‐salt), and high sodium‐fed Dahl salt‐sensitive rat (HS) models are commonly used. Multiple studies have been conducted, however, elevation in blood pressure in these models due to the reactivity of adrenergic vasoconstriction has not been well characterized in a centralized experiment. In this study, the pressor responses to periarterial nerve stimulation (PNS) or exogenous noradrenaline (NA) infusion were measured in the isolated mesenteric vascular bed with the intestinal tract to investigate the reactivity of mesenteric adrenergic vasoconstriction. The systemic arterial blood pressure of the hypertensive rat models was uniformly elevated compared with their respective controls. However, the changes in perfusion pressure in the mesenteric vascular bed in response to PNS and exogenous NA infusion were quite different depending on the model. The pressor responses to PNS in SHRs and Dahl S HS rats were significantly higher, and those in DOCA‐salt rats were significantly lower than those in the controls. The pressor responses to exogenous NA infusion in SHRs were significantly higher, and those in Dahl S HS rats were significantly lower than those in their respective controls. No difference was observed in the pressor responses to the exogenous NA between the DOCA‐salt and sham groups. These results demonstrate that the reactivity of adrenergic vasoconstriction is different for each type of experimental hypertensive model rat.     

Comparison of the effects of hydralazine and nifedipine on contractions and 45Ca influx of rat aorta.

The effect of the vasodilator hydralazine has been compared with nifedipine on KCl-(K+) (60 mM) and noradrenaline-(NA) (10 microM) induced 45Ca uptake and contractile responses in rat aorta arterial strips without endothelium. Hydralazine (0.5-10 mM) was equally effective in relaxing K(+)- (IC50 = 2.2 +/- 0.17 mM) and NA- (IC50 = 3.06 +/- 0.25 mM) induced tension, the degree of relaxation depending on the dose. Nifedipine totally inhibited K(+)- (IC50 = 3.16 +/- 0.28 nM) induced contractions with lower doses than were necessary to relax (up to 54.0 +/- 4.1% with supramaximal concentrations) NA-induced contractions (IC50 = 1.48 +/- 0.12 microM). In the experiments in a calcium-free medium, nifedipine (1 microM) had no effect on the NA- (10 microM) induced contractions whereas hydralazine (1 mM) strongly inhibited them. Nifedipine did not affect the basal uptake of 45Ca but the induced uptakes were reduced to 66.3 +/- 3.2% (K+) and 65.5 +/- 4.1% (NA) of their basal values. Hydralazine did not affect the basal uptake of 45Ca nor that induced by the two vascoconstrictor agents. These results suggest that nifedipine acts on the cell membrane by blocking the movements of calcium through the voltage-dependent and receptor-operated calcium channels, whilst hydralazine has an intracellular effect.     

Pinacidil monohydrate—a novel vasodilator: Review of preclinical pharmacology and mechanism of action

Pinacidil dose-dependently decreased blood pressure in normotensive, renal hypertensive, spontaneously hypertensive, and neurogenic hypertensive rats; in anesthetized cats; and in normotensive and renal hypertensive dogs by a mechanism associated with decreased total peripheral resistance and reflex tachycardia. Pinacidil was 2–4-fold more potent as an antihypertensive than hydralazine. Antihypertensive activity of pinacidil was not modified by α, β, cholinergic, or histaminergic receptor antagonists and was not associated with calcium channel blockade, activation of cyclooxygenase, or formation of the vasodilator, adenosine. In vitro, pinacidil relaxed blood vessels contracted with several agonists by a mechanism that was independent of an intact endothelium, did not increase either CAMP or cGMP, and was approximately 700 times more potent than minoxidil or hydralazine. Thus, pinacidil is a direct-acting vasodilator, whereas minoxidil and hydralazine must be metabolized to a vasoactive species or exert some indirect effect for anithypertensive activity in vivo. Pinacidil lowered blood pressure in decerebrated cats, and only trace amounts of radiolabeled pinacidil distributed to the brain, suggesting minimal central effects. Pinacidil was rapidly absorbed after oral administration in rats, dogs, and man, showing a plasma half-life of approximately 30 min in all three species with peak plasma levels within 30 to 60 min. The major biotransformation product of pinacidil in rats, dogs, and man was pinacidil-N-oxide; and renal excretion was the main elimination pathway for both agents. Pinacidil-N-oxide was less potent than pinacidil as an antihypertensive agent and showed approximately 10–100-fold less vasodilation than pinacidil in vitro. In summary, pinacidil is a clinically effective novel antihypertensive agent that nonselectively inhibits contractile responses of arterial and other smooth muscle.     

Synthesis and antihypertensive activity of novel 3-hydrazino-5-phenyl-1,2,4-triazines.

In an effort to develop antihypertensive agents with peripheral vasodilator activity, a series of 40 novel 3-hydrazino-5-phenyl-1,2,4-triazines (II) were synthesized and evaluated in the spontaneously hypertensive rat assay (SHR assay). Based on the performance of the structurally related standard, hydralazine (I), 15 triazines were active. Thirteen of these hypotensive triazines possessed LD50 values in the mouse greater than I (LD50 = 100 mg/kg); only one active triazine had an LD50 value greater than 300 mg/kg (11d). Four asymmetric triazines had moderate antihypertensive activity and LD50 values greater than 300 mg/kg (6b, 7c, 8f, and 9g). Based on the relationship between toxicity and antihypertensive activity, three triazines (8f, 9g, and 11d) were chosen for dose-responses studies in the SHR assay. None were as efficacious as I, but all three were less toxic, resulting in similar therapeutic indices relative to I.     

Characterization of receptors for endothelins in the perfused arterial and venous mesenteric vasculatures of the rat.

1. Endothelin-1 and -3 induced marked arterial and venous constrictions in the perfused mesenteric vasculature of the rat with endothelin-3 being at least 20 times less active than endothelin-1, on both arterial and venous sides of the vasculature. 2. Two ETB selective agonists, BQ-3020 and IRL 1620 (500 pmol), induced weak constrictions of the venous mesenteric vasculature and were inactive in the arterial side at doses up to 1000 pmol. 3. In mesenteric vasculatures precontracted with either methoxamine (arterial side) or the thromboxane A2-mimetic, U46619 (venous side), acetylcholine or bradykinin produced vasodilations of both arterial and venous vessels, whereas endothelin-3 induced vasodilations only on the arterial side. 4. A selective ETA receptor antagonist, BQ-123, blocked, in a concentration-dependent and reversible fashion, the vasoconstrictions induced by endothelin-1 on both sides of the mesenteric circulation (IC50; arterial side: 0.013 microM; venous side: 0.032 microM). 5. In contrast, the vasodilator responses induced by endothelin-3 on the arterial side of the precontracted mesenteric vasculature were not affected by BQ-123. 6. The present study illustrates the presence of ETA receptors which are responsible for vasoconstriction by endothelins in the arterial and venous mesenteric vasculatures. Furthermore, we suggest that the vasodilations induced by endothelin-3 in the arterial vasculature uniquely, are ETB receptor-mediated.     

Effects of pinacidil on serotonin-induced contractions and cyclic nucleotide levels in isolated rat aortae: comparison with nitroglycerin, minoxidil, and hydralazine

Pinacidil is a novel, clinically effective vasodilator used for the treatment of hypertension whose mechanism of action has not been precisely defined. In vitro, pinacidil (ED50 = 0.3 microM) was approximately 30-fold less potent than nitroglycerin and 700-fold more potent than minoxidil or hydralazine in relaxing rat aortic strip preparations. Aortic relaxations produced by nitroglycerin and acetylcholine were dramatically antagonized by methylene blue (10(-5) M), an inhibitor of soluble guanylate cyclase. In contrast, relaxation to hydralazine or minoxidil was unaffected and relaxation to pinacidil was only modestly inhibited (approximately threefold) by methylene blue (10(-5) M). Furthermore, aortic relaxation to pinacidil was similar in preparations with and without an intact endothelium. Relaxation induced by pinacidil (10(-7)-10(-4) M) was not associated with any elevation in either cyclic AMP (cAMP) or cyclic GMP (cGMP) levels in vitro, although nitroglycerin (10(-6) M) but not minoxidil (10(-3) M) or hydralazine (10(-3) M) significantly elevated cGMP levels. Thus, pinacidil was a potent relaxant agonist in vitro, in contrast to minoxidil and hydralazine, which were considerably weaker in this regard. Vascular relaxation produced by pinacidil was independent of an intact endothelium and was not associated with elevations in either cAMP or cGMP. These data are consistent with the proposal that the antihypertensive activity of pinacidil is due to nonspecific arterial vasodilation.     

Comparative evaluation of the in vitro effects of hydralazine and hydralazine acetonide on arterial smooth muscle.

1. Dose-response relationships to K+ were determined in isolated strips of rabbit aorta. 2. K+ contractures were induced by 30 mM K+ in paired strips from individual animals. The effects of hydralazine and hydralazine acetone hydrazone (hydralazine acetonide) on these contractures were studied. 3. Hydralazine and hydralazine acetonide both produced dose-dependent decreases of K+-induced tone. Threshold concentrations for hydralazine were 11.89 +/- 4.5 X 10(-5) M (mean +/- s.d.) and for hydralazine actonide 9.7 +/- 4.6 X 10(-5) M (0.5 less than P less than 0.4). 4. The magnitude of the effect of hydralazine acetonide was greater than that of hydralazine at all concentrations above threshold, as reflected in a significant difference (P less than 0.05) in the slopes of dose-response curves to the two treatments. The vasodilator effects of hydralazine and the acetonide were terminated by washout of the bath. 5. The differences in effect were not due to instability of hydralazine under in vitro conditions. 6. It is concluded that hydralazine acetonide has intrinsic activity on vascular smooth muscle which differs significantly from that of the parent compound and that this may contribute to the hypotensive effects which follow administration of the parent compound.     

Failure of acetaldehyde or acetate to mimic the splanchnic arteriolar or venular dilator actions of ethanol: direct in situ studies on the microcirculation.

The effects of acetaldehyde and sodium acetate on rat mesenteric arterioles (17-25 micron i.d.) and muscular venules (35-50 micron i.d.) were examined in situ, at the microcirculatory level, by use of a high-resolution closed circuit television microscope recording system. Local, intravenous or intra-arterial administration of acetaldehyde (1.8-3600 mumol) and sodium acetate (0.25-250 mumol) to mesenteric arterioles and muscular venules of the anaesthetized rat induced dose-dependent vasoconstriction. Systemic administration of a variety of pharmacological antagonists (i.e., phentolamine, diphenhydramine, methysergide, atropine and indomethacin) did not attenuate or prevent the dose-dependent vasoconstrictor actions of these metabolites of ethanol. Our findings do not support the concept that some or all of the peripheral vasodilator actions of ethanol can be attributed to its metabolites, acetaldehyde and acetate.     

Effects of hydrazine derivatives on vascular smooth muscle contractility,blood pressure and cGMP production in rats: comparison with hydralazine

Hydralazine is a hydrazine derivative used clinically as a vasodilator and antihypertensive agent. Despite numerous studies with the drug, its mechanism of action has remained unknown; guanylate cyclase activation and release of endothelial relaxing factors are thought to be involved in its vasodilator effect. Other hydrazine derivatives are known to stimulate guanylate cyclase and could therefore share the vasodilator activity of hydralazine, although such possibility has not been assessed systematically. In the present study, hydralazine, hydrazine, phenylhydrazine, and isoniazid were evaluated for vascular smooth muscle relaxation in rat aortic rings with and without endothelium, as well as after incubation with the guanylate cyclase inhibitor methylene blue. They were also tested for enhancement of cyclic guanosine monophosphate (cGMP) production by cultured rat aortic smooth muscle cells and for hypotension in the anesthetized rat. All hydrazines relaxed aortic rings, an action unaffected by endothelium removal and, in all cases except hydralazine, antagonized by methylene blue. Only phenylhydrazine increased cGMP production and only hydralazine markedly lowered blood pressure. It was concluded that hydralazine vascular relaxation is independent of endothelium and is not related to guanylate cyclase activation. The other hydrazines studied also elicit endothelium-independent relaxation, but the effect is related to guanylate cyclase. The marked hypotensive effect of hydralazine contrasts with its modest relaxant activity and is not shared by the other hydrazines. The fact that hydrazine and isoniazid produce methylene blue-sensitive relaxation, yet do not enhance cGMP production suggests the need for activating factors present in aortic rings but not in isolated cells.     

Mechanisms contributing to the differential haemodynamic effects of bombesin and cholecystokinin in conscious, Long Evans rats.

1. Long Evans rats were chronically instrumented with intravascular catheters and pulsed Doppler probes to assess changes in renal, mesenteric and hindquarters blood flows and vascular conductances in response to bombesin (2.5 micrograms kg-1, i.v.) and cholecystokinin (CCK) (0.5 and 5.0 micrograms kg-1, i.v.). 2. Bombesin caused an increase in heart rate and blood pressure, together with a transient renal vasoconstriction and prolonged mesenteric vasodilatation; there was an early hindquarters vasodilatation followed by vasoconstriction. 3. In the presence of phentolamine, bombesin caused a fall in blood pressure due to enhanced hindquarters vasodilatation; these effects were reversed by propranolol and hence were possibly due to circulating adrenaline acting on vasodilator beta 2-adrenoceptors. 4. During concurrent administration of phentolamine, propranolol and atropine, bombesin caused prolonged tachycardia and a rise in blood pressure. The renal vasoconstrictor and mesenteric vasodilator effects of bombesin were not reduced under these conditions and thus probably were direct and/or indirect non-adrenergic, non-cholinergic (NANC) effects. 5. CCK caused dose-dependent increases in blood pressure accompanied by renal, mesenteric and hindquarters vasoconstriction followed, after the higher dose, by vasodilatations. The lower dose of CCK increased heart rate but there was a bradycardia followed by a tachycardia after the higher dose. 6. Experiments with antagonists as described above indicated the pressor effect of CCK was mediated largely through alpha-adrenoceptors, as were the mesenteric and hindquarters vasoconstrictor effects; CCK exerted NANC negative chronotropic effects. 7. All the effects of CCK were markedly inhibited by L364,718. This observation, and the finding that L364,718 had no effect on the responses to bombesin, together with the dissimilarities in the regional haemodynamic effects of exogenous CCK and bombesin, indicate that the cardiovascular actions of the latter were not dependent on the release of endogenous CCK.     

Reduction of endothelial function with age in the mesenteric arterial bed of the normotensive rat.

1. Age-related changes in endothelial vasodilator function were studied in an in vitro preparation of the mesenteric arterial bed removed from male, normotensive, Wistar rats. 2. Animals were killed at 2, 12 or 22 months of age, the superior mesenteric artery was cannulated and the gut removed. The mesenteric arterial bed was perfused at a constant flow rate of 4 ml min-1 and perfusion pressure was taken as an index of arteriolar tone. 3. The muscarinic agonist, carbachol, antagonized noradrenaline-induced vasoconstriction in the presence, but not in the absence, of endothelium. This cholinoceptor agonist-induced release of endothelial-derived relaxing factor (EDRF) was impaired in 22 month old rats. 4. Noradrenaline-induced vasoconstriction increased following removal of endothelium suggesting that agonist-induced release of EDRF attenuates vasoconstrictor responses to noradrenaline measured in the presence of endothelium. 5. Removal of endothelium had less effect on noradrenaline-induced vasoconstriction in old rats suggesting once again that agonist-induced release of EDRF is impaired in old rats. 6. The noradrenaline dose-response curve established in the presence of endothelium was shifted to the left in 22 month old rats. 7. In conclusion, aging in the rat appears to lead to a reduction in endothelial vasodilator function in a resistance vessel.     

Antihypertensive and cardiovascular effects of the new calcium antagonist YH334

Antihypertensive effect of YH334 was examined in various experimental hypertension rat models and the systemic and regional hemodynamic profiles of the compound were investigated in conscious spontaneously hypertensive rats (SHR). The antihypertensive potency of YH334 is found to be more than 10 times stronger than that of nitrendipine in the all hypertensive models. The effective doses to lower the initial blood pressure by 20% (ED₂₀) of YH334 were 1.4 mg/kg in normotensive rats (NR), 0.7 mg/kg in SHR, 0.1 mg/kg in DOCA salt hypertensive rats (DHR) and 0.4 mg/kg in renal hypertensive rats (RHR), and the ED₂₀ values of nitrendipine were 15.8 mg/kg in NR, 7.1 mg/kg in SHR, 1.7 mg/kg in DHR and 4.8 mg/kg in RHR. The primary hemodynamic effect of YH334 was characterized by increasing CI and SVI and reducing TPRI of which hemodynamic profile is similar to that of nitrendipine. Both compounds seem to produce potent antihypertensive effects by lowering peripheral resistance in the skeletal muscles. In the organ bath study using isolated rabbit aorta, YH334 was found to be a potent voltage dependent calcium channels blocker without significant inhibitory effect on the receptor operated calcium channels like the most of other dihydropyridine type calcium antagonists. Furthermore, YH334 showed acute diuretic and natriuretic effects in conscious SHR, which may render the unnecessary restriction of sodium in the diet of those patients on long term hypertension therapy. This effect would provide an additional benefit to its potent antihypertensive activity.     

Comparative regional haemodynamic effects of the nitric oxide synthase inhibitors,S-methyl-L-thiocitrulline and L-NAME,in conscious rats

1. The regional haemodynamic effects of the putative nNOS inhibitor, S-methyl-L-thiocitrulline (SMTC), were compared with those of the nonselective NOS inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), in conscious, male Sprague-Dawley rats. 2. SMTC (0.3 mg kg(-1) bolus) produced a significant, short-lived, pressor effect associated with renal, mesenteric and hindquarters vasoconstriction; the same dose of L-NAME did not affect mean blood pressure (BP), although it caused bradycardia and mesenteric vasoconstriction. 3. At the highest dose tested (10 mg kg(-1)), L-NAME produced a significantly greater bradycardia and fall in mesenteric vascular conductance than SMTC, although the initial pressor response to SMTC was greater, but less sustained, than that to L-NAME. 4. Infusion of SMTC or L-NAME (3 mg kg(-1) h(-1)) induced rises in BP and falls in renal, mesenteric and hindquarters vascular conductances, but the effects of L-NAME were greater than those of SMTC, and L-NAME also caused bradycardia. 5. The renal vasodilator response to acetylcholine was markedly attenuated by infusion of L-NAME, but unaffected by SMTC. The hindquarters vasodilatation induced by salbutamol was attenuated by L-NAME, but not by SMTC. The mesenteric vasodilator response to bradykinin was modestly enhanced by SMTC, but not by L-NAME. The depressor and renal, mesenteric and hindquarters vasodilator responses to sodium nitroprusside were enhanced by L-NAME, whereas SMTC modestly enhanced the hypotensive and renal vasodilator effects of sodium nitroprusside, but attenuated the accompanying tachycardia. 6. The results are consistent with the cardiovascular effects of low doses of SMTC being attributable to nNOS inhibition.     

Augmented sensory-motor vasodilatation of the rat mesenteric arterial bed after chronic infusion of the P1-purinoceptor antagonist, DPSPX.

1. The effect of long-term antagonism of P1-purinoceptors on vascular function was examined in the perfused mesenteric arterial bed isolated from rats which had received constant infusion of either the non-selective P1-purinoceptor antagonist, 1-3-dipropyl-8-sulphophenylxanthine (DPSPX, 30 micrograms kg-1 h-1, i.p.) or saline for seven days. Sympathetic and sensory-motor neurotransmission, smooth muscle and endothelial function were assessed. 2. Basal tone was similar in mesenteric arterial preparations from control and DPSPX-treated rats. Continuous perfusion with methoxamine (7-70 microM) induced similar increases in tone in control and DPSPX-treated preparations. In the presence of guanethidine (5 microM), electrical field stimulation (EFS; 1-12 Hz, 60V, 0.1 ms, 30 s) elicited frequency-dependent vasodilatation due to activation of sensory-motor nerves. In tissues from DPSPX-treated rats the nerve-mediated vasodilator responses were markedly augmented at all frequencies. Maximal relaxation at 8 Hz was 38.34 +/- 4.76% (n = 5) in controls and 65.92 +/- 3.68% (n = 5) after DPSPX-treatment (P < 0.01). Adenosine (3 microM) inhibited the frequency-dependent sensory-motor neurotransmission similar in preparations from controls and DPSPX-treated rats. 3. In raised-tone preparations calcitonin gene-related peptide (CGRP; 5,15 and 50 pmol), the principal vasodilator transmitter of sensory-motor nerves in rat mesenteric arteries, produced similar relaxations in control and DPSPX-treated preparations. Vasodilator responses to the sensory neurotoxin capsaicin (50 and 500 pmol) were also similar between the groups. 4. Assay of tissue CGRP levels of the superior mesenteric artery by enzyme-linked immunosorbent assay showed no significant difference in tissue levels of CGRP in controls, 120.25 +/- 26.34 pmol g-1 tissue (n = 6) and with DPSPX-treatment, 82.12 +/- 24.42 pmol g-1 tissue (n = 6). 5. In raised-tone preparations dose-dependent endothelium-dependent vasodilatation to acetylcholine and ATP, and endothelium-independent vasodilatation to sodium nitroprusside were similar in control and DPSPX-treated preparations. 6. EFS (4-32 Hz, 90V, 1 ms, 30 s) elicited frequency-dependent vasoconstriction due to activation of sympathetic nerves which was similar in controls and in DPSPX-treated preparations. Adenosine (10 and 30 microM) inhibited sympathetic neurotransmission similarly in control and DPSPX-treated preparations. Dose-dependent vasoconstriction to noradrenaline (NA) and ATP, and to KCI (0.15 mmol) was similar between the groups. 7. High performance liquid chromatographic analysis of tissue NA showed no significant difference in NA content of the superior mesenteric artery from DPSPX-treated (1.38 +/- 0.09 ng mg-1, n = 6) and control rats (1.46 +/- 0.17 ng mg-1, n = 6). 8. In conclusion, in rats with hypertension due to 7 days treatment with the P1-purinoceptor antagonist, DPSPX, there is an increase in sensory-motor vasodilatation of the mesenteric arterial bed. There is no change in sympathetic nerve, endothelial or smooth muscle function. Augmented sensory-motor neurotransmission, which does not involve a change in postjunctional responsiveness to CGRP or in the CGRP content of sensory-motor nerves, could be a compensatory change in response to the DPSPX- induced hypertension.     

Histamine H(3) receptor-mediated modulation of perivascular nerve transmission in rat mesenteric arteries

The rat mesenteric artery has been shown to be innervated by adrenergic vasoconstrictor nerves and calcitonin gene-related peptide (CGRP)-containing (CGRPergic) vasodilator nerves. The present study was designed to investigate the involvement of histamine H(3) receptors in the neurotransmission of perivascular adrenergic and CGRPergic nerves. The mesenteric vascular beds without an endothelium isolated from male Wistar rats were perfused with Krebs solution and perfusion pressure was measured. In preparations with resting tension, the selective H(3) receptor agonist (R)-α-methylhistamine (α-methylhistamine; 10-100nM) significantly reduced periarterial nerve stimulation (2-8Hz)-induced vasoconstriction and noradrenaline release in the perfusate without an effect on the vasoconstriction induced by exogenously injected noradrenaline (0.5, 1.0nmol). In preparations with active tone produced by methoxamine (2μM) and in the presence of guanethidine (5μM), the periarterial nerve stimulation (1, 2Hz)-induced vasodilator response was inhibited by α-methylhistamine (0.1-1μM) perfusion without affecting vasodilation induced by exogenously injected CGRP (5pmol). Clobenpropit (histamine H(3) receptor antagonist, 1μM) canceled the α-methylhistamine-induced decrease in the periarterial nerve stimulation-induced vasoconstriction and noradrenaline release and periarterial nerve stimulation-induced vasodilation. These results suggest that the stimulation of H(3) receptors located in rat perivascular nerves inhibits presynaptically the neurotransmission of not only adrenergic nerves, but also CGRP nerves, by decreasing neurotransmitters.     

Vasodilator profile of synthetic atrial natriuretic factor

The vasodilator profile of synthetic atrial natriuretic factor (ANF) was characterized using isolated vascular preparations. Nanomolar concentrations of ANF relaxed rabbit aortic rings contracted by serotonin, histamine, methoxamine or angiotensin II. The synthetic peptide was most effective (IC50 = 1.3 X 10(-10) M) in relaxing the tonic, intrinsic contractions of the rabbit facial vein. ANF was poorly active against K+-contracted aortic rings or the phasic contractions of the rat portal vein. A similar vasodilator profile was obtained for sodium nitroprusside but not papaverine, hydralazine, adenosine or nifedipine. This first demonstration of the vascular activity of synthetic ANF depicts this substance as a nonselective vasodilator of agonist-induced contractions. The observed similarities in the vasodilator activity of ANF and sodium nitroprusside suggest a common mechanism of action.     

Anandamide induces endothelium-dependent vasoconstriction and CGRPergic nerve-mediated vasodilatation in the rat mesenteric vascular bed

An endogenous cannabinoid anandamide (N-arachidonoylethanolamide) has been shown to cause vasodilatation in vitro and a brief vasoconstriction followed by prolonged depressor response in vivo. This study investigated the vascular effects of anandamide and underlying mechanisms in rat mesenteric vascular beds. In preparations with an intact endothelium and active tone, anandamide at low concentrations (0.1 - 1 nM) caused a concentration-dependent decrease in perfusion pressure due to vasodilatation, but at high concentrations (10 nM - 1 μM) elicited an initial and sharp increase in perfusion pressure due to vasoconstriction followed by long-lasting vasodilatation in a concentration-dependent manner. Treatment with SR141716A [cannabinoid-1 (CB(1))-receptor antagonist] blunted both the vasoconstrictor and vasodilator responses. Also, removal of the endothelium and indomethacin (cyclooxygenase inhibitor), but not adrenergic denervation with 6-hydoxydopamine (adrenergic neurotoxin), markedly inhibited the vasoconstrictor response to anandamide, while these treatments did not affect vasodilatation. The vasodilatation, but not vasoconstriction, in response to anandamide was markedly attenuated by capsazepine [selective antagonist for transient receptor potential vanilloid-1 (TRPV1)], pretreatment with capsaicin [calcitonin gene-related peptide (CGRP)ergic-nerve depletor], or cold-storage denervation. These results suggest that in rat mesenteric vascular beds, anandamide causes CB(1)-receptor- and prostanoid-mediated endothelium-dependent vasoconstriction and perivascular capsaicin-sensitive CGRPergic nerve-mediated vasodilatation.