John A. Bellan

Penile erections induced by vasoactive intestinal peptide and sodium nitroprusside

SummaryThe use of vasoactive intestinal peptide (VIP), sodium nitroprusside (SNP), and the reference combination of papaverine, prostaglandin E1, and phentolamine was studied in 22 adult cats. The maximal erectile response (intracavernous pressure, penile length, and rigidity) was produced by intracavernous injection of a combination of 1.65 mg papaverine, 0.5μg PGE1, and 25μg phentolamine. This combination was considered as “control” in order to compare the effect of other agents. VIP and SNP increased the intracavernous pressure and caused erection in a dose-dependent manner with a maximal response obtained with 5μg VIP or 10μg SNP. The duration of peak erection and the total duration of drug effect were significantly shorter with VIP and SNP than with the reference combination (P<0.01). Epinephrine (30μg) reversed the effects of SNP and significantly shortened the duration of peak action and total effect (P<0.05). This study supports the use of an in vivo feline model for the evaluation of vasoactive agents and demonstrates that the intracavernous injection of either VIP or SNP can induce penile erection in the adult cat.

Prostacyclin, thromboxane A2, and prostaglandin E2 formation in atherosclerotic human carotid artery.

Prostaglandin (PG) formation In 16 atherosclerotic human carotid endarterectomy specimens was compared systematically with that of normal carotid artery from seven white pigs and six rhesus monkeys. Prostacyclin (PGI2) formation (plcomoles 6-keto- PGF1a/2 mln/100 μg homogenate protein plus 2 mM glutathlone [GSH]) of nonatheromatous Intlma adjacent proximal (276 ± 32, mean ± SEM) or distal (271 ± 14) to carotid plaque was comparable to that of normal carotid artery from white pig (272 ± 25, NS) and rhesus monkey (219 ± 41, NS), and was greater than stenotlc Intlma (156 ±17, p<0.01), sublntlmal plaque (168 ± 14, p<0.01), and ulceratlon (65 ± 16, p<0.01). GSH modulated PGI2 synthesis In all carotid specimens except areas of ulceratlon (p<0.05), but did not restore PGI2 formation In atheromatous fractions to basal level. No detectable arterial thromboxane A2 (TXA2) formation or GSH-dependent PGE2 Isomerase activity was observed. The decrement In atherosclerotic carotid artery PGI2 formation was focal (confined to the plaque) and may have been related to loss of effective GSH modulation. These conditions could contribute to a localized Imbalance between arterial PGI2 and platelet TXA2 with adverse vascular thromboregulatory consequences.

Selective and complete blockade of acetylcholine-induced relaxation in rabbit aortic rings by Nω-nitro-L-arginine but not by glybenclamide

This study addressed the possibility that acetylcholine-induced relaxation in the rabbit aorta is mediated by dual mechanisms: one N omega-nitro-L-arginine (NLA)-sensitive, the other glybenclamide-sensitive. Acetylcholine, nitroglycerin and BRL38227 (lemakalim), an activator of glybenclamide-sensitive potassium channels, were added to an organ bath containing rabbit aortic rings in a cumulative manner in the absence or presence of NLA and/or glybenclamide. NLA inhibited acetylcholine-induced relaxation and potentiated the relaxant response to nitroglycerin. BRL38227 caused a dose-dependent relaxation in rabbit aortic rings, and 30 microM glybenclamide produced essentially complete inhibition of this relaxation. Glybenclamide alone produced no inhibition of acetylcholine-induced relaxation. These results indicate that glybenclamide-sensitive potassium channels in the rabbit aorta play no role in mediating the relaxant response to acetylcholine, while NLA can produce a selective and essentially complete blockade of the relaxant response to acetylcholine in the rabbit aorta.

Concentration-activity profile of the modulation of cyclooxygenase product formation by reduced glutathione in microsomal fractions from the goat lung

Age-related changes in pulmonary formation of arachidonic acid (AA) metabolites are thought to play an important role in regulating cardiopulmonary function. This study addresses the potential role of reduced glutathione (GSH) in modulating cyclooxygenase product formation in the developing lung. Prostaglandin H2 (PGH2) metabolism was studied in microsomal fractions isolated from the lungs of unventilated fetal, neonatal and adult goats. GSH-dependent PGH2 to PGE2 isomerase activity in microsomal fractions from the perinatal (fetal and neonatal) goat lung was not saturable with respect to GSH and can respond to changes in GSH concentration over the range of 0.01 to 30 mM, which encompasses the full range the intracellular GSH levels reported in the literature. However, in fractions from the adult, a lower rate of PGE2 formation is observed at higher GSH concentrations. In addition, the tissue levels of GSH exhibited developmental stage-related differences with fetal being higher than neonatal or adult. The present observations may have physiologic relevance, in that decreases in pulmonary GSH levels after birth may contribute to decreases in plasma PGE2 levels by decreasing pulmonary PGE2 synthesis, thereby contributing to closure of the ductus arteriosus; conversely, increased GSH levels associated with hyperoxia may contribute to persistence of ductal patency. Formation of 6-keto-PGF1 alpha and of TXB2 (the stable metabolites of prostacyclin and TXA2) was decreased when PGE2 formation was increased by GSH activation of PGE2 isomerase in fractions isolated from all three developmental stages. A similar pattern of product formation was observed when AA was employed as substrate. These data suggest the possibility that changes in GSH concentration may modulate eicosanoid formation in cells that contain GSH-dependent PGE2 isomerase, as well as either or both prostacyclin or thromboxane synthase(s).

Prostacyclin and thromboxane A2 formation by atherosclerotic carotid artery: Comparison with normal aorta, saphenous vein, and platelets *

Prostacyclin (PGI2) and thromboxane A2 (TxA2) formation by whole-tissue segments of nine carotid endarterectomy specimens (CES), five normal aortic specimens (NAS), six saphenous vein specimens (SVS), and four platelet samples were determined by incubation with 10 mumol/L 1-14C-radiolabeled prostaglandin endoperoxide H2 (PGH2), and in other experiments with and without 10 mumol/L of CGS 13080, a TxA2 synthase inhibitor. PGI2 formation (expressed as picomoles 6-keto-PGF1 alpha/2-min incubation per sample) by nonatheromatous proximal intima of CES (307 +/- 23, mean +/- standard error) and distal intima of CES (260 +/- 22) was not statistically different; however, it was greater than atheromatous transitional plaque (159 +/- 13 pmol) (p less than 0.01) and ulceration regions (140 +/- 15 pmol) (p less than 0.01) of CES, NAS (204 +/- 16 pmol) (p less than 0.01), and SVS (165 +/- 9 pmol) (p less than 0.01). TxA2 formation (expressed as picomoles TxB2/2-min incubation per sample) by CES ulceration (51 +/- 2 pmol) was low but greater than proximal (17 +/- 2 pmol) (p less than 0.01), distal (19 +/- 3 pmol) (p less than 0.01), and transitional (23 +/- 3 pmol) (p less than 0.01) regions. TxA2 formation by NAS and SVS was not detected (less than 10 pmol). CGS 13080 inhibited TxA2 formation by CES below the limits of detection. Incubation of 1.9 x 10(5) intact platelets with 10 mumol/L of PGH2 formed a quantity of TxA2 equal to that of CES ulceration.(ABSTRACT TRUNCATED AT 250 WORDS)

Nisoldipine inhibits adrenergic responses in the hindquarters vascular bed of the cat

The effects of the calcium entry blocking agent nisoldipine on adrenergic vasoconstrictor responses were investigated in the hindquarters vascular bed of the cat under conditions of controlled blood flow. Nisoldipine dilated the hindquarters vascular bed and inhibited vasoconstrictor responses to Bay K 8644, a nifedipine analog which promotes calcium entry. During infusion of nisoldipine, vasoconstrictor responses to sympathetic nerve stimulation, norepinephrine, and tyramine were inhibited in a reversible manner. In addition to blocking responses to nerve-released and exogenous norepinephrine, the calcium entry antagonist decreased responses to methoxamine and BHT 933, alpha 1- and alpha 2-adrenoceptor agonists. Responses to methoxamine were reduced by prazosin, an alpha 1-adrenoceptor antagonist, but not by yohimbine, an alpha 2-adrenoceptor blocking agent, whereas responses to BHT 933 were decreased by yohimbine but not by prazosin. The results of these studies suggest that vasoconstrictor responses to neuronally released and exogenous norepinephrine, as well as to selective alpha 1- and alpha 2-adrenoceptor agonists, are dependent in part on an extracellular source of calcium in resistance vessels of the feline hindquarters vascular bed. The inhibitory effect of nisoldipine on vasoconstrictor responses to neuronally released norepinephrine may be important in the antihypertensive actions of calcium entry blocking agents.

Inhibition of pulmonary thromboxane A2 synthase activity and airway responses by CGS 13080

The effects of CGS 13080, a thromboxane (TXA2) synthase inhibitor, on airway responses to arachidonic acid (AA) were investigated in the anesthetized cat. Feline and human lung microsomal fraction exhibited prostaglandin I2 (PGI2, prostacyclin), and TXA2 synthase activities, and human platelet microsomal fractions exhibited TXA2 synthase activity. Cat and human lung microsomal fractions, but not human platelets, exhibited the presence of GSH-dependent PGE2 isomerase activity. CGS 13080 inhibited TXA2 synthase activity in all three microsomal fractions in a concentration-dependent manner. The increases in transpulmonary pressure and lung resistance and decreases in dynamic compliance in response to AA were decreased significantly by CGS 13080. These data suggest that the bronchoconstrictor actions of AA are mediated in large part by the formation of TXA2. The data further indicate that cyclooxygenase products other than TXA2 are involved in the bronchoconstrictor response to AA since meclofenamate had greater inhibitory activity than did CGS 13080. Moreover, the effects of CGS 13080 were due to inhibition of TXA2 synthase rather than an effect on TXA2 receptors, since airway responses to the TXA2 mimic, U46619, were not altered. The present data show that CGS 13080 inhibits TXA2 synthase activity without altering cyclooxygenase, PGI2 synthase, or GSH-dependent PGE2 isomerase activities. The data further indicate that in vivo administration of CGS 13080 may selectively increase PGI2 synthase activity.

Prostaglandin Endoperoxide Metabolism by the Human Carotid Artery

Prostacyclin (PGI2) synthesis is associated with maintenance of normal vascular function, and a fall in human vascular PGI2 synthesis has been associated with atherosclerosis (1,2). It has been proposed that a decrease in PGI2 production results in the loss or impairment of the vascular defense mechanism against platelet deposition which might favor thrombosis and the progression of atherosclerosis (1). A decrease in bioassayable PGI2 production by vascular sections from fatty streak (pre-atherosclerotic) and advanced atherosclerotic lesions has been reported (3). Little if any information is available concerning the activity of prostacyclin synthetase in vascular tissue immediately adjacent to advanced atherosclerotic plaques. An increase or decrease in the PGI2 generating capacity of these adjacent areas could represent a protective mechanism against or a predisposing factor for, respectively, the progression of atherosclerotic disease into these areas.

Influence of SQ 29,548 on vasoconstrictor responses in the hindquarters vascular bed of the cat

The effects of SQ 29,548, a thromboxane (TX) receptor blocking agent, on vasoconstrictor responses were investigated under conditions of controlled blood flow in the hindquarters vascular bed of the cat. Intravenous injection of SQ 29,548 at a dose of 0.1 mg/kg had no significant effect on systemic arterial pressure but caused a significant reduction in hindquarters perfusion pressure. Injection of the TXA2 mimics, U44069 and U46619, into the perfusion circuit caused dose-dependent increases in hindquarters perfusion pressure with U46619 being approximately 3 times more potent than U44069. Following the administration of SQ 29,548, pressor responses to both U44069 and U46619 were reduced significantly, and the dose-response curves for both TXA2 mimics were shifted to the right in a parallel fashion. SQ 29,548 had no significant effect on the dose-dependent increases in hindquarters perfusion pressure in response to angiotensin II or BAY K8644, a nifedipine analog which promotes calcium entry. The TXA2 receptor blocking agent had no significant effect on increases in hindquarters perfusion pressure in response to angiotensin II or BAY K8644, a nifedipine analog which promotes calcium entry. The TXA2 receptor blocking agent had no significant effect on increases in hindquarters perfusion pressure in response to sympathetic nerve stimulation or injections of norepinephrine and tyramine. These findings suggest that SQ 29,548 blocks responses to the TXA2 mimics in a competitive manner, and that this inhibitory effect is selective.(ABSTRACT TRUNCATED AT 250 WORDS)

Prostaglandins and Defects in Vascular Function

Atherosclerotic arterial lesions have been reported to exhibit altered arachidonic acid metabolism (1–4). These alterations have consistently involved decreases in arterial prostacyclin (prostaglandin I2 or PGI2) synthesis; arterial thromboxane A2 (TXA2) formation (3) and increased PGE2 formation (2) have also been associated with atherosclerotic lesions. Conversely, it has been recently reported that total urinary excretion of 2,3-dinor-6-keto-PGFlα, one of the metabolites of PGI2 produced. in vivo, is higher in patients with severe atherosclerosis than normal subjects (5). It was suggested that there was an increase in PGI2 formation due to stimulation of PGI2 synthetase as there may be increased platelet-endothelial interactions in patients with severe atherosclerosis. The present study reports that there are graduated decreases in human arterial PGI2 synthetase activity within atherosclerotic plaque but that these decreases are focal, as the arterial regions immediately adjacent to the plaque exhibit normal PGI2 synthetase activity.