Postsynaptic alpha 1- and alpha 2-adrenergic receptors in adrenergic control of capacitance vessel tone in vivo

Cooling-Evoked Hemodynamic Perturbations Facilitate Sympathetic Activity with Subsequent Myogenic Vascular Oscillations via Alpha2-Adrenergic Receptors

This study extends our previous work by examining the effects of alpha2-adrenoceptors under cold stimulation involving the increase of myogenic vascular oscillations as increases of very-low-frequency and low-frequency of the blood pressure variability. Forty-eight adult male Sprague-Dawley rats were randomly divided into four groups: vehicle; yohimbine; hexamethonium+yohimbine; guanethidine+yohimbine. Systolic blood pressure, heart rate, power spectral analysis of spontaneous blood pressure and heart rate variability and spectral coherence at very-low-frequency (0.02 to 0.2 Hz), low-frequency (0.2 to 0.6 Hz), and high-frequency (0.6 to 3.0 Hz) regions were monitored using telemetry. Key findings are as follows: 1) Cooling-induced pressor response was attenuated by yohimbine and further attenuated by hexamethonium+yohimbine and guanethidine+yohimbine, 2) Cooling-induced tachycardia response of yohimbine was attenuated by hexame-thonium+yohimbine and guanethidine+yohimbine, 3) Different patterns of power spectrum reaction and coherence value compared hexamethonium+yohimbine and guanethi-dine+yohimbine to yohimbine alone under cold stimulation. The results suggest that sympathetic activation of the postsynaptic alpha2-adrenoceptors causes vasoconstriction and heightening myogenic vascular oscillations, in turn, may increase blood flow to prevent tissue damage under stressful cooling challenge.

Role of calcium-dependent K+ channels in the regulation of arterial and venous tone by nitric oxide in pigs

Effects of inhibition of calcium-dependent potassium channels (K+Ca channels) on the regulation of arterial and venous tone by nitric oxide (NO) were studied in anaesthetized pigs following vagotomy and blockade of autonomic ganglia. Selective inhibition of K+Ca channels by charybdotoxin (CTX, 2 microg/kg iv) or iberiotoxin (IbTX, 1 microgram/kg) significantly augmented mean total peripheral resistance (TPR) to levels 30-60% above control. Venous and pulmonary vascular tone were assessed by changes in effective compliances of the venous (EVC) and pulmonary (EPC) vascular beds as calculated from changes in central venous and diastolic pulmonary arterial blood pressure during haemorrhagia (-5 ml/kg) and hypervolaemia (+5 ml/kg). Blockade of K+Ca channels significantly decreased both EVC (-20 to -30%) and EPC (-30 to -50%). Both CTX and IbTX significantly diminished the vasodilation caused by the NO-donor S-nitroso-N-acetylpenicillamine (SNAP) both during control conditions and following experimental vasoconstriction induced by systemic inhibition of NO-synthesis by NG-nitro-L-arginine methyl ester (L-NAME) or infusion of vasoconstrictor agonists. Dilator effects of the adenosine 3',5'-cyclic monophosphate (cAMP)-dependent agonist adenosine were only slightly reduced. However, blockade of K+Ca channels did not increase vasoconstriction induced by L-NAME significantly. These results suggest that activation of vascular K+Ca channels is an important mechanism by which NO attenuates the constrictor tone of resistance and capacitance vessels in vivo.

Vascular capacitance and cardiac output in pacing-induced canine models of acute and chronic heart failure

The relationship between stressed and total blood volume, total vascular capacitance, central blood volume, cardiac output (CO), and pulmonary capillary wedge pressure (Ppcw) was investigated in pacing-induced acute and chronic heart failure. Acute heart failure was induced in anesthetized splenectomized dogs by a volume load (20 mL/kg over 10 min) during rapid right ventricular pacing at 250 beats/min (RRVP) for 60 min. Chronic heart failure was induced by continuous RRVP for 2-6 weeks (average 24 +/- 2 days). Total vascular compliance and capacitance were calculated from the mean circulatory filling pressure (Pmcf) during transient circulatory arrest after acetylcholine at three different circulating volumes. Stressed blood volume was calculated as a product of compliance and Pmcf, with the total blood volume measured by a dye dilution. Central blood volume (CBV) and CO were measured by thermodilution. Central (heart and lung) vascular capacitance was estimated from the plot of Ppcw against CBV. Acute volume loading without RRVP increased capacitance and CO, whereas after volume loading with RRVP, capacitance and CO were unaltered from baseline. Chronic RRVP reduced capacitance and CO. All interventions, volume +/- RRVP or chronic RRVP, increased stressed and central blood volumes and Ppcw. Acute or chronic RRVP reduced central vascular capacitance. Cardiac output was increased when stressed and unstressed blood volumes increased proportionately as during volume loading alone. When CO was reduced and Ppcw increased, as during chronic RRVP or acute RRVP plus a volume load, stressed blood volume was increased and unstressed blood volume was decreased. Thus, interventions that reduced CO and increased Ppcw also increased stressed and reduced unstressed blood volume and total vascular capacitance.

Alpha-adrenoceptors

Major advances have been made in our understanding of the molecular structure and function of the alpha-adrenoceptors. Many new subtypes of the alpha-adrenoceptor have been identified recently through biochemical and pharmacological techniques and several of these receptors have been cloned and expressed in a variety of vector systems. Currently, at least seven subtypes of the alpha-adrenoceptor have been identified and the molecular structure and biochemical functions of these subtypes are beginning to be understood. The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. The mechanisms by which the alpha-adrenoceptor subtypes recognize different G-proteins, as well as the molecular interactions between receptors and G-proteins, are the topics of current research. Furthermore, the physiological and pathophysiological role that alpha-adrenoceptors play in homeostasis and in a variety of disease states is also being elucidated. These major advances made in alpha-adrenoceptor classification, molecular structure, physiologic function, second messenger systems and therapeutic relevance are the subject of this review.

Effect of the platelet-activating factor antagonist, TCV-309, and the cyclo-oxygenase inhibitor, ibuprofen, on the haemodynamic changes in canine experimental endotoxic shock

1. The present study was conducted in order to examine the effects of the platelet-activating factor (PAF) antagonist, TCV-309, and the cyclo-oxygenase inhibitor, ibuprofen, on the acute haemodynamic responses to endotoxin in anaesthetized dogs. 2. Endotoxin (2 mg kg-1, i.v.) induced a severe hypotension by decreasing both total peripheral resistance (TPR) and cardiac output. Endotoxin also decreased central venous pressure and increased effective vascular compliance (EVC), indicating a blood pooling in the capacitance vessels. 3. The endotoxin-induced hypotension but not the fall in cardiac output, was markedly attenuated by ibuprofen. Ibuprofen abolished the decrease in TPR and even caused a systemic vasoconstriction. Ibuprofen abolished the increase in EVC. 4. The hypotension caused by endotoxin was attenuated by TCV-309 to a lesser extent than ibuprofen. However, the reduction in cardiac output produced by endotoxin was markedly attenuated by the PAF antagonist. TCV-309 also abolished the increase in EVC. In contrast to ibuprofen, TCV-309 did not affect the decrease in TPR caused by endotoxin. 5. Combined treatment with ibuprofen and TCV-309 markedly attenuated the endotoxin-induced hypotension, but not the fall in cardiac output. Nevertheless, when compared with animals treated with ibuprofen alone, treatment with ibuprofen and TCV-309 partly attenuated the endotoxin-induced reduction in cardiac output and systemic vasoconstriction. 6. These data indicate that dilatation of both resistance vessels and capacitance vessels contributes to the endotoxin-induced hypotension. It is suggested that (i) both prostanoids and PAF are involved in dilatation of capacitance vessels, (ii) prostanoids, but not PAF cause dilatation of resistance vessels and(iii) PAF may partly contribute to prostanoid-independent reduction in cardiac output in acute canine experimental endotoxin shock.

Effect of NG-nitro-L-arginine on effective vascular stiffness in dogs

It has been reported that there is a continuous release of nitric oxide (NO) that contributes to the regulation of vascular tone in the arterial system. In contrast, the role of NO on vascular tone in the venous system is controversial. We examined the effect of NG-nitro-L-arginine (LNNA), an NO synthase inhibitor, on venous tone in dogs. Venous tone was evaluated by effective vascular stiffness (EVS), which was calculated from the changes in central venous pressure recorded simultaneously with changes in blood volume. LNNA (10 mg/kg, i.v.) increased EVS from 0.21 +/- 0.04 to 0.30 +/- 0.03 mmHg.kg/ml as well as increasing the systemic vascular resistance. NG-Nitro-D-arginine had no effect on EVS. The LNNA-induced increase in EVS was partly reversed by L-arginine, but not by D-arginine, indicating that the increase in EVS was attributable to a blockade of NO synthesis. Since the present study was conducted under ganglion blockade, nitroxidergic nerve terminals do not seem to be the source of NO in this case. These findings suggest that NO (endothelium-derived relaxing factor) is constantly released in the venous system and contributes to the regulation of total systemic venous tone.

Neurohormonal inhibition and hemodynamic unloading during prolonged inhibition of ANF degradation in patients with severe chronic heart failure

BACKGROUND

The purpose of this study was to investigate the therapeutic potential of prolonged inhibition of atrial natriuretic factor (ANF) degradation in patients with severe chronic heart failure.

METHODS AND RESULTS

The effects of repeated doses of the endopeptidase inhibitor candoxatrilat (150 mg i.v.) were examined over a 24-hour period in patients with severe chronic heart failure (New York Heart Association class III-IV). Plasma alpha-hANF(99-126) was elevated at baseline (235 +/- 59 pg/ml), increased 2.5-fold at 2 hours after the first dose, and remained significantly elevated throughout the 24-hour protocol. In contrast, pro-hANF(31-67) decreased from 3,151 +/- 616 to 2,072 +/- 362 pg/ml (p less than 0.05). Cardiac index (CI) increased only transiently after the first dose of candoxatrilat (CI, 2.11 +/- 0.2 to 2.67 +/- 0.28 l/min/m2, p less than 0.05). Sodium excretion increased sixfold (p less than 0.05) 2 hours after the first dose of candoxatrilat and remained significantly elevated throughout the protocol. Degree of natriuresis and diuresis in response to candoxatrilat was closely related to baseline cardiac output. Glomerular filtration rate and volume excretion did not change significantly. Pulmonary capillary wedge pressure fell from 23 +/- 3 to 18 +/- 3 mm Hg (p less than 0.05) and remained below baseline throughout the 24 hours. Arterial pressure, heart rate, and total peripheral resistance did not change significantly during the 24-hour period. Urinary cGMP excretion increased fivefold (p less than 0.05), whereas urinary ANF immunoreactivity and plasma cGMP levels remained unchanged. Excretion of prostacyclin metabolite 6-keto-PGF-1 alpha increased 3.3-fold (p less than 0.05). Plasma norepinephrine and epinephrine levels decreased significantly after candoxatrilat and remained suppressed over the 24-hour period. There was also a transient reduction in plasma vasopressin, aldosterone levels, and plasma renin activity. Hematocrit, total protein content, and plasma albumin concentrations did not change, indicating that no fluid shift into the extravascular space had occurred.

CONCLUSIONS

1) The inhibition of ANF degradation causes sustained drop in left and right atrial pressures that appears to be mediated by an inhibition of neurohumoral activity; 2) concomitant inhibition of bradykinin breakdown (which in turn stimulates renal prostacyclin synthesis) contributes to natriuresis; 3) the close correlation between renal response and baseline cardiac index indicates that an inadequate renal perfusion secondary to low cardiac output diminishes the efficacy of this treatment modality. This spectrum of action would be advantageous for a first-line diuretic agent early in the course of disease rather than in patients with advanced chronic heart failure.

Orthostatic hypotension occurs following α2-adrenoceptor blockade in chronic prazosin-pretreated conscious spontaneously hypertensive rats

1. Studies were performed to evaluate whether chronic prazosin treatment alters the alpha 2-adrenoceptor function for orthostatic control of arterial blood pressure in conscious spontaneously hypertensive rats (SHR). 2. Conscious SHR (male 300-350 g) were subjected to 90 degrees head-up tilts for 60 s following acute administration of prazosin (0.1 mg kg-1 i.p.) or rauwolscine (3 mg kg-1 i.v.). Orthostatic hypotension was determined by the average decrease (%) in mean arterial pressure (MAP femoral) over the 60-s tilt period. The basal MAP of conscious SHR was reduced to a similar extent by prazosin (-23%(-)-26% MAP) and rauwolscine (-16%(-)-33% MAP). However, the head-up tilt induced orthostatic hypotension in the SHR treated with prazosin (-16% MAP, n = 6), but not in the SHR treated with rauwolscine (less than +2% MAP, n = 6). 3. Conscious SHR were treated for 4 days with prazosin at 2 mg kg-1 day-1 i.p. for chronic alpha 1-adrenoceptor blockade. MAP in conscious SHR after chronic prazosin treatment was 14% lower than in the untreated SHR (n = 8). Head-up tilts in these rats did not produce orthostatic hypotension when performed either prior to or after acute dosing of prazosin (0.1 mg kg-1 i.p.). Conversely, administration of rauwolscine (3 mg kg-1 i.v.) in chronic prazosin treated SHR decreased the basal MAP by 12-31% (n = 4), and subsequent tilts induced further drops of MAP by 19-23% in these rats. 4. The pressor responses and bradycardia to the alpha 1-agonist cirazoline (0.6 and 2 micrograms kg-1 i.v.), the alpha 2-agonist Abbott-53693 (1 and 3 micrograms kg-1 i.v.), and noradrenaline (0.1 and 1.0 micrograms kg-1 i.v.) were determined in conscious SHR with and without chronic prazosin pretreatment. Both the pressor and bradycardia effects of cirazoline were abolished in chronic prazosin treated SHR (n = 4) as compared to the untreated SHR (n = 4). On the other hand, the pressor effects of Abbott-53693 were similar in both groups of SHR, but the accompanying bradycardia was greater in SHR with chronic prazosin treatment than without such treatment. Furthermore, the bradycardia that accompanied the noradrenaline-induced pressor effect in SHR was similar with and without chronic prazosin treatment despite a 47-71% reduction of the pressor effect in chronic alpha 1-receptor blocked SHR.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of chronic rapid ventricular pacing on total vascular capacitance

BACKGROUND

Rapid right ventricular pacing (RRVP) at 250 bpm for 3-6 weeks produces chronic heart failure manifested by a reduction in cardiac output and increases in right atrial, pulmonary artery, and capillary wedge pressures.

METHODS AND RESULTS

One week after splenectomy and pacemaker placement, vascular capacitance, unstressed volume, and compliance were determined in 19 anesthetized dogs from pressure-volume curves using transient circulatory arrests induced by acetylcholine. Nine dogs were restudied 31 +/- 1 days later without RRVP, and 10 dogs underwent RRVP at 250 bpm and were restudied at 23 +/- 8 and 38 +/- 8 days in cardiac failure and after 1 and 2 weeks of postpacing recovery. Control animals had no changes in vascular capacitance or compliance. Dogs undergoing RRVP exhibited a marked increase in mean circulatory filling pressure (5.4 +/- 0.4 to 10.5 +/- 1.5 mm Hg) during the development of cardiac failure with a reduction in unstressed volume (81.9 +/- 5.7 to 43.9 +/- 8.1 ml.kg-1) without changing total vascular compliance. Total blood volume decreased (95.4 +/- 6.2 to 66.7 +/- 6.5 ml.kg-1) primarily due to a reduction in packed cell volume. The pressure gradient for venous return and overall venous resistance was unaltered. Central blood volume as a proportion of total blood volume increased (9.3 +/- 1.7% to 16.0 +/- 2.7%). Arterial compliance and capacity and pulmonary vascular compliance were reduced. In the 2-week postpacing period, except for a reduced cardiac response to a volume load, all of these parameters returned to baseline values.

CONCLUSIONS

Chronic RRVP induced cardiac failure with a marked reduction in total vascular capacitance due to a reduction in unstressed volume without altering compliance. The rise in mean circulatory filling pressure was limited by a reduction in total blood volume.

[Effects of lidocaine on arterial and venous circulation of limbs in man]

The effects of intravenous lidocaine on limb arteries and veins were investigated in a placebo-controlled study. Seven young healthy volunteers, 23 to 28-years-old, were included. Electrocardiogram, arterial pressure and arm and leg blood flows were recorded continuously. Systolic and diastolic blood pressures were measured in the left arm by finger photoplethysmography. Limb blood flow and the limb venous system were studied by venous occlusive plethysmography. The venous parameters studied were venous tone, lowest closing pressure, venous volume at 30 mmHg, and venous distensibility. After an initial bolus of 1.5 mg.kg-1 lidocaine had been given, 30, and then 60, micrograms.kg-1.min-1 were given for one hour each. Plasma noradrenaline and serum lidocaine titres were measured before giving the lidocaine, and at the end of each one hour period. Placebo consisted in a two hour infusion of 0.25 ml.min-1 normal saline. Lidocaine titres were 1.64 +/- 0.40 microgram.ml-1 after one hour, and 2.55 +/- 0.69 microgram.ml-1 after two hours. Lidocaine increased vascular resistances in both the forearm (+81% to +93%) and the calf (+38% to +57%). There was a concomitant increase in mean arterial blood pressure (+21% to +28%) without any change in heart rate. There was a significant dose-dependent increase in plasma noradrenaline levels during the second period of the lidocaine infusion with respect to the preinfusion period and the same period during the placebo infusion. Venous capacitance measured before any infusion had been started was greater in the leg than in the arm.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitrate tolerance in epicardial arteries or in the venous system is not reversed by N-acetylcysteine in vivo, but tolerance-independent interactions exist

N-acetylcysteine is assumed to reverse nitrate tolerance by replenishing depleted intracellular sulfhydryl groups, but data on interactions of N-acetylcysteine and nitrates in patients with stable angina are controversial and disappointing. Therefore, we studied the effect of N-acetylcysteine on nitrate responsiveness of epicardial arteries and of the venous system (assessed as changes in effective vascular compliance) in dogs (n = 12) during long-term nitroglycerin treatment (1.5 micrograms/kg/min i.v. for 5-6 days). In dogs with nitroglycerin-specific tolerance (shift of venous or epicardial artery dilation to 15-17-fold higher dosages), N-acetylcysteine (100 mg/kg i.v.) had no dilator effect and did not alter the dose-response relations of nitroglycerin. Yet, in nontolerant dogs (n = 17), N-acetylcysteine augmented (1.5-2.0-fold) the dilation of epicardial arteries and the reduction of peripheral vascular resistance induced by 0.5-1.5 micrograms/kg/min nitroglycerin. In vitro, the augmentation of purified guanylate cyclase activity by nitroglycerin (10-100 microM) was potentiated by N-acetylcysteine (0.01-1.0 mM) in saline or in canine plasma, but N-acetylcysteine alone was ineffective. We conclude that 1) N-acetylcysteine does not restore nitroglycerin responsiveness in tolerant epicardial arteries or veins in vivo, 2) a small, tolerance-independent augmentation of nitroglycerin-induced dilation may result from N-acetylcysteine-induced extracellular formation of a stimulant of guanylate cyclase from nitroglycerin.

Preferential venoconstriction by cyclooxygenase inhibition in vivo without attenuation of nitroglycerin venodilation

Because prostacyclin is a rather potent venodilator in vivo, we analyzed the effect of cyclooxygenase inhibition on venous tone in 14 anesthetized dogs during ganglionic and beta-adrenergic blockade and atraumatic conditions. Effective vascular stiffness (a reciprocal of effective vascular compliance) as a variable of integrated venous tone was 0.30 +/- 0.01 mm Hg.kg/ml (n = 35) and was augmented up to twofold by diclofenac (1, 3, and 10 mg/kg i.v.), ibuprofen (6 and 60 mg/kg), or indomethacin (5 mg/kg) parallel to augmentations in central venous pressure, while the rise in arterial pressure was less than half of the increase induced by equivenoconstrictor dosages of norepinephrine. After preconstriction by indomethacin or diclofenac, nitroglycerin (1.5 micrograms/kg/min) lowered effective vascular stiffness (by 24 +/- 2% or 23 +/- 5%, respectively), similarly as during preconstriction by norepinephrine (by 24 +/- 4%). Long-term cyclooxygenase inhibition (diclofenac 2 x 1 mg/kg/day for 4 days) did not modify arterial pressure, heart rate, or hematocrit levels in conscious dogs at rest, but it lowered plasma volume to 52.5 +/- 1.9 ml/kg (sham treatment: 59.1 +/- 1.6 ml/kg, p less than 0.05, n = 4). In conclusion, venoconstriction by clinical dosages of cyclooxygenase inhibitors does not interfere with the venodilator action of nitroglycerin and is compensated chronically by adjustments of plasma volume.

Direct effects of alpha 2-adrenergic receptor stimulation on intravascular systemic capacity in the dog

The role of alpha 2-adrenergic receptor stimulation in the regulation of systemic vascular capacity and venous return, a major determinant of cardiac output, is not well understood. With the influence of the central nervous system isolated from the systemic circulation, the direct peripheral vascular effects of two specific, chemically distinct alpha 2-adrenergic receptor agonists, UK 14,304 and B-HT 920, were investigated in 19 dogs on total cardiopulmonary bypass with constant arterial perfusion and central venous pressure. Five-minute intra-arterial infusions of UK 14,304 (200 micrograms/min) resulted in increased arterial resistance (mean arterial pressure increased 18 +/- 4 [SEM] mm Hg; p less than 0.01) and a decrease in systemic vascular capacity (81 +/- 20 ml; p less than 0.01). This decrease in systemic vascular capacity appears to result from vasoconstriction, since there was no decrease in transhepatic resistance to portal flow and no significant change in hepatic vein flow to suggest redistribution of arterial blood flow. Yohimbine abolished both the arterial and systemic capacity effects, whereas prazosin did not. Intra-arterial administration of B-HT 920 (200 theta grams/min) in five dogs produced similar changes in arterial resistance and systemic capacity. These findings provide direct evidence for beta 2-adrenergic control, not only of arterial resistance but also of systemic vascular capacity, which in the intact animal would increase venous return to the heart.