calcimycin and Uremia

Platelets of patients with uremia develop a defective platelet function and have a decreased production of thromboxane B2 (TxB2). Activated platelets generate thromboxane from free arachidonate that is previously released from the membrane phospholipids (PLs) by phospholipases. Phospholipase A2 (PLA2) release up to 70% of the arachidonate in normal platelets, and to date, the activity of this enzyme in uremia is unknown. This work studied the PLA2 activity in the platelets of nine uremic patients and nine healthy volunteers. Washed platelets were labelled with [(14)C]arachidonic acid and activated with calcium ionophore A-23187 (4 microgr/ml). Lipids were resolved by TLC and identified by autoradiography. The distribution of [(14)C]arachidonic acid in the five major platelet phospholipids was found to be normal. Uremic platelets released more radioactivity than normal platelets (19.0 +/- 5.2% versus 11.3 +/- 1.6%, P = 0.001). The production of both, radioactive thromboxane B2 and hydroxyheptadecatrienoic acid was normal (2.6 +/- 1.2% and 3.5 +/- 1.6% of total radioactivity respectively), but the formation of the lipoxygenase metabolite hydroxyeicosatetraenoic acid was increased with respect to the controls (12.9 +/- 4.6% vs 7.0 +/- 1.3% of total radioactivity, P = 0002). In conclusion, platelets of patients with uremia have an increased activity of phospholipase A2 and produce increased amounts of hydroxyeicosatetraenoic acid, an inhibitor of the platelet function.

Topics: Arachidonic Acid; Blood Platelets; Calcimycin; Case-Control Studies; Fatty Acids, Unsaturated; Humans; Kidney Failure, Chronic; Phospholipases A; Phospholipases A2; Platelet Function Tests; Thromboxane B2; Uremia

Bleeding complications in uraemia are not uncommon. The pathogenesis of haemorrhage in uraemia is still a matter of controversy and the pattern of bleeding suggests a defect of primary haemostasis. Platelet aggregation and biochemistry, including calcium levels, have been studied; however, the results are controversial. We have examined platelet aggregation, platelet-free calcium and calmodulin in platelet-rich plasma because of the significant role of calcium and calmodulin in regulating platelet and other cells' functions. Platelet aggregation in uraemic subjects was similar to that of controls. Platelet basal free cytosolic calcium and platelet calcium in response to 10 microM Ca++ ionophore A23187 in eight subjects with uraemia were 117 +/- 33 nM and 2025 +/- 398 nM (mean +/- SEM) respectively. By contrast in seven matched healthy controls basal calcium and ionophore-stimulated calcium values were 47 +/- 14 nM and 1354 +/- 414 nM, significantly less than in the patients with uraemia (P less than 0.05). The sensitivity of uraemic platelets to A23187 was similar to that of controls. Calmodulin activity in platelet-rich plasma of 12 subjects with uraemia showed no significant difference from that of controls [1.86 +/- 0.29 micrograms/ml (mean +/- SEM) and 2.0 +/- 0.37 micrograms/ml (mean +/- SEM) respectively]. We conclude that despite elevation of platelet calcium in uraemia, which may be due to a plasma factor such as parathyroid hormone, platelet aggregation is normal and bleeding in uraemia is more likely to be due to other factors, including the effect of reduced haematocrit on platelet endothelial interaction. Disturbances in platelet calcium cannot explain the bleeding manifestations in uraemia but warrant further investigation in order to identify the pathogenic mechanisms responsible.

Topics: Adolescent; Adult; Aged; Aged, 80 and over; Bleeding Time; Blood Platelets; Calcimycin; Calcium; Calmodulin; Female; Humans; Male; Middle Aged; Platelet Aggregation; Uremia

Endothelium-derived relaxing factor, now identified as nitric oxide (NO), is a labile humoral agent formed by vascular endothelial cells from L-arginine. NO mediates the action of substances that induce endothelium-dependent relaxation and plays a role in regulating blood pressure. In this study we investigated whether NO is involved in the pathogenesis of the bleeding tendency associated with renal failure. Rats with extensive surgical ablation of renal mass develop renal insufficiency due to progressive glomerulosclerosis. Like uremic humans, rats with renal mass reduction and uremia have a bleeding tendency that manifests itself by a prolonged bleeding time. We found that N-monomethyl-L-arginine (L-NMMA), a specific inhibitor of NO formation from L-arginine, completely normalized bleeding time when given to uremic rats. L-NMMA injection also increased ex vivo platelet adhesion but did not affect ex vivo platelet aggregation induced by adenosine diphosphate, arachidonic acid, and calcium ionophore A23187. The shortening effect of L-NMMA on bleeding time was completely reversed by giving the animals the NO precursor L-arginine, but not D-arginine, which is not a precursor of NO. It thus appears that NO is a mediator of the bleeding tendency of uremia.

Topics: Animals; Arginine; Bleeding Time; Calcimycin; Endothelium, Vascular; Hemorrhage; Male; Nitric Oxide; omega-N-Methylarginine; Platelet Adhesiveness; Platelet Aggregation; Rats; Rats, Inbred Strains; Uremia

We have examined platelet aggregation in patients with chronic uremia using ADP, thrombin and calcium ionophore A23187 as inducers. The study was performed on patients treated conservatively, by hemodialysis and peritoneal dialysis. Platelet aggregation was most significantly depressed in patients treated conservatively and by hemodialysis. Different mechanisms are responsible for platelet dysfunction.

Topics: Adenosine Diphosphate; Adult; Calcimycin; Female; Humans; Male; Middle Aged; Peritoneal Dialysis; Platelet Aggregation; Renal Dialysis; Stimulation, Chemical; Thrombin; Uremia

Parathyroid hormone (PTH) is a polypeptide which in different in vitro systems raises intracellular cyclic AMP (cAMP) levels via adenyl cyclase activation and stimulates Ca2+ transport across cell membranes. We tested whether, on the basis of this mechanism, PTH would inhibit human platelet aggregation. The latter was tested in vitro by a photometric technique. Platelet aggregation induced by the calcium ionophore A 23187 was inhibited by PTH at concentrations (0.5-3 USP U/ml) similar to those effective in other in vitro systems. Higher concentrations of PTH were required to prevent aggregation initiated by adenosine-5'-diphosphate, arachidonic acid, or platelet-aggregating factor. The terminal synthetic fragment 1-34 b PTH was ineffective against all aggregation stimuli. The antiaggregating effect of PTH was potentiated by verapamil and theophylline and was additive to that of PGI2. However, PTH did not appear to increase platelet cAMP levels and was not counteracted by an inhibitor of platelet adenyl cyclase. It is therefore unlikely that PTH inhibits platelet aggregation through an adenyl cyclase stimulated increase of cAMP. Since PTH levels are markedly increased in uremic plasma, it might contribute to the defective platelet function and the bleeding tendency frequently occurring in uremic patients.

Topics: Adenylyl Cyclases; Blood Platelets; Calcimycin; Calcium; Cyclic AMP; Humans; In Vitro Techniques; Parathyroid Hormone; Platelet Aggregation; Theophylline; Thromboxane B2; Uremia; Verapamil

Myocardiopathy is common in uremia, but its cause in unknown. Excessive entry of calcium in heart cells by catecholamines has been shown to cause necrosis of myocardium. The high blood levels of parathyroid hormone (PTH) in uremia may also enhance entry of calcium into heart cells and exert deleterious effects on the heart. We examined the effect of PTH on rat heart cells grown in culture. Both amino-terminal (1-34) PTH and intact (1-84) PTH, but not the carboxy-terminal (53-84) PTH produced immediate and sustained significant rise in beats per minute and the cells died earlier than control. The effect was reversed if PTH was removed from medium, and was abolished by inactivation of the hormone. There was a dose-response relationship between both moieties of PTH and the rise in heart beats, but the effect of 1-84 PTH was significantly greater than that of 1-34 moiety. PTH stimulated cyclic AMP production within 1 min, and cyclic AMP remained significantly elevated thereafter. The effect of PTH required calcium, was mimicked by calcium ionophore, was prevented by verapamil and was not abolished by alpha- or beta-adrenergic blockers. PTH action was additive to phenylephrine and synergistic with isoproterenol. Sera from uremic parathyroidectomized rats did not effect heart beats, but sera from uremic rats with intact parathyroid glands or from uremic-parathyroidectomized rats treated with PTH had effects similar to PTH. Data indicate that (a) heart cell is a target organ for PTH and may have receptors for the hormone; (b) PTH increases beating rate of heart cells and causes early death of cells; (c) PTH effect appears to be due to calcium entry into heart cells; (d) the locus of action through which PTH induces calcium entry is different from that for catecholamines; and (e) uremic serum has no effect unless it contains PTH. Data suggest that myocardial damage may occur in uremia due to prolonged exposure to very high blood levels of PTH, and assign new dimensions to PTH toxicity in uremia.

Topics: Animals; Calcimycin; Calcium; Catecholamines; Cyclic AMP; Heart; In Vitro Techniques; Myocardium; Parathyroid Hormone; Rats; Uremia; Verapamil