Effects of pinacidil on arterial and venous resistances and mean circulatory filling pressure in rats

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.

Venous dilator effect of apelin, an endogenous peptide ligand for the orphan APJ receptor, in conscious rats

Apelin is an endogenous depressor peptide for the G protein-coupled APJ receptor. Our hypothesis is that apelin is a venodilator, and it reduces mean circulatory filling pressure (MCFP; index of venous tone). Dose-response curves of apelin (10, 20 and 40 nmol/kg) or vehicle (0.9% NaCl) were constructed in two groups each of conscious, unrestrained rats: unblocked rats and rats pretreated with mecamylamine (Mec; ganglionic blocker) and noradrenaline (NA; to restore vascular tone). The vehicle had no effects in the unblocked or ganglionic-blocked rats. Apelin decreased mean arterial pressure (MAP) and increased heart rate (HR), but it did not alter mean circulatory filling pressure in the unblocked rats. In the ganglionic-blocked rats, apelin did not alter heart rate but decreased mean arterial pressure and mean circulatory filling pressure. These results show that apelin is an arterial and venous dilator in vivo. The depressor effect of apelin is accompanied by tachycardia which is abolished by ganglion blockade.

Effect of nociceptin/orphanin FQ on venous tone in conscious rats

Nociceptin (orphanin FQ) is an endogenous agonist for the opioid receptor-like1 (ORL1) receptor. We investigated the effects of nociceptin on mean circulatory filling pressure, an index of venous tone. The effects of nociceptin (10, 30 nmol/kg, i.v.) and the vehicle (0.9% NaCl) on mean arterial pressure, heart rate and mean circulatory filling pressure were examined in two groups each of conscious rats: rats with, or without, ganglionic blockade through pretreatment with mecamylamine (1 mg/kg, i.v.) and noradrenaline (2 microg/kg min, i.v.). In the unblocked rats, both doses of nociceptin decreased mean arterial pressure and heart rate, and the high dose also decreased mean circulatory filling pressure. In the ganglionic-blocked rats, nociceptin did not alter heart rate but caused greater reductions of mean arterial pressure and mean circulatory filling pressure. The vehicle had no effects in any group. Therefore, nociceptin is a depressor agent with prominent direct venodilator and bradycardic action.

Venodilator action of an organotransition-metal nitrosyl complex

Nitrovasodilators, such as nitroglycerin, cause endothelium-independent dilatation of arterial and capacitance vessels via the release of nitric oxide (NO). This study examined the venodilator effect of CpCr(NO)(2)Cl (organotransition-metal nitrosyl complex) relative to that of nitroglycerin in conscious, unrestrained rats. Organotransition-metal nitrosyl complexes have releasable NO directly attached to metal centres. The dose-response effects of CpCr(NO)(2)Cl and nitroglycerin on the mean arterial pressure and the mean circulatory filling pressure (index of the body venous tone) were obtained in rats continuously infused with either normal saline or noradrenaline. The results show that both CpCr(NO)(2)Cl and nitroglycerin reduced the mean arterial pressure in rats with normal or elevated vasomotor tone. However, maximum depressor response of CpCr(NO)(2)Cl was greater than that of nitroglycerin. In vehicle-treated rats, both compounds increased the mean circulatory filling pressure. In rats with elevated vasomotor tone through the infusion of noradrenaline, both agents reduced the mean circulatory filling pressure. These results show that CpCr(NO)(2)Cl is an efficacious depressor and venodilator agent.

Endothelin ET(B) receptors counteract venoconstrictor effects of endothelin-1 in anesthetized rats

The inhibitory effects of BQ 788 (3 mg/kg, i.v., ET(B)-receptor antagonist) on endothelin-1 (ET-1)- or IRL 1620 (ET(B)-receptor agonist)-induced changes in mean arterial pressure (MAP), mean circulatory filling pressure (MCFP, driving force of venous return), arterial resistance (RA), venous resistance (RV) and cardiac output (CO) were characterized in 6 groups of pentobarbital-anesthetized rats. ET-1 or IRL 1620 (0.5, 1 and 2 nmol/kg, i.v.) dose-dependently increased MAP, RA, RV and MCFP and decreased CO. Maximum changes in RA, RV and CO elicited by ET-1 were greater than those by IRL 1620. Equimolar doses of ET-1 and IRL 1620 also caused similar initial transient decreases in MAP. BQ 788 alone slightly elevated MCFP, but did not alter other variables. The ET(B)-blocker abolished all changes elicited by IRL 1620, but only partially inhibited its responses on MCFP, showing the presence of BQ 788-insensitive receptors. BQ 788 also abolished ET-1's depressor response, partially inhibited its effect on MCFP, and markedly augmented its effects on RA, RV and CO. Thus, ET(B)-receptors counteract the sustained constrictor effects of ET-1 on arterial and venous resistance vessels Our results indicate a substantial arterial and venous dilator role for ET(B) receptors.

Subtypes of endothelin receptors that mediate venous effects of endothelin-1 in anaesthetized rats

1. The subtypes of endothelin receptors that mediate the effects of endothelin-1 (ET-1) on mean arterial pressure (MAP), heart rate (HR), mean circulatory filling pressure (MCFP), arterial resistance (RA), cardiac output (CO) and venous resistance (RV) were characterized in 9 groups of pentobarbitone-anaesthetized rats via the injection of ET-1 in the absence and presence of bosentan (Ro 47-0203, ETA- and ETB-receptor antagonist), PD 142893 (ETA- and ETB-receptor antagonist) or FR 139317 (ETA-receptor antagonist), as well as injection of the ETB-receptor agonist, IRL 1620. 2. Cumulative i.v. bolus injections of ET-1 or IRL 1620 (0.5, 1 and 2 nmol kg-1) dose-dependently increased MAP (ET: by 22, 34 and 44; IRL: 8, 17 and 28 mmHg), RA (ET: 62, 108 and 162; IRL: 51, 63 and 86% over baseline), RV (ET: 70, 132 and 179; IRL: 81, 89 and 98% over baseline) and MCFP (ET: 1.1, 1.8 and 1.9; IRL: 0.9, 1.0 and 1.2 mmHg) and reduced CO (ET: -18, -35 and -44; IRL: -24; -26; -25% below baseline). Equimolar doses of ET-1 and IRL 1620 caused similar initial transient depressor responses. Saline did not modify any haemodynamic variables in the time-control group. 3. Bosentan (10 mg kg-1, i.v.) inhibited ET-induced increases in MAP, RV, RA and MCFP and decrease in CO. PD 142893 (22 mg kg-1, i.v.) abolished ET-induced changes on MAP, RV, RA and CO, but did not alter effects on MCFP. Bosentan alone did not cause haemodynamic changes, but PD 142893 alone elevated MCFP (0.9 +/- 0.3 mmHg at 1 h after injection) and did not alter other variables. Both antagonists abolished the initial depressor effects of ET-1. 4. FR 139317 (1 mg kg-1, i.v.) partially inhibited the increases in MAP, RV, RA and MCFP and decreases in CO elicited by ET-1, but did not alter the transient depressor response of ET-1. 5. The results show that both ETA- and ETB-receptors mediate the arterial and venous constrictor effects of ET-1. Bosentan is more efficacious than PD 142893 in inhibiting the venous effects of ET-1.

Effects of endothelin-1 on arterial and venous resistances in anaesthetized rats

The effects of endothelin-1 and vehicle (0.9% NaCl) on mean arterial pressure, heart rate, mean circulatory filling pressure, systemic arterial resistance, cardiac output and venous resistance were studied in four groups of pentobarbitone-anaesthetized rats, either in presence or absence of phentolamine. I.v. bolus injections of endothelin-1 at 0.5, 1 and 2 nmol/kg dose dependently increased mean arterial pressure (22, 34 and 40 mmHg), arterial resistance (33, 93 and 122% over baseline), venous resistance (40, 117 and 143% over baseline) and mean circulatory filling pressure (1.0, 1.7 and 1.8 mmHg), but decreased heart rate (-16, -21 and -17 beats/min) and cardiac output (-6, -28 and -35% below baseline). The vehicle did not significantly alter any of these variables. During the continuous infusion of phentolamine (300 microg/kg per min), endothelin-1 caused similar increases in arterial resistance, venous resistance and mean circulatory filling pressure, similar reduction in cardiac output but significantly greater pressor and bradycardic responses, suggesting that the arterial and venous constrictor effects of endothelin-1 are not due to sympathetic activation and the stimulation of alpha-adrenoceptors. The results show that endothelin-1 raised mean arterial pressure via the increment in systemic arterial resistance, since cardiac output was markedly reduced. This decrease in cardiac output was mediated by increases in arterial as well as venous resistances. The vasoconstrictor and venoconstrictor effects of endothelin-1 were independent of sympathetic tone.