Ganglion-blocked Rats Research Articles

Effect of intermedin/adrenomedullin-2 on venous tone in conscious rats

We investigated the effect of intermedin/adrenomedullin-2 (3 and 10 nmol/kg, i.v.), a member of the calcitonin gene-related peptide family, relative to the vehicle (0.9% NaCl) on mean circulatory filling pressure (index of venous tone) in conscious rats: intact (unblocked) or ganglionic blocked through treatment with mecamylamine (10 mg/kg, i.v.) and noradrenaline (4 microg/kg/min, i.v.). In intact rats, both doses of intermedin/adrenomedullin-2 reduced mean arterial pressure (-14+/-3, -30+/-3 mmHg), but did not alter mean circulatory filling pressure; the high dose also increased heart rate. In ganglionic-blocked rats, both doses decreased mean arterial pressure (-22+/-3, -46+/-5 mmHg) and the high dose also decreased mean circulatory filling pressure (-2.81+/-0.82 mmHg), but neither dose affected heart rate. The vehicle did not have any effects in any of the groups. In addition, intermedin/adrenomedullin-2 did not have any effect on blood volume in both intact and ganglion-blocked rats. The results show that intermedin/adrenomedullin-2 is a dilator of arterial resistance and capacitance vessels.

Influence of urocortin and corticotropin releasing factor on venous tone in conscious rats

The corticotropin releasing factor (CRF) family includes CRF and urocortin. The effects of urocortin and CRF (0.3, 1, 3 and 10 nmol/kg, i.v.) relative to those of vehicle (0.9% NaCl) on mean arterial pressure and mean circulatory filling pressure (index of venous tone) were examined in conscious, unrestrained rats that were either intact (unblocked) or ganglion-blocked by treatment with mecamylamine (10 mg/kg, i.v.) followed by noradrenaline (4 μg/kg/min, i.v.) to increase vasomotor tone. Both urocortin and CRF dose-dependently decreased mean arterial pressure in intact rats and ganglion-blocked rats. The depressor action of urocortin was greater than that of CRF at all doses. In intact rats, neither compound reduced mean circulatory filling pressure. In ganglion-blocked rats, urocortin and the highest dose of CRF decreased mean circulatory filling pressure. In conclusion, both urocortin and CRF are vasodepressor agents with venodilator action.

Pharmacological dissection of vascular effects caused by activation of protease-activated receptors 1 and 2 in anesthetized rats.

SPECIFIC AIMSTo clarify the role played by protease activated receptors 1 and 2 (PAR-1 and PAR-2) in the control of blood pressure, we have evaluated changes in blood pressure induced by a peptide activating the receptor PAR-1 (PAR-1AP, SFLLRNPND) and a peptide activating the receptor PAR-2 (PAR-2AP, SLIGRL) in naive and ganglion-blocked anesthetized rats. To investigate the role played by nitric oxide (NO), the effect of PAR-1AP and PAR-2AP in ganglion-blocked rats was evaluated before and after L-NAME administration.PRINCIPAL FINDINGS1. Both PAR-1AP and PAR-2AP cause a biphasic change in MABP in ratsIntravenous (i.v.) injection of PAR-1AP (1 mg/kg) into naive anesthetized rats caused a biphasic response characterized by hypotension (13±2 mmHg; n=6), immediately followed by hypertension (21±3 mmHg; n=6), returning to baseline value in 3 min; i.v. injection of PAR-2AP (1 mg/kg) caused a hypotension (28±5 mmHg; n=5) lasting about 2 min. Hypotension was followed by a slowly developing hypertension, reaching...

Effects of calcitonin gene-related peptide on vascular resistance in rats: role of sex steroids.

It has been demonstrated in reflex-intact animals that the sensitivity to calcitonin gene-related peptide (CGRP) is increased during pregnancy and that this action is mediated by sex steroids but not by nitric oxide (NO). We assessed the effects of CGRP in the following groups of anesthetized ganglion-blocked rats: 1) pregnant, 2) ovariectomized, and 3) ovariectomized and treated with estradiol and progesterone. Changes in mean arterial pressure (MAP) were assessed after the administration of varying doses of CGRP. Decreases in MAP after CGRP administration were significantly greater in pregnant rats and ovariectomized rats administered sex steroids than in ovariectomized controls. The CGRP antagonist CGRP8-37 produced a pressor response of similar magnitude in both pregnant and ovariectomized rats. We also assessed the effects of CGRP and the modulating role of NO in the isolated uterine vascular bed preparation. CGRP reduced perfusion pressure to a greater degree in ovariectomized animals treated with sex steroids than in ovariectomized animals. This response was attenuated by pretreatment with an NO synthesis inhibitor. CGRP8-37 produced a similar increase in perfusion pressure in both groups. We conclude that 1) the increased vascular sensitivity observed during pregnancy or after treatment with sex steroids is in part mediated by NO, and 2) CGRP8-37 has a vasoconstrictor action of its own.

AV3V lesions attenuate the cardiovascular responses produced by blood-borne excitatory amino acid analogs.

Systemic injections of the excitatory amino acid (EAA) analogs, kainic acid (KA) and N-methyl-D-aspartate (NMDA), produce a pressor response in conscious rats that is caused by a centrally mediated activation of sympathetic drive and the release of arginine vasopressin (AVP). This study tested the hypothesis that the tissue surrounding the anteroventral part of the third ventricle (AV3V) plays a role in the expression of the pressor responses produced by systemically injected EAA analogs. Specifically, we examined whether prior electrolytic ablation of the AV3V region would affect the pressor responses to KA and NMDA (1 mg/kg iv) in conscious rats. The KA-induced pressor response was smaller in AV3V-lesioned than in sham-lesioned rats (11 +/- 2 vs. 29 +/- 2 mmHg; P < 0.05). After ganglion blockade, KA produced a pressor response in sham-lesioned but not AV3V-lesioned rats (+27 +/- 3 vs. +1 +/- 2 mmHg; P < 0.05). The KA-induced pressor response in ganglion-blocked sham-lesioned rats was abolished by a vasopressin V1-receptor antagonist. Similar results were obtained with NMDA. The pressor response to AVP (10 ng/kg iv) was slightly smaller in AV3V-lesioned than in sham-lesioned ganglion-blocked rats (45 +/- 3 vs. 57 +/- 4 mmHg; P < 0.05). This study demonstrates that the pressor responses to systemically injected EAA analogs are smaller in AV3V-lesioned rats. The EAA analogs may produce pressor responses by stimulation of EAA receptors in the AV3V region, or the AV3V region may play an important role in the expression of these responses.

Zaprinast, a type V phosphodiesterase inhibitor, dilates capacitance vessels in anaesthetised rats

The effects of zaprinast (a type V phosphodiesterase inhibitor) on mean arterial pressure, heart rate, cardiac output, mean circulatory filling pressure, arterial and venous resistances were compared to those of sodium nitroprusside in three groups, each of intact or ganglion-blocked, Inactin-anaesthetised rats. In intact rats, zaprinast (1.5, 3.0 mg kg −1 min −1) and sodium nitroprusside (8.0, 64.0 μg kg −1 min −1) dose-dependently reduced mean arterial pressure and arterial resistance, but did not alter cardiac output and venous resistance. Both increased heart rate, with the effect of zaprinast less than that of sodium nitroprusside. Mean circulatory filling pressure was elevated by both doses of zaprinast but only the high dose of sodium nitroprusside. In rats given mecamylamine (3.7 μmol kg −1, i.v. bolus) and noradrenaline (7.3 nmol kg −1 min −1), zaprinast and sodium nitroprusside elicited dose-dependent reductions in mean arterial pressure, arterial and venous resistances, and mean circulatory filling pressure. Both increased cardiac output, with the effect of zaprinast greater than that of sodium nitroprusside at the low dose. Zaprinast but not sodium nitroprusside reduced heart rate. Our results indicate that zaprinast, similar to sodium nitroprusside, dilates both resistance and capacitance vessels in ganglion-blocked rats infused with noradrenaline to restore vasomotor tone. Zaprinast but not sodium nitroprusside has a direct, negative chronotropic effect on the heart.

Lack of autonomic contributions to tonic nitric oxide-mediated vasodilatation in unanesthetized free-moving rats.

To clarify the controversial issue of whether autonomic influences modulate vascular nitric oxide-mediated vasodilatation or even directly contribute to production of nitric oxide (NO) via nitroxidergic fibers. Chronic venous and arterial catheters were implanted in Wistar-Kyoto rats (n = 65) for continuous blood pressure measurement, drug administration and blood sampling. Tonic NO-dependent vasodilatation in the conscious free-moving animal was evaluated as the pressor response to inhibition of NO synthesis by intravenous L-monomethylarginine (a 100 mg/kg intravenous bolus plus 0.5 mg/kg per min infusion for 30 min). Experiments were performed under control conditions, chemical sympathectomy by 6-hydroxy-dopamine, ganglionic blockade by hexamethonium, and surgical denervation of sino-aortic baroreceptors. Baseline mean arterial pressure was 100+/-4 mmHg (mean +/- SEM) in control rats and 73+/-3, 62+/-5, and 105+/-10 mmHg in sympathectomized, ganglion-blocked, and denervated rats, respectively. The peak increase in mean arterial pressure after administration of L-monomethylarginine was 38+/-3 mmHg in control rats and 51+/-3, 50+/-6, and 63+/-10 mmHg in sympathectomized, ganglion-blocked, and denervated rats, respectively. Epinephrine and norepinephrine levels in rats of separate groups of unanesthetized control, sympathectomized and ganglion-blocked animals were measured by high-performance liquid chromatography from an arterial blood sample, the results indicating drastic reductions in levels of both catecholamines in the ganglion-blocked (but not in the sympathectomized) rats compared with those in the control rats. Tonic NO-dependent vasodilatation can normally be maintained in the unanesthetized unrestrained rat irrespective of autonomic or humoral adrenergic influences.

Effects of adenosine and adenosine analogues on mean circulatory filling pressure and cardiac output in anesthetized rats.

The effects of adenosine and adenosine analogues, 2-[p-(2-[carboxyethyl) phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS 21680) and N6-2-(4-aminophenyl)-ethyladenosine (APNEA), on mean arterial pressure, cardiac output, mean circulatory filling pressure, arterial resistance, venous resistance and heart rate in untreated or treated (with ganglion-blockers mecamylamine and atropine) pentobarbital-anesthetized rats were examined. Infusion of adenosine (100, 300 and 900 microg/kg/min), CGS 21680 (0.1, 0.3 and 0.9 microg/kg/min) or APNEA (1.0, 3.0 and 9.0 microg/kg/min) reduced mean arterial pressure and arterial resistance in all groups. Adenosine and APNEA also reduced mean circulatory filling pressure, venous resistance and heart rate in untreated animals. Furthermore, APNEA but not adenosine reduced cardiac output. In contrast, CGS 21680 increased cardiac output and heart rate but did not have any effect on mean circulatory filling pressure or venous resistance. In ganglion-blocked rats, APNEA reduced cardiac output, mean circulatory filling pressure and heart rate, while adenosine did not have any effect on these parameters. In addition, APNEA and adenosine reduced arterial resistance but were unable to alter venous resistance while CGS 21680 reduced mean circulatory filling pressure and arterial resistance but did not further affect cardiac output, heart rate and venous resistance in ganglion-blocked animals. The results of the present study suggest that adenosine and APNEA dilate arterioles and vein, whereas CGS 21680 causes arterial dilatation but not venodilatation in untreated animals due to hypotension-induced sympathetic activation. A possible explanation for the present observations could be differences in the distribution of vascular A2 versus A3 adenosine receptors in the venous circulation.

Cardiorenal consequences of dual angiotensin converting enzyme and neutral endopeptidase 24.11 inhibition in transgenic rats with an extra renin gene.

The cardiovascular consequences of mixed angiotensin converting enzyme and neutral endopeptidase (ACE/NEP) inhibition with alatriopril/alatrioprilat were compared with the consequences of endopeptidase (NEP) inhibition alone with (S)-thiorphan/ecadotril by determining the acute effects of the compounds on hemodynamic, hormonal, and renal parameters in hypertensive transgenic rats harboring an additional mouse renin gene (TGR(mRen2)27). Infusion of alatrioprilat and (S)-thiorphan in anesthetized TGR decreased blood pressure in a dose-dependent manner, but heart rate remained unchanged. The renal excretion of water, sodium, and cGMP also increased dose-dependently, with nearly the same maximal effects after infusion of (S)-thiorphan and alatrioprilat. At the end of infusion, plasma ANP and cGMP were elevated both after (S)-thiorphan and after alatrioprilat, whereas plasma renin activity increased only after alatrioprilat. The ACE inhibition effect was studied in ganglion-blocked rats receiving a continous infusion of angiotensin I. Alatrioprilat decreased the mean blood pressure dose-dependently, but about 30 times higher concentrations were needed to produce the same effects as the ACE inhibitor captopril. At a dose of 30 mg/kg p.o., ecadotril, the orally active prodrug of (S)-thiorphan, decreased the systolic blood pressure in conscious TGR by 22 mmHg for 6 h, whereas alatriopril (100 mg/kg p.o.) also reduced the systolic pressure in these rats with a maximal reduction of 22 mmHg. In addition, ecadotril and alatriopril significantly increased the urinary excretion of sodium. In contrast, ACE inhibition with captopril decreased the excretion of sodium dose-dependently in conscious TGR. In conclusion, combined ACE/NEP inhibition produced a comparable lowering of blood pressure and improvement in renal function as those with NEP inhibition in TGR. Dual ACE/NEP inhibition may therefore be useful in cardiovascular conditions such as hypertension or heart failure.

In vivo evidence of an endothelin-induced vasopressor tone after inhibition of nitric oxide synthesis in rats.

Continuous production of nitric oxide (NO) from endothelial cells permanently inhibits the synthesis and the vasoconstrictor effects of endothelin. Thus, inhibition of NO synthesis might unmask a vasopressor response to endothelin. To assess whether endothelin contributes to the pressor response induced by inhibition of NO synthesis, we tested whether bosentan, a nonpeptide antagonist of ETA and ETB endothelin receptors, affected the hypertensive response induced by the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME). Anesthetized rats received increasing doses of L-NAME (0.1 to 3 mg.kg-1) in the absence or the presence of bosentan (3 mg.kg-1 IV 15 minutes before L-NAME). Bosentan itself did not affect blood pressure. L-NAME induced a dose-dependent increase in mean arterial pressure (percent increase from baseline after 3 mg.kg-1, 25 +/- 5%), and this was reduced by bosentan (13 +/- 3%; P < .05) or by the selective ETA antagonist BQ-123 (3 mg.kg-1: controls, 25 +/- 4%; BQ-123, 14 +/- 5%; P < .01). In contrast, bosentan did not affect the pressor response to phenylephrine (1 to 100 micrograms.kg-1). The response to L-NAME (3 mg.kg-1) was also reduced by bosentan in ganglion-blocked (chlorisondamine 2.5 mg.kg-1: controls, 89 +/- 10%; bosentan, 45 +/- 7%) or pithed rats (controls, 165 +/- 9%; bosentan, 85 +/- 12%; P < .01). Bosentan also inhibited the pressor response to another inhibitor of NO synthesis, NG-nitro L-arginine (3 mg.kg-1) in normal (controls, 24 +/- 5%; bosentan, 10 +/- 3%; P < .01) or ganglion-blocked (controls, 86 +/- 13%; bosentan, 25 +/- 8%; P < .01) rats. Finally, L-NAME induced a modest increase in plasma levels of endothelin-1 (controls, 26.8 +/- 4.1 pg.mL-1; L-NAME, 38.5 +/- 3.3 pg.mL-1; P < .05). These experiments demonstrate that inhibition of NO synthesis unmasks a tonic pressor influence of endothelin, suggesting that this peptide could play a major role in pathophysiological situations associated with an impaired formation of NO.

Role of the baroreflex in beta 2-sympathomimetic induced tachycardia in male rats

The aim of the study was to determine whether tachycardia which is associated with the use of β 2-sympathomimetic tocolytic agents is caused by baroreflex activation or by direct stimulation of cardiac β-adrenoceptors. In conscious male rats, tachycardiac responses following intravenous injection of hexoprenaline, ritodrine and fenoterol were compared following (i) bilateral sinoaortic denervation (SAD) or (ii) sham-operation, and (iii) ganglionic-blockade using hexamethonium. Dose-ranges were chosen to result in similar reductions in diastolic blood pressure (DAP). Furthermore, following ganglionic blockade, the relative contribution of β 1-adrenoceptor stimulation was assessed using the selective β 2-receptor antagonist ICI 118,551. In intact rats, increases in HR induced by all beta-adrenoceptor agonist were comparable. In SAD and ganglion-blocked rats, DAP fell more pronounced at even lower doses. The corresponding increases in HR were approximately 3 times smaller than in intact rats but not different between agents. During ganglionic blockade ICI 118,551 significantly inhibited HR responses to fenoterol and hexoprenaline but not ritodrine. The conclusion is that in intact male rats, baroreflex activation is the major determinant of tachycardia following injection of ritodrine, fenoterol or hexoprenaline. Increasing the β 2-selectivity of these drugs will not limit the tachycardic effects.

Studies on the hypotensive mechanisms of NZ-105, a new dihydropyridine derivative, in rats and rabbits

1. Intravenous administration of NZ-105 caused a slow-onset and long-lasting hypotension in anesthetized SHR. 2. Centrally administered NZ-105 (0.04 mg/kg) slightly decreased blood pressure. 3. The hypotension of NZ-105 (0.1 mg/kg, i.v.) was not affected by atropine, propranolol, diphen-hydramine plus cimetidine, aminophylline or indomethacin. 4. In ganglion-blocked rats, NZ-105 (0.003-0.3 mg/kg, i.v.) inhibited the pressor response to several hypertensive agents to a similar degree. 5. In pithed SHR, NZ-105 (0.03 mg/kg, i.v.) showed the same degree of hypotensive action as in non-treated SHR. 6. NZ-105 did not inhibit reflex responses to tilting in conscious rabbits. 7. Thus NZ-105 exerts its hypotensive action through the mechanisms of peripheral origin.

In Vitro and In Vivo Studies with a Series of Hexapeptide Endothelin Antagonists

The effects of different amino acids incorporated into the 16 and 17 positions of the C-terminal hexapeptide of ET-1 were examined. Structure-activity relationships (SAR) of the ET receptor antagonists PD 142893 [Ac-(D-Dip16-L-Leu17-L-Asp-L-Ile-L-Ile-L-Trp) (D-Dip = 3,3-D-diphenylalanine)] and PD 145065 [Ac-(D-Bhg16-L-Leu17-L-Asp-L-Ile-L-Ile-L-Trp) (D-Bhg = 5H-dibenzyl[a,d]cycloheptene-10,11-dihydro-glycine)] uncovered certain requirements for high potency. The disodium salt of PD 145065 has 4.0 and 15 nM binding affinity (IC50 values) for the ETA (rabbit renal artery vascular smooth-muscle cells) and ETB receptor (rat cerebellum), respectively. The compound is also an antagonist of ET-1- and SRTX-6c-stimulated vasoconstrictor activity, with pA2 values of 6.9 (rabbit femoral artery, ETA assay) and 7.1 (rabbit pulmonary artery, ETB assay). The tripeptidic ETA antagonist FR 139317 was found to be less active in the rabbit femoral artery, with a pA2 value of 6.0, and inactive in the rabbit pulmonary artery. Substitution of acidic and basic residues at position 17 in PD 142893 and PD 145065 indicates differences in selectivity. Incorporation of bulky non-natural amino acids at position 16 has led to potent nonselective analogues, including Ac-D-Bheg16-L-Leu-L-Asp-L-Ile-L-Ile-L-Trp [D-Bheg (5H-dibenzo[a,d]cycloheptene glycine)]. The in vivo effects of single-bolus doses of selected ET antagonists on depressor and pressor responses to ET-1 in anesthetized ganglion-blocked rats were evaluated.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of sympathetic nerve activity in the generation of vascular nitric oxide in urethane-anesthetized rats.

The aim of the present study was to examine the involvement of the sympathetic nervous system in the generation or release of vascular nitric oxide. In urethane-anesthetized rats, the administration of the novel nitric oxide synthesis inhibitor L-N-nitro arginine (LNA) (0.02 mmol/kg i.v.) increased mean arterial pressure and renal, mesenteric, and hindquarter vascular resistances. The intravenous administration of L-arginine (60 mg/kg plus 12 mg/kg/min i.v.) produced small reductions in arterial pressure and vascular resistances and abolished the hemodynamic effects of LNA. Pretreatment with the ganglion blocking agent chlorisondamine lowered mean arterial pressure and vascular resistances, abolished the LNA-induced pressor and renal vasoconstrictor response, and attenuated the increases in mesenteric and hindquarter resistances. In contrast, the vasodilator hydralazine lowered mean arterial pressure and vascular resistances to levels equivalent to that of ganglionic blockade; however, the subsequent administration of LNA still produced significant increases in arterial pressure and regional vascular resistances. In ganglion-blocked rats in which pressure and vascular resistances were returned to normal levels by infusion of arginine vasopressin or phenylephrine, the pressor and vasoconstrictor effects of LNA were restored. However, phenylephrine was significantly more efficacious and markedly exaggerated the action of LNA. These results suggest that the sympathetic nervous system plays an important role in modulating the synthesis or release of vascular nitric oxide through the effects of 1) normal sympathetic discharge, 2) humoral activation of alpha-adrenergic receptors, and 3) vascular tone per se.

Cardiocirculatory Responses to AII and AVP in Conscious Rats

Cardiac and peripheral circulatory responses to changes in afterload with angiotensin II (AII) and vasopressin (AVP) were investigated in ganglion-blocked (hexamethonium) conscious rats. Cardiac output (CO) was measured by thermodilution. Both hormones were infused at a dose adjusted to increase mean arterial pressure 70% above baseline. AVP (11.4 +/- 2.2 ng/kg/min, n = 6) decreased CO from 43.4 +/- 2.3 to 34.1 +/- 2.9 ml/min/100 g (p less than 0.001), whereas AII (33.4 +/- 7.4 ng/kg/min, n = 7) increased CO from 38.7 +/- 2.6 to 44.9 +/- 3.4 ml/min/100 g (p less than 0.01). Heart rate did not change with the increase in afterload with either vasoconstrictor. To study whether the different effects of AII and AVP on CO may be explained by their different actions on the venous system, changes in venous tone were evaluated by measuring mean circulatory filling pressure (MCFP) and determining the pressure gradient for venous return (PGVR). AVP changed neither MCFP nor PGVR, whereas AII increased both these parameters, 20.7 +/- 2.8% (p less than 0.01) and 20.3 +/- 6.4% (p less than 0.01), respectively, above control. We also examined the effects of AII and AVP on ventricular dynamics: left ventricular systolic pressure and left ventricular dP/dtmax increased as aortic pressure was increased in a similar manner with both vasoconstrictors. However, AVP induced a greater increase in left ventricular end diastolic pressure than AII. Our results indicate that AII induces an increase in preload by its effect on venous tone, which is adequate to increase cardiac output. The decrease in cardiac output induced by increasing afterload with AVP can be explained by two mechanisms: an inadequate venous return and a failure of the left ventricle to overcome the increased afterload.

Rat model for evaluating inhibitors of human renin

A rat model that provides a rapid method for the in vivo evaluation of potential inhibitors of human renin has been developed and validated. Recombinant human renin was infused intravenously into anesthetized, nephrectomized, ganglion-blocked rats. The resulting blood pressure had an approximate 60 mm Hg human renin-dependent component. The angiotensin I to angiotensin II converting enzyme inhibitor, captopril, and the renin inhibitor, ditekiren (U-71038), were capable of abolishing this component after oral administration. Oral administration of ditekiren to rats receiving human renin infusions evoked dose-dependent hypotensive responses that were greater in magnitude and longer in duration than those elicited in rats receiving hog renin infusions. Observations made in the renin-infused rats reflected the results of in vitro kinetic studies that had indicated a greater binding affinity of ditekiren for human renin than for hog renin.

In vitro and in vivo activity of chymotrypsin-activated big endothelin (porcine 1–40)

We investigated whether big endothelin (porcine 1–40) had contractile activity in isolated rat aorta or pressor activity when injected intravenously into the anesthetized rat. When isolated rat aorta was exposed to a 100 nM concentration of pig endothelin, 4.8% of a maximal KCl contraction was observed, compared to 131% of KClmax when paired aortic rings were exposed to an equivalent concentration of synthetic endothelin. Likewise, big endothelin had very weak pressor activity when injected intravenously into anesthetized, ganglion-blocked rats at 10 nmol/kg. When big endothelin was incubated with chymotrypsin, native endothelin and other peptide fragments were formed. Chymotrypsin-treated big endothelin produced an endothelin-like contraction when applied to isolated rat aortic rings, and a characteristic endothelin-like effect on blood pressure in vivo. Our results indicate that the biological activity of endothelin could be effectively blocked by inhibiting the enzyme which converts big endothelin to endothelin.

Pressor sensitivities to vasopressin, angiotensin II, or methoxamine in diabetic rats.

We investigated the pressor sensitivities to vasopressin, angiotensin II, and methoxamine of intact and ganglion-blocked rats that had been treated 21 days earlier with streptozotocin or saline. No differences in blood pressure or heart rate responses to vasopressin or angiotensin II were found between the intact groups when these peptides were administered intravenously in equimolar doses. After ganglion blockade a significant enhancement in pressor responsiveness to both vasopressin and angiotensin II was observed in the control groups, but in the streptozotocin-treated animals no enhancement in pressor sensitivity to vasopressin was found. Furthermore, although a significant augmentation of the responses to angiotensin II was observed, it was smaller than that seen in the ganglion-blocked control group. Neither group showed enhanced pressor responsiveness to methoxamine. These results indicate that the previously observed diminished contributions from endogenous vasopressin and the renin-angiotensin system to blood pressure recovery following ganglion blockade in streptozotocin-treated rats may have been due, at least in part, to diminished pressor responsiveness.

An orally active inhibitor of renin.

A potent renin inhibitor, U-71038 (Boc-Pro-Phe-N-MeHis-Leu psi[CHOHCH2]Val-Ile-Amp), was tested for oral effectiveness. Enzyme kinetic studies indicated that U-71038 was a competitive inhibitor of hog renin with an inhibitor constant (Ki) value of 12 nM. Intravenous as well as oral administration of U-71038 to anesthetized, ganglion-blocked rats infused with hog renin elicited dose-related hypotensive responses. Intravenous administration of U-71038 to conscious, sodium-depleted monkeys caused dose-related decreases of blood pressure and plasma renin activity without affecting heart rate. Similarly, the oral administration of U-71038 at 50 mg/kg to conscious, sodium-depleted monkeys elicited a pronounced hypotension and decrease in plasma renin activity that persisted for 5 hours. The hypotensive responses elicited by intravenous and oral administration of U-71038 to hog renin-infused rats and sodium-depleted monkeys were shown to be due entirely to inhibition of the renin-angiotensin system. A comparison of the results obtained after the intravenous administration of U-71038 with the results obtained after the oral administration of U-71038 implied that at least 10% of the orally administered U-71038 must have been absorbed to cause the observed effects in hog renin-infused rats and sodium-depleted monkeys. The studies demonstrated that an inhibitor of renin with a long duration of action and with oral effectiveness is a feasible entity.

α1- and α2-Adrenoceptor-Mediated Pressor Responses

We investigated the effect of cardiovascular depression on the pressor responses to the alpha 1-adrenoceptor selective agonist methoxamine, and the alpha 2-adrenoceptor selective agonist B-HT 920 in anesthetized ganglion-blocked rats. The calcium channel blocking drug nifedipine preferentially inhibited the effect of B-HT 920, as has been reported by other authors. Lowering the starting blood pressure by hemorrhage, by nitroprusside infusion, or by additional pentobarbitone also preferentially inhibited the pressor effect of B-HT 920. These selective effects of vascular depression on B-HT 920 are consistent with predicted interactions between functional antagonists and a partial (low-efficacy) agonist. This was tested in part by reducing the maximum effect of methoxamine by phenoxybenzamine treatment. Under these conditions, methoxamine behaved like B-HT 920 in that it was sensitive to inhibition by nitroprusside infusion. By analogy, the vasodepressive effect of calcium channel blocking drugs could be responsible for the preferential inhibition of the vasoconstrictor responses to alpha 2-adrenoceptor agonists. It is concluded that a differential reliance on influx of extracellular Ca2+ by alpha 1- and alpha 2-adrenoceptors may not be the only explanation of the selective effect of calcium channel blocking drugs.