Regional hemodynamic effects of antihypertensive renomedullary lipids in conscious rats

Involvement of brain mineralocorticoid receptor in salt-enhanced hypertension in spontaneously hypertensive rats

We recently showed that brain mineralocorticoid receptors (MRs) are involved in blood pressure and kidney function control in normotensive Wistar rats. We now assessed the involvement of brain MRs in spontaneously hypertensive rats (SHR), in which the presence of adrenocorticoids has been shown to be required for the development of hypertension. The effect of a single intracerebroventricular (ICV) injection of an MR antagonist (RU28318) on systolic blood pressure (SBP) and renal function was examined in conscious adult SHR and Wistar-Kyoto rats (WKY) maintained on a standard-sodium diet (0.4% Na(+)). In WKY, a long-lasting decrease in SBP was caused by the ICV injection of 10 ng RU28318 as previously reported in Wistar rats, associated with increased urinary excretion of water and electrolytes. In SHR maintained on the standard diet, the ICV injection of RU28318 (10 or 100 ng) had no effect on cardiovascular and renal functions. However, the ICV injection of 10 ng RU28318 in SHR after 3 weeks of high sodium intake (8% Na(+)) caused a long-lasting decrease in SBP. The effect was present at 8 hours (DeltaSBP 34+/-2 mm Hg), persisted at 24 hours (DeltaSBP 29+/-1 mm Hg), and disappeared at 48 hours after the injection. The hypotension was not associated with changes in heart rate, urinary excretion of water and electrolytes, and plasma renin activity, whereas renal denervation did not affect the decrease in SBP. A more pronounced decrease in SBP (49+/-3 mm Hg at 8 hours) was observed with 100 ng RU28318. This dose of the antagonist was without effect after subcutaneous administration. Thus, brain MRs appear to participate in the maintenance of hypertension in conscious adult SHR sensitized by sodium loading.

Role of platelet-activating factor in cardiovascular pathophysiology

Platelet-activating factor (PAF) is a phospholipid mediator that belongs to a family of biologically active, structurally related alkyl phosphoglycerides. PAF acts via a specific receptor that is coupled with a G protein, which activates a phosphatidylinositol-specific phospholipase C. In this review we focus on the aspects that are more relevant for the cell biology of the cardiovascular system. The in vitro studies provided evidence for a role of PAF both as intercellular and intracellular messenger involved in cell-to-cell communication. In the cardiovascular system, PAF may have a role in embryogenesis because it stimulates endothelial cell migration and angiogenesis and may affect cardiac function because it exhibits mechanical and electrophysiological actions on cardiomyocytes. Moreover, PAF may contribute to modulation of blood pressure mainly by affecting the renal vascular circulation. In pathological conditions, PAF has been involved in the hypotension and cardiac dysfunctions occurring in various cardiovascular stress situations such as cardiac anaphylaxis and hemorrhagic, traumatic, and septic shock syndromes. In addition, experimental studies indicate that PAF has a critical role in the development of myocardial ischemia-reperfusion injury. Indeed, PAF cooperates in the recruitment of leukocytes in inflamed tissue by promoting adhesion to the endothelium and extravascular transmigration of leukocytes. The finding that human heart can produce PAF, expresses PAF receptor, and is sensitive to the negative inotropic action of PAF suggests that this mediator may have a role also in human cardiovascular pathophysiology.

Renal medullary blood flow and renal medullary antihypertensive mechanisms

It has long been recognised that the kidneys take part in blood pressure control via both their exocrine and endocrine functions. An endocrine antihypertensive function of the renal medulla has been proposed. The renal medullary depressor substances ("medullipins"), are released in response to increased renal perfusion pressure. It has been suggested that the release of "medullipin" is controlled via changes in renal medullary blood flow. Recent observations also suggest that renal medullary blood flow is involved in the control of the pressure/natriuretic-diuretic action of the kidney. In this review we outline a unified hypothesis for blood pressure control via a combination of the plasma volume regulating pressure-natriuresis mechanism and the powerful antihypertensive actions of the "medullipins" (i.e. vasodilatation, inhibition of sympathetic drive and a diuretic action). It is hypothesised that the activity of both these systems are under control by renal medullary blood flow.

Lesions in rostral ventromedial or rostral ventrolateral medulla block neurogenic hypertension

Neurogenic hypertension results from the removal of inhibitory baroreceptor afferent input to vasomotor systems in the central nervous system. We sought to determine whether the bilateral destruction of neurons in the rostral ventrolateral or rostral ventromedial medulla, made using microinjections of N-methyl-D-aspartic acid (30 nmol in 200 nL), would block the acute increase in arterial pressure after sinoaortic deafferentation in pentobarbital-anesthetized rats. Bilateral lesions of the rostral ventrolateral or rostral ventromedial medulla decreased mean arterial pressure (107 +/- 4 to 78 +/- 5 and 115 +/- 3 to 94 +/- 3 mm Hg, respectively). In rostral ventrolateral or rostral ventromedial medulla lesioned rats, sinoaortic deafferentation failed to increase arterial pressure. Sham lesions or lesions placed rostral to the rostral ventrolateral or rostral ventromedial medulla did not significantly lower arterial pressure. Subsequent sinoaortic deafferentation significantly increased mean arterial pressure (109 +/- 3 to 145 +/- 4 and 109 +/- 5 to 141 +/- 3 mm Hg, respectively). In eight rats we used an infusion of angiotensin II to return arterial pressure to control levels after lesion of the rostral ventrolateral (n = 4) or rostral ventromedial (n = 4) medulla. In these animals, sinoaortic deafferentation failed to increase arterial pressure. We conclude that neurons in the rostral ventrolateral and rostral ventromedial medulla are involved in the normal maintenance of arterial pressure and the development of hypertension after sinoaortic deafferentation in pentobarbital-anesthetized rats.

Hypertension induced by brain grafts from fetal spontaneously hypertensive rats

Hypothalami from fetal rats were grafted into the third ventricle of four strains of adult rats. Grafts from spontaneously hypertensive rats (SHR), in contrast to grafts from Wistar-Kyoto (WKY) rats, induced an elevation of systolic blood pressure and a thickening of the media of resistance arteries, along with corresponding alterations in the contractile properties of these vessels. However, no cardiac hypertrophy was observed. The resistance arteries of rats grafted with hypothalamic from SHR also displayed functional alterations that were similar to what is typically found in the resistance arteries of young prehypertensive SHR, ie, an increase in the sensitivity to cocaine and an impairment in the ability to relax in the presence of acetylcholine. This suggests that the brain may play a causal role in these alterations. Histological examination of sections of brains grafted with previously labeled tissue revealed that (1) there was no brain area that was systematically infiltrated by grafts from SHR and not by grafts from WKY rats; (2) the volume of the transplants appeared larger 2 weeks after the graft than the volume of the tissue originally implanted; and (3) grafts from SHR were slightly larger, displayed more individual foci, and extended farther along the anteroposterior axis than grafts from WKY rats. In addition, glial cultures derived from the hypothalami of SHR had a higher in vitro growth rate than equivalent cultures from WKY rats. It is therefore possible that the ability of brain grafts from SHR to induce hypertension is related to a higher proliferative and/or migratory potential of nonneuronal cells within the hypothalamus.

Pressure range for release of renomedullary depressor substance in rabbits

We investigated the relation between renal perfusion pressure and the release of a renal vasodepressor substance in vivo to determine whether this substance was released at physiological pressures. We perfused the left kidneys of anesthetized rabbits using an extracorporeal circuit that allowed renal perfusion pressures to be set at 65 mm Hg (control) and increased to 95, 125, 155, or 185 mm Hg for 30-minute experimental periods. Systemic blood pressure did not change significantly when renal perfusion pressure was maintained at 65 mm Hg throughout. When renal perfusion pressure was increased to 95, 125, 155, or 185 mm Hg, systemic blood pressure fell significantly at rates of 0.17 +/- 0.04, 0.79 +/- 0.31, 0.60 +/- 0.11, and 2.18 +/- 0.79 mm Hg/min, respectively (P < .05). Restoration of renal perfusion pressure to 65 mm Hg abruptly reversed the falls in systemic blood pressure in each group. There was a natriuresis and diuresis that were both pressure related and progressive in the face of each constant level of increased renal perfusion pressure. In summary, there was a continuum of arterial vasodepressor responses across a renal perfusion pressure range from resting pressure to 185 mm Hg. We suggest that the threshold level for the release of significant amounts of a renal medullary depressor substance, probably medullipin, is just above normal arterial blood pressure and that the rate of release increases with increasing arterial pressure.

Renal vascular and sympathetic nerve responses to hypotension induced by platelet-activating factor in anesthetized dogs

This experiment was designed to determine renal sympathetic and renal vascular responses to platelet-activating factor (PAF)-induced hypotension in anesthetized dogs with and without systemic baroreceptor denervation. The left kidney was perfused at a constant flow, and renal perfusion pressure and efferent left renal sympathetic nerve activity were measured simultaneously. Intrarenal injection of PAF (1.25-5.0 x 10(-2) micrograms/kg, n = 6) produced a dose-dependent increase in renal perfusion pressure without any change in systemic blood pressure. An intravenous injection of PAF (10 micrograms/kg) to intact animals (n = 7) caused an initial increase in renal nerve activity (157 +/- 14%) followed by a gradual reduction below baseline (72 +/- 7%) with concomitant systemic hypotension (from 116 +/- 7 to 46 +/- 6 mmHg). Renal perfusion pressure increased significantly from 84 +/- 2 to 161 +/- 33 mmHg concomitant with an increase in renal nerve activity at 1 min and was maintained at this elevated level throughout the experiment. Similar responses of renal nerve activity and renal perfusion pressure were found in animals with complete systemic baroreceptor denervation (n = 7). These results suggest that renal vascular response during PAF-induced hypotension may presumably be mediated by a direct vasoconstrictor effect of PAF on the renal vasculature and that baroreceptor reflex is not involved in either renal sympathetic or renal vascular changes.

Hypothalamic-midbrain dysregulation syndrome: hypertension, hyperthermia, hyperventilation, and decerebration

Certain decerebrate lesions of brain stem or hypothalamus induce pharmacologically reversible hypertension and hyperthermia in animals. We observed three young patients with episodic decerebration, hyperthermia, hypertension, and hyperventilation during recovery from comas of different etiologies. The shared pathology on neurologic examinations and computed tomographic scans was hypothalamic-mesencephalic dysfunction, suggesting a diencephalic-brain-stem disconnection syndrome or brain-stem release mechanism. Propranolol was the most effective drug tested, but only two patients responded, one dramatically. This novel clinical syndrome may have localizing and therapeutic significance in pediatric coma that needs to be further defined in future studies.

Vasorelaxant effect of C16-PAF and C18-PAF on renal blood flow and systemic blood pressure in the anesthetized rat

The renal vasoactive and systemic hypotensive effects of platelet activating factor (C16:0-PAF and C18:1-PAF) were examined in anesthetized male Wistar rats. Bolus injections of C16-PAF (0.5-25 ng/kg) and C18-PAF (2.5-200 ng/kg) into the arterial circulation of the kidney produced increases in renal blood flow (6-15%) before causing dose-dependent systemic hypotension (2-64 mmHg). The dose-response curves for renal blood flow and systemic blood pressure generated by intrarenal C18-PAF administration were approximately 7 fold to the right of the dose-response curves generated by C16-DPAF. Intrarenal injections of vehicle or the biologically inactive enantiomer C16-DPAF (25-200 ng/kg) did not affect renal blood flow or systemic blood pressure. These results suggest that C16:0-PAF is a more potent renal vasodilator and hypotensive lipid than C18:1-PAF.

Reproducible vasodilatation by platelet-activating factor in blood- and Krebs-perfused rat kidneys is albumin-dependent

The vasodilator effect of platelet-activating factor (PAF) and its reproducibility was determined in rat kidneys perfused in situ with blood or in isolated kidneys perfused with Krebs solution or Krebs plus 0.25% bovine serum albumin (BSA) at 3 ml/min. Dilatation was measured as inhibition of the increase in perfusion pressure produced by stimulation of the renal nerves or by infusion of noradrenaline. PAF, in saline and 0.25% BSA, was infused into the perfusate at 0.05 ml/min to produce eight incremental consecutive concentrations from 3 x 10(-11) to 10(-7). Two sets of PAF infusions were made during nerve-stimulated responses followed by one set during noradrenaline infusion. With blood perfusion, PAF consistently produced dose-dependent dilatation and 3 x 10(-9) M reduced nerve-stimulated responses to 52% of control. This effect was reproduced by a second infusion. However in Krebs-perfused kidneys the effect of the first PAF infusion was not consistent, with responses either not affected or reduced only at the lower doses so that the mean maximum decrease was only 10% and the vasodilatation was not dose-dependent. The second PAF infusion invariably had no effect. On the other hand perfusion with Krebs and 0.25% BSA produced consistent and dose-dependent inhibition of vasoconstriction which was reproduced by a second infusion. PAF was effective at 10-fold lower doses in Krebs-albumin perfused compared to blood-perfused kidneys. These results indicate that exogenous PAF is a potent and reproducible dilator of the rat renal vasculature perfused with blood or Krebs-albumin solution but not albumin-free Krebs solution.

Biologic contrasts between medullipin I and vasoactive glyceryl compounds

Medullipin I causes a delayed onset depressor response when injected intravenously into rats. The glyceryl compounds selachyl alcohol (SA) and monoolein (MO) cause similar vasodepression. The neutral lipid 1-O-hexadecyl-2-acetyl-sn-glycerol (HAG) was suggested by Blank et al to be medullipin I (Med I, formerly ANRL). Biologic comparisons were made between Med I and various glyceryl compounds, including SA, MO, HAG, alkyl glyceryl ethers of phosphatidyl choline (termed APRL by us), diacylated SA, and the n-butyl boronic acid derivative of SA and MO. The n-butyl boronic acid derivative of Med I also was evaluated. The delay in onset of the depressor response to Med I was reduced by the injection of Med I into the portal vein; that of SA and MO was not. Med I, SA, and MO were activated by the liver, while APRL and HAG were not. Tween 20 inhibited Med I, SA, and MO, but not APRL and HAG. Proadifen (SKF 525A) inhibited Med I, but not SA and MO. The n-butyl boronic acid derivatives of SA, MO, and Med I were inactive. Med I, like SA and MO, appeared to have two hydroxyl groups in close proximity. It was concluded that Med I is neither HAG, APRL, SA, nor MO.

Inhibition of vasoconstriction by platelet activating factor in the in situ blood perfused rat mesentery

1. Perfusion pressure was measured in the in situ mesentery of anaesthetized rats perfused with blood at a constant 2 mL/min. 2. Increases in perfusion pressure were produced by mesenteric peri-arterial nerve stimulation at 10 Hz for 5 s at 2 min intervals and by bolus intra-arterial injections of the vasoconstrictors noradrenaline, angiotensin II and 5-hydroxytryptamine. 3. The intra-arterial infusion of platelet-activating factor (PAF) to produce a blood concentration of 3 X 10(-10) mol/L inhibited all responses to a similar extent. Intra-arterial prazosin (1-5 X 10(-9) mol/L), however, preferentially reduced responses to nerve stimulation and noradrenaline. 4. PAF at concentrations from 3 X 10(-11) to 10(-9) mol/L produced increasing inhibition of vasoconstrictor responses to nerve stimulation. The dose-response to PAF was shifted to the right by the concomitant intra-arterial infusion of the PAF antagonist SRI 63-441. 5. PAF at very low concentrations in vivo inhibits mesenteric vasoconstriction, produced by sympathetic nerve stimulation or various agonists, by a PAF-receptor mediated vasodilatation of the mesenteric vasculature.

Effects of endothelin on regional hemodynamics in conscious rats

Endothelin is a potent vasoactive peptide in anesthetized rats and isolated vascular smooth muscle. This study was performed to describe the hemodynamic effects of endothelin in conscious, freely moving rats. Endothelin (0.067-2 nmol/kg i.v.) produced long-lasting, dose-dependent increases in arterial pressure, mesenteric and, to a lesser degree, hindquarters vascular resistances and decreases in heart rate. We suggest that endothelin may play an important role in regulation of arterial pressure by modulating peripheral vasomotor tone.

Antagonism of the vasodilator effects of a platelet activating factor precursor in anesthetized spontaneously hypertensive rats

The hemodynamic effects of platelet activating factor (PAF), PAF antagonists and a precursor of PAF, 1-palmityl-2-acetyl-glycerol (PAG), were examined in pentobarbital-anesthetized spontaneously hypertensive rats to determine whether functionally significant amounts of PAF are produced via the cholinephosphotransferase pathway of PAF synthesis in vivo. Intravenous bolus doses of PAF, PAG and nitroprusside elicited hypotension and active mesenteric vasodilatation. Responses to PAG were slower in onset and longer in duration than those of PAF and nitroprusside. The specific PAF antagonists, CV-3988 and SRI 63-675, attenuated PAG- and PAF-, but not nitroprusside-induced changes in blood pressure and mesenteric flow/resistance. In contrast, captopril, which blocked the effects of angiotensin I, did not influence the hypotension caused by PAG, PAF and nitroprusside. The results suggest that the vasodilator effects of PAG are attributable to PAF produced from this alkylacetylglycerol, and the renin-angiotensin system does not appear to influence the biotransformation of PAG to PAF or the hypotensive action of PAF.

Cardiovascular pharmacology of thyrotropin releasing hormone

Thyrotropin releasing hormone (TRH) and its receptors are present in the cardiovascular nuclei of the brain as well as in the intermediolateral cell column of spinal cord. Anatomical, neurophysiological, functional and pharmacological studies suggest that TRH is a neurotransmitter/neuromodulator in the central nervous system. Administration of TRH to experimental animals or human subjects induces pressor and tachycardic responses and increases plasma levels of catecholamines. These effects are likely to be mediated by a central nervous system activation of the sympathoadrenomedullary system with no involvement of vasopressin or renin-angiotensin system. In the conscious rat, the TRH-induced pressor response is accompanied by an increment in cardiac output and a distinct change in organ blood flow, a hindquarter skeletal muscle vasodilation accompanied by renal and mesenteric vasoconstriction. The role of TRH in hypertension has not been studied. However, the extremely potent pressor and vasoconstrictor properties of TRH makes this tripeptide a candidate for neurotransmitters/modulators involved in the development and/or maintenance of hypertension. The role of TRH in the therapy of shock is at present controversial. Though preliminary experimental work raised hopes and expectations for therapeutic usage of TRH in shock and trauma, the more recent studies have shown no effect or a detrimental effect for TRH in some experimental shock states.

L-platelet activating factor induces changes on renal vascular resistance, vascular reactivity, and renin release in the isolated perfused rat kidney

Rat kidneys were isolated and perfused with a modified Krebs-Henseleit buffer containing 4% albumin. Perfusate recirculated except during L-platelet activating factor (L-PAF), angiotensin II (ang II), and norepinephrine (NE) infusions. L-PAF caused a dose-dependent decrease in renovascular resistance (RVR): -6 +/- 3% at 10(-9)M, -12 +/- 6% at 10(-8)M, -18 +/- 3% at 10(-7) and -20 +/- 7% at 10(-6)M. L-PAF increased immunoreactive PGE (iPGE) and thromboxane (iTXB) release into the venous effluent from 2.4 +/- 0.2 to 3.9 +/- 0.4 ng/min (p less than 0.05) and from 2.1 +/- 0.4 to 3.5 +/- 0.5 ng/min (p less than 0.05), respectively. Vasodilation by L-PAF (10(-7) M) in the presence of indomethacin (INDO) (5 microM) was enhanced compared to the non-INDO response (RVR change: L-PAF = -18 +/- 3% vs. L-PAF = -26 +/- 3%; p less than 0.05). As a control for specificity, the was infused at 10(-9) M, 10(-8) M, and 10(-7) M. None of these concentrations changed renal vascular resistance. To study the vascular receptor responsible for L-PAF-induced vasodilation, dose-response curves to NE and ang II were established with and without L-PAF (10(-7) M). The NE dose-response curve was unchanged by L-PAF, whereas the ang II dose-response curve was shifted to the right by one order of magnitude. In kidneys pretreated with INDO (5 microM), the L-PAF-induced shift of the ang II dose-response relation was increased to 2-3 orders of magnitude.(ABSTRACT TRUNCATED AT 250 WORDS)

The reno-hepatic axis of blood pressure control: further studies

A reno-hepatic axis of blood pressure (BP) control has been proposed primarily by two developments: the lag period between the injection of the antihypertensive neutral renomedullary lipid (ANRL) into the vena cava and the beginning of the drop of the BP is significantly reduced by injection of this lipid into the portal vein; and a reno-portal shunt (RPS) accelerates markedly the drop of the BP following unclipping the Goldblatt hypertensive rat. In the present studies, three maneuvers were performed: perfusion of the isolated, blood-drained rat liver with plasma containing ANRL and delivery of the plasma into the jugular vein of a hypertensive rat; injection of captopril, establishment of a RPS of a hypertensive rat (one-kidney, one-clip) followed by unclipping; and establishment of a RPS in a normal rat. The blood-drained liver was capable of decreasing significantly the lag period of ANRL. Captopril did not potentiate the antihypertensive action of the kidney when RPS was coupled with unclipping. Following the RPS, the BP of the normal rat dropped modestly over a 3-hour period. These results further support the concept of reno-hepatic axis of BP control.

Platelet activating factor vasoconstriction of dog kidney. Inhibition by alprazolam

Systemic administration of platelet activating factor (PAF; acetyl glyceryl ether phosphorylcholine) reduces renal blood flow, but the mechanism responsible for that effect has not been defined. To address that problem, we determined the effects on renal blood flow of PAF administered directly into the renal artery in pentobarbital (30 mg/kg)-anesthetized dogs. Bolus injections of PAF (0.2-0.8 microgram) caused transient renal vasoconstriction, reducing renal blood flow by 20 to 60% without altering systemic blood pressure; lyso-PAF (1 microgram) had no effect. The effects of PAF on renal blood flow were not altered by alpha-adrenergic blockade (phentolamine, 3 mg/kg) or by angiotensin II receptor blockade ([Sar1,Ala8]angiotensin II, 6 micrograms/kg/min), but they were increased in magnitude and duration by meclofenamate (5 mg/kg), a cyclooxygenase inhibitor. Methysergide (3 mg/kg), a serotonin antagonist, slightly reduced PAF effects, but a specific blocker of vascular serotonin receptors did not. Renal venous plasma platelet density was not altered by infusion of PAF into the renal artery at a dose (1-2 micrograms/min) that caused a sustained 20% renal blood flow decrease. Alprazolam, a benzodiazepine that competitively inhibited PAF-induced aggregation in canine platelet-rich plasma, also inhibited the renal vasoconstrictor action of PAF (0.8 mg/min, into the renal artery) but did not alter renal vasoconstrictor effects of norepinephrine or angiotensin II.

Inhibitory effect of the contralateral kidney on the renal pressor reflex evoked by ipsilateral renal artery stenosis

We previously identified a sympathoexcitatory pressor reflex that is evoked by acute (90 min) unilateral renal artery stenosis (RST). The reflex is strongly expressed in conscious rats when the renin-angiotensin system and baroreflexes are diminished by captopril and sinoaortic denervation, and is eliminated by denervation of the stenosed kidney. In the present study, dorsal rhizotomy inhibited the reflex, establishing the role of afferent renal nerves (ARN). During sustained (10 h) RST, the reflex reversed within 4 hours after onset of RST, but persisted after removal of the contralateral kidney. Hematocrit measurements did not support pressure diuresis as a mechanism for inhibition by the contralateral kidney. Other studies suggest that ARN signals and/or release of antihypertensive renomedullary lipid from the contralateral kidney are not involved in inhibition of the reflex. These data suggest that the contralateral kidney can oppose, at least acutely by an as yet undefined mechanism, an ARN-dependent pressor reflex evoked by ipsilateral renal artery stenosis.

The effects of PAF-acether on the cardiovascular system and their inhibition by a new highly specific PAF-acether receptor antagonist BN 52021

BN 52021, a new specific PAF-acether receptor antagonist, was evaluated on several cardiovascular models. BN 52021 antagonized PAF-acether-induced extravasation in rats. Inhibition of the hypotensive action of PAF-acether was obtained by administration of the antagonist, given preventively or curatively. In isolated guinea-pig hearts, BN 52021 inhibited the vasoconstriction induced by PAF-acether whereas a small inhibition was observed with papaverine. On the other hand, phosphodiesterase inhibitors were very effective against coronary vasoconstriction induced by vasopressin while BN 52021 was without effect. PAF-acether increased the tonus of rat isolated portal vein; this effect was inhibited by BN 52021, without any reduction in basal myogenic activity. In this model Ca2+ antagonists (D 600, diltiazem) showed a small inhibitory effect but they strongly reduced basal myogenic activity. Neither PAF-acether nor BN 52021 modified phenylephrine-induced contraction of the isolated rabbit aorta with or without endothelium demonstrating that endothelium-dependent relaxing factor is not related to PAF-acether. Our results suggest that BN 52021 specifically block the cardiovascular effects of PAF-acether. This agent may thus be an useful tool for a better understanding of the role of PAF-acether in hemodynamic changes involved in anaphylaxis or shock.

Sympathetic inhibition after surgical reversal of renovascular hypertension in rats. Role of vagal nerves

The present study was undertaken to study the changes in efferent renal sympathetic nerve activity (RSNA) and heart rate (HR) during the acute fall in blood pressure after surgical reversal of two-kidney, one-clip renovascular hypertension in rats, and to explore if cardiopulmonary vagal afferents can influence sympathetic outflow in this situation. In 14 rats with a MAP of 199 +/- 6 mmHg the renal artery clip was surgically removed and after 90 min MAP had decreased to 109 +/- 7 mmHg. The HR had then decreased by 26 +/- 15 beats min-1 (P less than 0.05) and RSNA did not increase as expected, but was somewhat reduced (84 +/- 8% of control). In contrast, lowering pressure with nitroprusside to the same extent produced large and significant increases in HR and RSNA in seven other renal hypertensive rats. Acute bilateral vagotomy was performed in seven of the declipped rats, which induced an immediate increase in MAP (+35 +/- 10 mmHg, P less than 0.05), HR +28 +/- 10 beats min-1, P less than 0.05) and RSNA (+51 +/- 19%, P less than 0.05). A few minutes afterwards the vagotomy pressure again started to fall and was, after another 90 min 107 +/- 10 mmHg together with slight reductions in HR and RSNA. Another group of six rats were initially exposed to bilateral vagotomy. In these vagotomized rats declipping also induced hypotension (204 +/- 10 to 95 +/- 7 mmHg, P less than 0.001), together with tendencies of decreases in HR (425 +/- 17 to 397 +/- 17 beats min-1) and in RSNA (-4 +/- 6%).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (platelet activating factor) on cardiac function in perfused guinea-pig heart

The direct cardiac action of 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (PAF) was studied in isolated perfused guinea-pig heart preparations. PAF produced a fall in left ventricular pressure, decreases in the rate of rise of the left ventricular pressure (dp/dt) and coronary flow, but had no effect on heart rate. These results indicate that PAF is a cardiodepressant with inotropic selectivity and this effect on heart is blocked by CV-3988, a specific PAF antagonist.

Mechanism(s) of the hypotensive effect of synthetic 1-O-octadecyl-2-O-acetyl-glycero-3-phosphorylcholine

The mechanism of the hypotensive effect of 1-O-octadecyl-2-O-acetyl-glycero-3-phosphorylcholine (C18- AGPC ) was examined. Synthetic C18- AGPC caused dose-dependent hypotension in conscious rats. The activity was almost the same in DOCA and renal hypertensive rats. This suggests that it is not a renin inhibitor. Hypotension also appeared in pithed rats. This suggests that the effect is not due to a central mechanism. Hypotension did not result from platelet aggregation or bronchial constriction. Since C18- AGPC suppressed not only the pressor response to noradrenaline but also to angiotensin II and vasopressin, and furthermore, did not disturb the dose-response curve of noradrenaline in the isolated aorta, the possibility of the agent being an alpha-adrenergic antagonist is ruled out. In the PGF2 alpha-contracted rat aorta. C18- AGPC caused marked vasodilation, which disappeared after removal of the endothelium. Perfusion pressure decreased in the blood-perfused rat hindquarters but not in the Tyrode solution-perfused ones. C18- AGPC induced a positive inotropic effect in isolated rat atrium. The hypotensive effect of synthetic C18- AGPC seems to be mainly due to endothelium-dependent vasodilation.