Peter A. Kot

Inhibition of myointimal proliferation of the rat carotid artery by the peptides, angiopeptin and BIM 23034☆

Proliferation of vascular smooth muscle is an early and major event in the formation of an atherosclerotic lesion. Here we report for the first time the inhibitory effects on myointimal proliferation of the rat carotid artery by a synthetic peptide, angiopeptin, and its closely related congener, BIM 23034. Proliferation was initiated in the carotid artery of anesthetized rats by air-drying of the endothelium. After 15 days the rats were killed and the carotid artery was pressure-fixed and subjected to morphologic analysis for evaluation of the degree of myointimal thickening. Five synthetic somatostatin-like peptides were tested by pretreating rats (20 and 50 micrograms/kg/rat s.c. daily) for 2 days prior to and for 5 days after the endothelial injury. Angiopeptin and the closely related octapeptide (BIM 23034) significantly inhibited myointimal thickening. Angiopeptin was also effective when the pretreatment period was reduced from 2 days to 30 min. The inhibitory effect of angiopeptin was further confirmed in an additional experiment involving [3H]thymidine incorporation. In this experiment angiopeptin (100 micrograms/kg/day s.c.) was also administered for 2 days prior to and five days following the endothelial injury and it significantly inhibited thymidine uptake. All the peptides tested inhibit the release of growth hormone. However, only angiopeptin and BIM 23034 inhibited myointimal proliferation. Thus the effect of angiopeptin and its congener is unlikely to be mediated through growth hormone. Since angiopeptin inhibits myointimal proliferation it may have clinical utility in preventing restenosis following angioplasty and coronary artery by-pass procedures.

Effects of arachidonic acid on systemic arterial pressure, myocardial contractility and platelets in the dog.

Summary The bisenoic prostaglandin precursor, arachidonic acid (AA), in a single dose intravenously, produced a marked vasodepres-sor response in dogs, and a weak and variable effect on myocardial contractility. This response differed from the vasodepressor effect of PGE2 in that the onset of effect was delayed (15 sec for AA, 4.5 sec for PGE2) and PGE2 always caused a pronounced increase in myocardial contractility. Arachidonic acid caused thrombocytopenia and increased aggregability of platelets. All AA effects were inhibited by aspirin. The monoenoic prostaglandin precursor, dihomo-γ-linolenic acid, in doses equivalent to that of AA, had no effects. The data suggest that AA exerts its effects through conversion to an intermediate in the biosynthesis of PGE2 and not PGE2 itself. This work was supported, in part, by the Educational Foundation of America.

Cardiovascular and Platelet Responses in the Dog to the Monoenoic Prostaglandin Precursor Dihomo-y-linolenic Acid

Summary The monoenoic prostaglandin precursor, dihomo-y-linolenic acid (DGLA), in a single dose intravenously (2.0 mg/kg) in dogs, produced a biphasic alteration in systemic arterial pressure (SAP) with a predominant and marked depressor effect. This SAP response is approximately equi-depressor to the effect of PGE1 5 μg/kg. DGLA had a positive inotropic effect, causing a greater increase in myocardial contractility than PGE1 in an equidepressor dose. The effect of DGLA on MC was not altered by ganglion blockade or β-adrenergic blockade. Aspirin blocked the sustained depressor response to DGLA but not an initial drop in SAP and increase of MC of very short duration. Aspirin had no effect on PGE1 or PGF1α responses. DGLA caused no thrombocytopenia, but caused a decrease in sensitivity to platelet aggregation. Control fatty acid injections produced variable effects with no resemblances to DGLA responses. It is concluded that DGLA produces direct depressor and positive inotropic responses as well as responses which may be due to conversion to an endo-peroxide formed in the biosynthesis of prostaglandins. In contrast, in equidepressor doses, arachidonic acid (AA), the bisenoic prostaglandin precursor, produces a delayed, single-phase depressor effect which may be due to endoperoxide formation alone. Further, the effect of AA on MC is reflex and is blocked by hexamethonium.

Cardiovascular responses to three prostaglandin endoperoxide analogs in the dog.

Summary Three stable analogs of prosta-glandin endoperoxides have been studied in limited quantities for their effects on the canine cardiovascular system. They are potent systemic pressor agents and powerful pulmonary vasoconstrictors. They directly increase myocardial contractile force. These responses are not altered by indomethacin. In their blood pressure and myocardial effects they are considerably more potent than arachidonic acid and may be estimated to be more potent than the primary bisenoic PGs, PGE2 and PGF20. The cyclic ether endo-peroxide analogs of Bundy are approximately twice as potent in their effects on these parameters as the azo analog of Corey et al.

Cardiovascular Responses to PGD2 in the Dog

Summary The cardiac and peripheral vascular effects of PGD2 and PGE2 were investigated in dogs with an intact circulation and during left ventricular bypass. PGE2 had a positive inotropic effect, whereas PGD2 produced a negative inotropic effect on the heart that was independent of reflex effects or changes in ventricular preload or afterload. Both PGE2and PGD2 produced a systemic depressor response which was due to the direct peripheral vasodilating action of these compounds.

Effects of ganglionic and beta-adrenergic blockade on cardiovascular responses to the bisenoic prostaglandins and their precursor arachidonic acid.

Summary Arachidonic acid (AA) 300 μg/kg, and PGE2, 5 μg/kg consistently produced a decrease in systemic arterial pressure in anesthetized dogs. PGF2α, 5 μg/kg, produced a pressor response. All three compounds increased myocardial contractile force, but the magnitude of the change following AA was less prominent. After ganglionic blockade, the depressor response to AA and PGE2 persisted and the pressor response to PGF2α was augmented. Myocardial contractile force did not increase following AA in ganglion-blocked animals indicating that the cardiac responses observed before hexamethonium were mediated by the baroreceptor reflexes. A much larger dose of AA (900 μg/kg) resulted in a small positive inotropic effect on the heart. This possibly represents a direct cardiac effect of AA, or may be indicative of increased biosynthesis of an intermediate endoperoxide, or PGE2 and PGF2α. Both PGE2 and PGF2α have a direct positive inotropic effect on the heart. The persistent cardiac effects of PGE2 and PGF2 after β-adrenergic blockade suggests that these compounds may not interact with the β-receptors of the myocardium.

THROMBOXANE B2 INHIBITS THE PULMONARY INACTIVATION OF PROSTAGLANDIN E2 IN THE DOG

1 The systemic vasodepressor response to intravenously administered prostaglandin E2 (PGE2, 0.3, 1.0 and 3.0 μg/kg) is potentiated during intravenous infusion of thromboxane B2 (TXB2, 1.0 μg kg−1 min−1) in the anaesthetized dog. 2 The augmented haemodynamic response returns toward control values following cessation of the TXB2 infusion. 3 The systemic haemodynamic responses to intra‐arterially administered PGE2, PGF2α and PGI2 as well as intravenously administered PGF2α and PGI2 are not altered by TXB2 infusion. 4 This study suggests that TXB2 inhibits the pulmonary inactivation of PGE2. 5 Arachidonic acid metabolites may interact, producing haemodynamic responses differing from their individual effects.

Endothelin-1: Possible implications in pulmonary vascular disease

The vasoactive properties of endothelin-1 (ET-1) in the animal model very with the tone of the pulmonary vessels, the dose level of ET-1, and the maturation of the vessels. The action of ET-1 is mediated by endothelium-derived nitric oxide, prostaglandins, and electrolytes. Plasma levels of ET-1 are elevated in pulmonary hypertension in both animals and humans. ET-1 antagonists may prove useful in treating pulmonary hypertension in children and adults.

Effects of Intracellular and Extracellular Calcium Blockers on the Pulmonary Vascular Responses to PGF2α and U46619

In this study, TMB-8, an intracellular calcium antagonist, and verapamil, an extracellular calcium antagonist, were used simultaneously to elucidate the role of calcium in the pulmonary vasopressor response induced by PGF2α and U46619. The pulmonary vasoconstrictor action of these two agonists was evaluated in the canine isolated lung lobe preparation. Lobar arterial pressure was constantly monitored and changes in arterial pressure were recorded as a percentage from baseline. Control responses to PGF2α (42.0±8.2%) and U46619 (47.2±7.0%) were obtained prior to the administration of TMB-8 and verapamil. After administration of TMB-8 and verapamil, the PGF2α (7.4±3.1%) and U46619 (28.8±6.2%) responses were significantly attenutated. We conclude that the PGF2α pressor response is dependent on a TMB-8-sensitive intracellular calcium pool and a verapamil-sensitive slow-channel calcium influx. In contrast, the degree of attenuation of the U46619 response was similar to the vasopressor response in the presence of verapamil alone, as described previously. This indicates a direct dependence on extracellular calcium. An additional source of calcium insensitive to verapamil and TMB-8 may also be activated and contribute to the pulmonary vasoconstrictor action. These results suggest that each agonist possesses a mechanism of action distinctly different from the other.

Sources of Increased in vivo Cyclooxygenase Product Release Following Whole-Body Irradiation of Rats

Ionizing radiation alters cyclooxygenase product synthesis in a time- and dose-related manner. Rats exposed to 10 or 20 Gy whole-body gamma radiation showed a significant increase (p <.05) in urine immunoreactive thromboxane B2 (iTXB2) 4-120 hours after 10 Gy as well as 4 and 12 hours after 20 Gy exposure. Irradiation with 2 Gy had no effect. The source(s) of this radiation-induced increase in iTXB2 excretion was studied by regional shielding. Rats were anesthetized and exposed to sham radiation, 15 Gy whole-body radiation, or a dose of 20 Gy radiation with either the abdomen or thorax shielded. Four hours after exposure, the animals were re-anesthetized and urine samples collected. Unshielded animals exposed to a 15 Gy dose of radiation showed a 2.5-fold (p <.05) increase in iTXB2 excretion. Abdominal shielding attenuated the radiation-induced increase in iTXB2 excretion by 41%, but thoracic shielding prevented the increase in iTXB2 excretion. Thus, the thoracic organs are an important source of the radiation-induced increase in iTXB2 excretion, and the abdominal organs may also contribute to the increased in vivo release of iTXB2. In the next series of experiments, the individual contribution of the kidneys and lungs to the increased excretion of cyclooxygenase products was studied. Rats were subjected to 20 Gy whole-body radiation and 4 hours after exposure, either the kidneys or lungs were isolated and perfused with a cell-free medium. Radiation did not alter urine acidification by the isolated perfused kidney. However, the concentration of urine from irradiated kidneys was 18.3% (p <.05) less than urine from control kidneys. Whole-body gamma radiation also elicited a 2.2-fold (p <.05) and 3.6-fold (p <.05) increase in the excretion of iPGE2 and 6-keto prostaglandin F1α (i6KPGF1α) from isolated perfused kidneys. The excretion rate of iTXB2 from perfused kidneys after irradiation was not significantly different from sham-irradiated controls. On the other hand, isolated perfused rat lungs released 100% (p <.05) more iTXB2 following irradiation than lungs from sham-irradiated animals. The release of i6KPGF1α was also significantly elevated. These studies showed a regional release of cyclooxygenase products following irradiation. The elevated excretion rate of iTXB2 appears to be primarily due to an increased pulmonary release of this arachidonate metabolite.