Effect of acetylcholine on vascular capacity in the dog.

Acetylcholine produces venoconstriction of isolated vein strip preparations. However, the effect of acetylcholine on overall vascular capacity is not known. To investigate this effect and to elucidate the mechanisms involved, 38 anesthetized dogs were placed on total cardiopulmonary bypass, splenectomized, and given intraarterial infusions of acetylcholine. Almost all of the effect on vascular volume was found to be in the splanchnic circulation, because in four eviscerated animals there was no significant change in capacity. In the animals in which the mesenteric arteries were cannulated to provide constant inflow, and the hepatic vein was cannulated to measure splanchnic venus outflow, acetylcholine infusion for 5 and 21 min increased splanchnic vascular capacity in all animals by 107 +/- 28 (SEM)ml (p less than 0.01) and 291 +/- 132 ml (p less than 0.05), respectively. This increase in splanchnic vascular volume was associated with a rapid and sustained increase in transhepatic resistance to portal blood flow for the duration of the infusions (p less than 0.01). In the animals in which the portal vein was vented proximal to the liver, no significant volume change occurred in the splanchnic vasculature with acetylcholine infusion. Increasing hepatic venous pressure to elevate portal venous pressure to the same level as that achieved with acetylcholine resulted in a similar increase in splanchnic vascular volume. Atropine, but not adrenergic blockade, blocked the acetylcholine-induced volume retention, indicating that the effect of acetylcholine was direct. Substantial volume retention was also achieved by stimulation of the distal ends of the sectioned cervical vagi. Thus, acetylcholine administration directly increases transhepatic resistance and is associated with a pooling of volume in the splanchnic vasculature that would, in the intact animal, result in a decrease in venous return to the heart.     

Effect of ouabain on total vascular capacity in the dog.

Whereas the cardiac effects of digitalis glycosides have been extensively studied, less is known of the extracardiac effects of the drug, in particular the effects on vascular capacity. We investigated the effects of parenteral ouabain on vascular capacity in the dog with particular emphasis on transhepatic resistance and its interaction with splanchnic and total intravascular capacity. We studied 49 dogs on total cardiopulmonary bypass in which the splanchnic and extrasplanchnic circulations could be separately perfused and drained, and the portal vein could be vented to systemic venous pressure. The results indicate: (a) ouabain produces a net central displacement of blood at 30 min after administration of 150 +/- 70 ml (SEM), (b) this displacement occurs despite a substantial increase in transhepatic resistance, although the early rise in transhepatic resistance may delay the net displacement of blood, and (c) the decrease of overall vascular capacity is due to an effect of ouabain on the capacitance vessels of both the splanchnic and extrasplanchnic circulations. The peripheral vascular capacity effects of ouabain may therefore contribute to overall cardiac performance.     

Evidence that prostacyclin modulates the vascular actions of calcium in man.

Increases in extracellular calcium (Ca++) can alter vascular tone, and thus may result in increased blood pressure (Bp) and reduced renal blood flow (RBF). Ca++ can stimulate prostaglandin E2 (PGE2) and/or prostacyclin (PGI2) release in vitro, which may modulate Ca++ vascular effects. However, in man, the effect of Ca++ on PG release is not known. To study this, 14 volunteers received low-dose (2 mg/kg Ca++ gluconate) or high-dose (8 mg/kg) Ca++ infusions. The low-dose Ca++ infusion did not alter systemic or renal hemodynamics, but selectively stimulated PGI2, as reflected by the stable metabolite 6-keto-PGF1 alpha in urine (159 +/- 21-244 +/- 30 ng/g creatinine, P less than 0.02). The same Ca++ infusion given during cyclooxygenase blockade with indomethacin or ibuprofen was not associated with a rise in PGI2 and produced a rise in Bp and fall in RBF. However, sulindac, reported to be a weaker renal PG inhibitor, did not prevent the Ca++ -induced PGI2 stimulation (129 +/- 33-283 +/- 90, P less than 0.02), and RBF was maintained despite similar increases in Bp. The high-dose Ca++ infusion produced an increase in mean Bp without a change in cardiac output, and stimulated urinary 6-keto-PGF1 alpha to values greater than that produced by the 2-mg/kg Ca++ dose (330 +/- 45 vs. 244 +/- 30, P less than 0.05). In contrast, urinary PGE2 levels did not change. A Ca++ blocker, nifedipine, alone had no effect on Bp or urinary 6-keto-PGF1 alpha levels, but completely prevented the Ca++ -induced rise in Bp and 6-keto-PGF1 alpha excretion (158 +/- 30 vs. 182 +/- 38, P greater than 0.2). However, the rise in 6-keto-PGF1 alpha was not altered by the alpha 1 antagonist prazosin (159 +/- 21-258 +/- 23, P less than 0.02), suggesting that calcium entry and not alpha 1 receptor activation mediates Ca++ pressor and PGI2 stimulatory effects. These data indicate a new vascular regulatory system in which PGI2 modulates the systemic and renal vascular actions of calcium in man.     

Inhibition of epinephrine-exacerbated coronary thrombus formation by prostacyclin in the dog

We studied the effect of intravenous prostacyclin (PGI2) infusion on epinephrine exacerbation of platelet-mediated acute coronary thrombus formation in an in vivo canine model of coronary artery stenosis. Platelet thrombi form in mechanically stenosed dog circumflex coronary arteries, producing cyclical reductions in coronary blood flow (CRF) as measured with an electromagnetic flowmeter probe. In the present study nine of 10 dogs exhibited spontaneously occurring CRF. With epinephrine (E) infusions of 10 micrograms/min CRF frequency increased 64% (p less than 0.025), CRF magnitude increased 27.1% (p less than 0.05), and the rate of flow decline increased 112.8%, indicating that the rate of thrombus formation increased with E infusion (p less than 0.005). When an identical E infusion was accompanied by simultaneous PGI2 infusion (150 ng/kg/min), CRF were abolished in nine of 10 dogs and markedly inhibited in the other. After the 10 min of simultaneous E and PGI2 infusion, E infusion alone was continued. There was a recurrence of CRF within 2.49 +/- 0.89 min after cessation of PGI2 infusion. However, CRF frequency in the subsequent 10 min was less than the frequency of CRF during the initial E infusion (p less than 0.10) and not significantly different from that of the control period (no E infusion). The rate of flow decline and magnitude during the final E infusion after cessation of PGI2 infusion were not significantly different from those of the initial E infusion. Prostacyclin infusion in the coronary care unit may be potentially beneficial in cases of acute myocardial ischemia where elevated catecholamines are a thrombogenic stimulus.     

Effect of flow and vascular heterogeneity on glucose metabolism in isolated dog hearts

In non-ischaemic myocardium glucose uptake is assumed to be proportional to blood flow. We investigated the effect of regional vascular heterogeneity on glucose metabolism at various flow rates in the isolated blood-perfused dog hearts. Aortic bolus injections contained an intravascular reference tracer (albumin) and two of three glucoses: L-glucose (an extracellular tracer), D-glucose and 2-deoxy-D-glucose. Flow ranged from 0.5 to 2 4 ml min(-1) g(-1). Vascular heterogeneity was calculated from the albumin outflow dilution curve. A three-region convection-diffusion from the abumin outflow dilution curve. A three-region, convection-diffusion model was fitted to outflow dilution curves to estimate glucose metabolic rate (consumption). The results of 2-deoxy-D-glucose experiments showed that the lumped constant was dependent on flow, glucose metabolic rate was proportional to flow and dependent on the heterogeneity of the myocardial vasculature. The results support the views that without accounting the regional flow heterogeneity, glucose metabolic rate will be underestimated.     

Effect of prostacyclin on renal kallikrein release in man

The aim of this research was to study exogenous prostacyclin effect on urinary kallikrein excretion (UKK) in man, to define whether prostacyclin-induced renin release and/or endogenously released cyclooxygenase products were responsible for prostacyclin-induced enhancement of UKK, to determine furosemide effect on UKK. Prostacyclin was infused in eight healthy men and repeated after propranolol and indomethacin treatment. Prostacyclin caused a dose-dependent increase of UKK. Pretreatment with propranolol and indomethacin did not affect prostacyclin-induced enhancement of UKK, although it reduced absolute values of plasma renin activity. Furosemide increased UKK and simultaneously urinary 6-keto-prostaglandin F1 alpha. We conclude that prostacyclin induces an increase in UKK in a dose-dependent manner; furosemide-induced renal prostacyclin synthesis is temporally related to enhancement of UKK; partial dissociation of UKK from plasma renin activity under propranolol and indomethacin treatment and in response to furosemide might suggest a direct effect of prostacyclin on UKK.     

Mechanism of vascular actions of prostacyclin in the rat isolated perfused mesenteric arteries

Prostacyclin (PGI2) did not alter the basal perfusion pressure in the isolated rat mesenteric arteries perfused with Krebs' solution, but produced a biphasic effect in arteries preconstricted with norepinephrine or arginine vasopressin: constriction, then prolonged dilation. Both these components of PGI2 effect were diminished in arteries denuded of their endothelia by a 10 min perfusion with distilled water or p-bromophenacyl bromide (10 microM). The present study elucidates the mechanism of these PGI2 actions. Indomethacin (0.28 microM) SQ 29548 (1 microM, thromboxane A2 receptor antagonist), saralasin (1 microM, angiotensin II receptor antagonist) or the free radical scavengers, superoxide dismutase (60 U/ml) and catalase (40 U/ml) did not inhibit the initial vasoconstriction, suggesting it was not mediated through endothelially generated thromboxane A2, angiotensin II or oxygen-derived free radicals. However, ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (50 microM; Ca++ chelating agent), 8-(diethyl-amino)octyl 3,4,5-trimethoxy benzoate (10 microM; intracellular Ca++ antagonist), or neomycin (5 mM; phospholipase-C inhibitor) abolished the vasoconstriction. Ouabain (0.5 mM) did not affect the vasodilation, but perfusion with excess (50 mM) or 0 K+ Krebs' solution abolished it, suggesting this PGI2 action involves changes in membrane K+ conductance via a mechanism independent of Na+/K+ adenosine triphosphatase. Vasodilation evoked by BRL 34915 (K+ channel activator) was similarly attenuated under these conditions, but not by ouabain. Furthermore, procaine (1 mM; nonspecific K+ channel inhibitor), but not apamin (0.5 microM) or tetraethylammonium (10 mM) blocked PGI2- and BRL 34915-induced vasodilation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of nitroglycerin on human vascular prostacyclin and thromboxane A2 generation

We examined the effects of NTG on human saphenous and umbilical vein PGI2 and TXA2 generation. Vascular rings from both types of vessels generated TXA2 in addition to PGI2. The treatment of vascular rings with NTG in concentrations of 5 to 1000 ng/ml and subsequent incubation with AA caused a significant increase in PGI2 in the supernatants, as identified by bioassay (platelet aggregation inhibition) and by measurement of 6-keto-PGF1 alpha (stable hydrolysis product of PGI2). The maximum increase in PGI2 was observed with therapeutic concentrations of NTG, i.e., 5 to 10 ng/ml. The levels of vessel wall-generated TXB2 (stable metabolite of TXA2) were not affected by NTG treatment. A prior incubation of vascular rings with indomethacin abolished the increase in PGI2 after NTG treatment. In contrast, incubation of vascular rings with OKY 1581 (selective TXA2 inhibitor) resulted in a significant additional increase in PGI2 release but no change in TXB2 levels after NTG treatment. These studies indicate important effects of NTG on vascular PGI2 generation but not on TXA2 generation.     

Effect of experimental azotemia on renal clearance of furosemide in the dog.

The clearance of furosemide (F), whose renal tubular transport shares the classical characteristics of the organic acid system, was determined in dogs with varying degrees of azotemia and compared with tetraethylammonium (TEA), an organic base. Two normal and eight azotemic dogs [blood urea nitrogen (BUN), 12-273] were studied. Azotemia was produced by bilateral uretero-venous anastomoses. The left renal vein and ureter were cannulated and renal blood flow (RBF) was measured by electromagnetic flowmeter. Simultaneous left renal clearances (C) of subpharmacological doses of TEA-14C and furosemide-14C were determined at seven 30-minute intervals. Initial loading doses were followed by continuous maintenance infusions. For TEA, clearance (1.5 ml/min-g +/- 0.2 S.E.M.) and extraction (E) (0.83 +/- 0.02) are independent of the degree of azotemia. Renal plasma flow (RPF), calculated as CTEA/ETEA, agreed closely with directly measured RPF (2.0 ml/g-min +/- 0.3). RPF was independent of azotemia. To allow for individual differences in the animals in RPF, the ratio CTEA/CF was used. CF (1.07-0.17 ml/min-g) and EF (0.54-0.06) decreased as a linear function of the increase in uremic serum: (see article). Furosemide and its principle metabolite were greater than or equal to 97% of the furosemide portion of the radioactivity. The metabolite did not increase with time in either plasma or urine. After acute administration of exogenous urea to two dogs (BUN 170 and 253) CTEA/CF was unrelated to BUN. Thus, the CF decreases proportionately with progressive azotemia and is not related to RBF, exogenous urea or metabolite. This suppression of renal tubular secretion of furosemide may partially account for reduced therapeutic efficacy of furosemide in azotemia.     

Role of the vascular endothelium in the contractile response to prostacyclin in the isolated rat aorta

Prostacyclin (PGI2) relaxes vascular smooth muscle in several species but, in high doses, PGI2 has been reported to contract several isolated arteries. Vascular endothelium is known to be obligatory for the vasodilatory responses to acetylcholine and several other substances. We therefore investigated the contractile effect of various prostanoids on rat abdominal aorta in which the endothelium was left intact or was removed. PGI2 (4-2000 ng/ml), 6-keto-prostaglandin (PG) E1, PGE1 and PGE2 (4-800 ng/ml) contracted both intact and de-endothelialized aortic segments in a dose-dependent manner. PGI2 (8-2000 ng/ml) increased the force generated by aortic rings with intact endothelium from 77.3 +/- 24.6 to 685 +/- 99.2 mg. The response to similar doses of PGI2 in aortic rings with the endothelium removed was reduced significantly (22.7 +/- 14.1 to 260 +/- 116.4 mg). This contractile response to PGI2 in both intact and de-endothelialized aortic rings was abolished by indomethacin pretreatment (20 micrograms/ml for 30 min) and was also blocked completely by the thromboxane receptor antagonist SQ 29548 (100 ng/ml). In contrast, the thromboxane synthase inhibitor OKY 1581 (2.5 micrograms/ml) did not significantly reduce the contractile response to PGI2. Unlike PGI2, the force generated by PGE2 (4-800 ng/ml) in aortic rings with intact endothelium (0-550.0 +/- 107.2 mg) was not significantly different from that generated by aortic rings without endothelium (35.0 +/- 23.6 to 650.0 +/- 193.2 mg).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of glucagon on glucose production during insulin deficiency in the dog.

The aim of the present experiments was to determine the effects of basal glucagon on glucose production after induction of prolonged insulin lack in normal conscious dogs fasted overnight. A selective deficiency of insulin or a combined deficiency of both pancreatic hormones was created by infusing somatostatin alone or in combination with an intraportal replacement infusion of glucagon. Glucose production (GP) was measured by a primed constant infusion of [3H-3]glucose, and gluconeogenesis (GNG) was assessed by determining the conversion rate of circulating [14C]alanine and [14C]lactate into [14C]glucose. When insulin deficiency was induced in the presence of basal glucagon the latter hormone caused GP to double and then to decline so that after 4 h it had returned to the conrol rate. The conversion of alanine and lactate into glucose, on the other hand, increased throughout the period of insulin lack. Withdrawal of glucagon after GP had normalized resulted in a 40% fall in GP, a 37% decrease in GNG, and a marked decrease in the plasma glucose concentration. Induction of insulin deficiency in the absence of basal glucagon resulted in an initial (30%) drop in GP followed by a restoration of normal GP after 2--3 h and moderately enhanced glucose formation from alanine and lactate. It can be concluded that (a) the effect of relative hyperglucagonemia on GP is short-lived; (b) the waning of the effect of glucagon is attributable solely to a diminution of glycogenolysis because GNG remains stimulated; (c) basal glucagon markedly enhances the GNG stimulation apparent after induction of insulin deficiency; and (d) basal glucagon worsens the hyperglycemia pursuant on the induction of insulin deficiency both by triggering an initial overproduction of glucose and by maintaining the basal production rate thereafter.     

Release of prostacyclin after erythrocyte adhesion to cultured vascular endothelium

Endothelial cell damage is considered to be the initial step in the genesis of thrombosis and atherosclerosis. Recently, the adhesion of erythrocytes from patients with diabetes or sickle cell anemia to endothelial cells was found to be increased and correlated with the severity of vascular complications. We have measured by radioimmunoassay the release of 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) as an index of prostacyclin (PGI2) production, during red cell adhesion to endothelial cells in culture. The amount of 6-keto-PGF1 alpha released after incubation with normal red cells was similar to that observed with buffer (1.07 +/- 0.32 nmol/10(6) endothelial cells). However, after the adhesion of erythrocytes from patients with diabetes or sickle cell anemia, the amount of 6-keto-PGF1 alpha produced was significantly increased (P less than 0.01) and was correlated with the extent of erythrocyte adhesion (P less than 0.05). Tritium-labeled PGI2 was found to bind to erythrocytes, and the binding was time and concentration dependent. PGI2 release was inhibited by the cyclooxygenase inhibitor (flurbiprofen), whereas red cell adhesion remained unchanged. Fibrinogen potentiated erythrocyte adhesion and PGI2 production. The increase in PGI2 production after the adhesion of red cells from patients with diabetes or sickle cell anemia to endothelial cells indicates that endothelium may be damaged by abnormal erythrocyte adhesion.