Impaired cation transport in thymocytes of rats with chronic uremia includes the Na+/H+ antiporter

Endogenous cardiotonic steroids in kidney failure: a review and an hypothesis

In response to progressive nephron loss, volume and humoral signals in the circulation have increasing relevance. These signals, including plasma sodium, angiotensin II, and those related to volume status, activate a slow neuromodulatory pathway within the central nervous system (CNS). The slow CNS pathway includes specific receptors for angiotensin II, mineralocorticoids, and endogenous ouabain (EO). Stimulation of the pathway leads to elevated sympathetic nervous system activity (SNA) and increased circulating EO. The sustained elevation of circulating EO (or ouabain) stimulates central and peripheral mechanisms that amplify the impact of SNA on vascular tone. These include changes in synaptic plasticity in the brain and sympathetic ganglia that increase preganglionic tone and amplify ganglionic transmission, amplification of the impact of SNA on arterial tone in the vascular wall, and the reprogramming of calcium signaling proteins in arterial myocytes. These increase SNA, raise basal and evoked arterial tone, and elevate blood pressure (BP). In the setting of CKD, we suggest that sustained activation/elevation of the slow CNS pathway, plasma EO, and the cardiotonic steroid marinobufagenin, comprises a feed-forward system that raises BP and accelerates kidney and cardiac damage. Block of the slow CNS pathway and/or circulating EO and marinobufagenin may reduce BP and slow the progression to ESRD.

Mechanisms through which high glucose concentration raises [Ca2+]i in renal proximal tubular cells

BACKGROUND

The basal levels of cytosolic calcium ([Ca2+]i) of renal proximal tubular cells of rats with streptozotocin-induced diabetes are elevated. It is possible that this phenomenon is mediated by the hyperglycemia, which may cause both increased calcium influx into and/or decreased calcium efflux out of these cells.

METHODS

We examined whether high glucose concentration in vitro causes acute rise in [Ca2+]i of freshly isolated renal proximal tubular cells and explored the pathways that are involved in such an event.

RESULTS

There were dose and time dependent increments in [Ca2+]i of renal proximal tubular cells exposed to high concentrations of glucose. A similar effect was observed with equimolar concentrations of mannitol or choline chloride but not urea. A substantial part of the rise in [Ca2+]i was inhibited when the media contained verapamil, nifedipine, amlodipine or ryanodine and when the cells were placed in a calcium free media. Inhibitors of G protein(s) (GDPbetaS or pertussis toxin), inhibitors of cAMP-protein kinase A pathway (RpcAMP or H-89), inhibitors of protein kinase C (staurosporine or calphostin) and inhibitor of Na+-H+ exchanger (HOE 694) blocked the rise in a dose dependent manner. High glucose concentration also caused a decrease in ATP content of these cells and a reduction in the Vmax of their Ca2+ATPase.

CONCLUSIONS

The results are consistent with the formulation that the osmotic activity (cell shrinkage) of the high glucose concentration may activate a stretch receptor with subsequent stimulation of various cellular pathways including G protein(s), cAMP-protein kinase A and phospholipase C systems and calcium channels. Activation of these cellular pathways permits both calcium influx into renal tubular cells and mobilization of calcium from their intracellular stores. Further, a decrease in calcium efflux secondary to the reduction in the Vmax of Ca2+ ATPase may occur. It is possible that the rise in [Ca2+]i is critical for the stimulation of the events that lead to restoration of cell volume to normal.

Sodium pump and Na+/H+ activities in uremic erythrocytes. A microcalorimetric and pH-metric study

The sodium pump and Na+/H+ antiport activities in red blood cells from uremic hemodialyzed patients were measured concomitantly. The patients selected (n = 35) were normotensive and free of intercurrent illness known to affect Na transport. The Na pump activity of intact red blood cells in suspension in their own plasma was measured by flow microcalorimetry. The Na+/H+ antiport activity of the erythrocytes from the same patients was determined by a titrimetric technique. The mean global value of the sodium pump was lower in uremics than in controls (13.3 +/- 0.6 vs. 11.3 +/- 0.8 mW/l cells, P < 0.05). The Na+/H+ antiport maximal activity was decreased in uremics (2.9 +/- 0.3 vs. 4.6 +/- 0.5 mmol H+/l cells/h, P < 0.05). Our results thus confirm that uremia per se can affect sodium transport. Moreover it has been shown that a decrease in Na+/H+ antiport activity is able to counteract an impairment of sodium pump. The decrease found in this study could thus explain, at least in part, the absence of hypertension in the patients studied despite their decreased sodium pump activity.

Pathways through which glucose induces a rise in [Ca2+]i of polymorphonuclear leukocytes of rats

Basal levels of [Ca2+]i are elevated in diabetes mellitus. Such an abnormality is most likely due to both increased calcium influx into cells and decreased efflux of this ion out of the cells. The present study examined the cellular pathways that are responsible for hyperglycemia-induced acute rise in polymorphonuclear leukocytes (PMNL), and explored whether such a rise is due to increased calcium entry into PMNL and/or to calcium release from their intracellular stores. There were dose dependent and time dependent rises in the [Ca2+]i of PMNL exposed to high concentrations of glucose. Similar effects were observed when the PMNL were exposed to high concentrations of choline chloride or mannitol. A substantial part of the rise in [Ca2+]i was inhibited when the media contained verapamil or nifedipine or when the PMNL were placed in calcium free media, and the rise in [Ca2+]i was completely abolished when the PMNL were placed in calcium free media containing ryanodine. GDP beta S or pertussis toxin almost completely prevented the glucose-induced rise in [Ca2+]i of PMNL. Rp-cAMP, H-89 or staurosporine produced significant inhibition of the rise in [Ca2+]i. High concentrations of glucose produced a dose dependent shrinkage of PMNL volume over a period of two hours. The volume of PMNL, however, was normal after 24 hours in vitro incubation studies as well as after 1, 2 and 12 days of streptozotocin-induced hyperglycemia in rats. The results are consistent with the formulation that the osmotic activity (cell shrinkage) of the high glucose concentrations activates G protein(s) which then stimulates the adenylate-cAMP-protein kinase A pathway, phospholipase C system and calcium channels. The stimulation of these cellular pathways permits both calcium influx into the PMNL as well as mobilization of calcium from their intracellular stores. Both of these events contribute to the acute rise in their [Ca2+]i. It is possible that the rise in [Ca2+]i is critical for the stimulation of the events that lead to the generation and accumulation of inorganic osmolytes to restore cell volume to normal.

Experimental acidemia and muscle cell pH in chronic acidosis and renal failure

To test whether muscle intracellular pH (pHi) decreases when extracellular pH falls, 31P-nuclear magnetic resonance was utilized in rats made acidotic by infusion of HCl, gavage-feeding NH4Cl, or induction of chronic renal failure (CRF). A 2- or 4-h HCl infusion did not lower muscle pHi, even though serum bicarbonate fell to 5 mM. With chronic acidemia, blood pH was 7.15 +/- 0.01 vs. 7.38 +/- 0.02 in pair-fed controls, and muscle pHi was 7.09 +/- 0.01 and 7.14 +/- 0.02, respectively (P < 0.01). pHi in muscle of CRF rats (7.16 +/- 0.01) did not differ from sham-operated, pair-fed controls (7.19 +/- 0.01) despite a blood pH of 7.23 +/- 0.05 in CRF vs. 7.39 +/- 0.01 in controls. Because ion transport is abnormal in CRF, we examined whether recovery of pHi is impaired when muscles of six CRF and control rats were exercised to tetany by stimulation of the sciatic nerve. Neither pHi nor the recovery of pHi differed between CRF and control rats. We conclude that pHi is maintained in muscle in uremia and that signals other than changing pHi must be necessary to disrupt metabolism.