George Z. Fadda

Impaired Phagocytosis in Dialysis Patients: Studies on Mechanisms

Dialysis patients have increased susceptibility to infection and this is, in part, due to impaired phagocytic and bactericidal activities of polymorphonuclear leukocytes (PMNL). The mechanisms responsible for the reduced phagocytosis are not known. Dialysis patients have elevated blood levels of parathyroid hormone (PTH), and available data indicate that PMNL is a target cell for PTH. Chronic exposure to excess PTH may cause accumulation of calcium in PMNL which in turn could adversely affect cellular events leading to their dysfunction. We studied phagocytosis, resting levels of cytosolic calcium [( Ca2+]i), ATP content and the rise in [Ca2+]i in response to ligation of Fc gamma RIII receptors with 3G8 monoclonal antibody in PMNL from 37 dialysis patients and 48 normal subjects. The PMNL from the dialysis patients displayed impaired phagocytosis, elevated resting levels of [Ca2+]i, decreased ATP content and a smaller rise in [Ca2+]i in response to various doses of 3G8 monoclonal antibody as compared to values obtained in PMNL of normal subjects. Our results suggest that derangements in cellular metabolism and possibly an abnormality in Fc gamma RIII interaction with antibody and/or the consequences of such interaction are responsible, at least in part, for the impaired phagocytosis of PMNL of dialysis patients. Our data are consistent with the notion that excess PTH may play an important role in the processes leading to impaired phagocytosis.

Chronic Renal Failure is a State of Cellular Calcium Toxicity

Studies on the metabolic profile of many cells have shown that chronic renal failure (CRF) is associated with a significant elevation in the basal levels of cytosolic calcium ([Ca2+]i). This latter abnormality is, in major part, responsible for the organ dysfunction in CRF. The initial step leading to the eventual increase in the basal level of [Ca2+]i is parathyroid hormone (PTH)-mediated increased calcium influx into cells. This is followed by decreased extrusion of calcium out of cells due to reduced activity of the enzymes responsible for pumping calcium out of the cells. The combination of increased entry and decreased exit of calcium results in elevation of [Ca2+]i. Prevention of secondary hyperparathyroidism in CRF or blocking of the effect of PTH by a calcium channel blocker results in normalization of [Ca2+]i and restoration of cell function. Thus, the available data are consistent with the notion that CRF is a state of cellular calcium toxicity, which underlies many of the metabolic and functional derangements in CRF.

Impaired Glucose-Induced Calcium Signal in Pancreatic Islets in Chronic Renal Failure

Basal level of cytosolic calcium ([Ca2+]i) is elevated in islets of rats with chronic renal failure (CRF). The high [Ca2+]i level was implicated in the impaired insulin secretion of CRF, and its effect is due, in part, to a reduction in ATP content and impaired glucose metabolism by the islets. However, elevated [Ca2+]i may interfere with insulin secretion via another pathway. Exposure of the islets to glucose causes an acute rise in [Ca2+]i which generates events leading to insulin secretion. It is possible that a sustained rise in [Ca2+]i interferes with the magnitude of glucose-induced calcium signal and the ratio between this signal and basal [Ca2+]i. We examined this question in normal, CRF, normocalcemic CRF-PTX rats and in CRF rats treated with verapamil (CRF-V). Basal [Ca2+]i was higher (p less than 0.01) in CRF (130 +/- 7.0 nM) than in normal (82 +/- 5.5 nM), CRF-PTX (75 +/- 3.6 nM) and CRF-V rats (84 +/- 3.8 nM). Glucose-induced calcium signal (95 +/- 10.4 nM) and the ratio between this signal and basal [Ca2+]i (0.73 +/- 0.07) in CRF rats were lower (p less than 0.01) than in normal (153 +/- 14.4 nM; 1.90 +/- 0.24), CRF-PTX (130 +/- 16.7 nM; 1.75 +/- 0.25) and CRF-V (124 +/- 5.8 nM; 1.90 +/- 0.12) rats despite high PTH in the latter. The data indicate that a sustained rise in [Ca2+]i of islets interferes with the glucose-induced calcium signal which in turn plays a role in impaired insulin secretion.

ACUTE ADMINISTRATION OF INTACT PARATHYROID HORMONE BUT NOT ITS AMINO-TERMINAL FRAGMENT STIMULATES VOLUME OF PANCREATIC SECRETION

Chronic renal failure is associated with structural and functional abnormalities of the exocrine system of the pancreas. Certain data suggest that the excess parathyroid hormone (PTH) in these patients may participate in the genesis of these pancreatic derangements. However, direct evidence that PTH exerts a direct effect on the exocrine pancreatic system is not well documented. The present study examined the effects of the intact molecule (1-84 PTH) and of the amino-terminal fragment (1-34 PTH) of the hormone on the basal output of pure pancreatic juice (PPJ) volume and pancreatic protein secretion in the rat. 1-84 PTH but not 1-34 PTH significantly (p less than 0.01) stimulated the output of PPJ volume without an effect on protein secretion. The magnitude of this stimulatory effect of 1-84 PTH depended on the dose of the hormone administered, and it was related to its biological activity, since inactivation of PTH abolished its action on the output of PPJ volume. The simultaneous administration of the calcium channel blocker, verapamil, reduced but did not abolish the stimulatory effect of PTH on the volume of PPJ output. The data demonstrate that: (1) the ductal cells of the pancreatic acini are targets for PTH and (2) the action of the hormone on these cells is mediated, at least in part, via PTH-induced entry of calcium into the target cells.

Verapamil Reverses Glucose Intolerance in Preexisting Chronic Renal Failure: Studies on Mechanisms

Glucose-induced insulin secretion is impaired in chronic renal failure (CRF), and this abnormality is due to the elevation of cytosolic calcium [Ca2+]i and other derangements in pancreatic islet metabolism. Verapamil given to rats from day 1 of CRF prevented the rise in [Ca2+]i of islets and the impairment in insulin secretion. However, it is not known whether verapamil can reverse the abnormalities of islet function and metabolism in animals with preexisting renal failure. Such a documentation has important clinical implications for the treatment of carbohydrate intolerance in patients with CRF. The present study examined this question. After 6 weeks of CRF, rats were randomized into two subgroups and maintained for additional 6 weeks. One subgroup received intraperitoneal injections of verapamil (0.1 micrograms/kg body weight twice daily) and the other received vehicle only. At the time of randomization, there were no significant differences between the two subgroups in their body weight, plasma levels of calcium, phosphorus and creatinine, serum parathyroid hormone and creatinine clearance. Similarly, at the time of sacrifice (12 weeks), there were no significant differences in these parameters except for a modestly lower plasma level of creatinine and modestly higher creatinine clearance.(ABSTRACT TRUNCATED AT 250 WORDS)

Elevated Basal Levels of Cytosolic Calcium of Thymocytes in Chronic Renal Failure

The basal levels of cytosolic calcium ([Ca2+]i) in rats and/or humans with chronic renal failure (CRF) are elevated in many cells including brain synaptosomes, pancreatic islets, polymorphonuclear leukocytes, platelets and B and T cells. This rise in [Ca2+]i has been attributed to the state of secondary hyperparathyroidism of CRF. These observations have led to the proposition that CRF is a state of cellular calcium intoxication mediated by excess parathyroid hormone (PTH). The documentation of a high basal level of [Ca2+]i in other cells is needed to provide further support for this postulate. The present study evaluated the basal levels of [Ca2+]i of thymocyte, which are targets for PTH action, in normal, CRF, and CRF parathyroidectomized (CRF-PTX) rats. We also examined whether CRF affects the phenotype expression (Thy-1, CD4 and CD8) in thymocytes. The results showed that the basal levels of [Ca2+]i in thymocytes from CRF rats (81 +/- 3.7 nM) are significantly (p < 0.01) higher than those in normal animals (60 +/- 2.9 nM). PTX of CRF animals prevented the elevation in the basal levels of [Ca2+]i of thymocytes; in these animals, the levels were 59 +/- 2.8 nM. Neither CRF nor the elevation in [Ca2+]i of thymocytes affected their phenotype expression.

Effect of Erythrocyte Deformability on Renal Hemodynamics and Plasma Renin Activity

Increased blood viscosity has been previously noted in a subgroup of patients with essential hypertension with concomitant high plasma renin activity (PRA). It has been suggested that the cause of hyperviscosity in hypertensives is the presence of circulating red blood cells (RBCs) that were rendered less deformable by significant alterations in their cationic milieu, namely an increase in intracellular concentration of calcium and sodium. The relation between RBC deformability and PRA however is not clear. Our study was conducted to examine this issue. RBC deformability was reduced experimentally, and its effects on renal blood flow, renal artery resistance, glomerular filtration rate and PRA were investigated in experimental (n = 8) and control (n = 4) groups of dogs. Blood was collected from the animals before the experiments and incubated with 0.025% glutaraldehyde. These hardened RBCs were administered to the animals through exchange transfusions. Following the exchange transfusion with the hardened RBCs, there were no changes in renal blood flow, renal artery resistance, and the creatinine clearance. The only change observed was an increase in PRA. In the control group, all parameters that were determined remained unchanged. The data are consistent with the notion that the presence of circulating hardened RBCs may by itself increase PRA, and this effect can be important in some types of hypertension and some other disorders in which impaired deformability of RBCs have been reported.

Phosphate Depletion Reduces Potassium-Induced Insulin Secretion

Abstract Potassium-induced insulin secretion is impaired in rats with chronic renal failure and a sustained rise in cytosolic calcium ([Ca2+]i). It has been found that the calcium signal (Δ[Ca2+]i) and the Δ[Ca2+]i/basal [Ca2+]i in these animals in response to potassium are smaller than those in normal rats and that these defects may underlie, at least in part, the reduced potassium-induced insulin secretion, since the latter depends on an appropriate rise in [Ca2+]i. Since phosphate depletion (PD) is another model associated with a rise in the basal level of [Ca2+]i of pancreatic islets, it provides another metabolic setting for investigating the interaction between high [Ca2+]i of islets and their response to potassium. We examined the potassium-induced insulin secretion, the potassium-induced calcium signal, and the Δ[Ca2+]i/basal [Ca2+]i in islets of PD rats with and without elevated [Ca2+]i. The levels of the basal [Ca2+]i in the islets of PD rats were significantly (P < 0.01) higher than those in pair-weighed (PW) animals and those in PD and PW rats treated with verapamil, which has been shown to prevent the rise in [Ca2+]i in islets of PD rats. Both initial and total insulin secretion, the calcium signal, and the Δ[Ca2+]i/basal [Ca2+]i in the islets of PD rats were significantly (P < 0.01) smaller than those in the other three groups of animals. There were no significant differences in basal levels of [Ca2+]i and in calcium signal, Δ[Ca2+]i/basal [Ca2+]i, and insulin secretion among PW rats, verapamil-treated PD rats, and verapamil-treated PW rats. The results are consistent with the notion that elevated resting levels of [Ca2+]i interfere with the magnitude of the calcium signal and the ratio of calcium signal to basal [Ca2+]i, and these derangements, at least in part, underlie the impaired potassium-induced insulin secretion in PD.

Carbohydrate Intolerance and Impaired Pancreatic Insulin Release in Chronic Renal Failure: Role of Excess Blood Levels of Parathyroid Hormone

Patients with chronic renal failure display abnormalities in carbohydrate metabolism (1–5). They almost always have resistance to the peripheral action of insulin (5, 6), while insulin secretion could be normal (4,7), increased (8,9) or decreased (3). Glucose intolerance is, therefore, usually encountered in uremic patients in whom both impaired tissue sensitivity to insulin and impaired secretion of the hormone co-exist (5,10).