Stimulation of ATP-driven Ca2+ pump in the basal-lateral plasma membranes of kidney cortex during compensatory renal growth

Marinobufagenin may mediate the adverse impact of salty diets on renal calcium retention by impairing the efficiency of renal tubular sodium–calcium exchange

For reasons yet to be clarified, salt loading and plasma volume expansion decrease renal calcium retention; consequently, high-salt diets are thought to increase risk for osteoporosis and renal stones. These measures also can evoke increased adrenal production of the natriuretic factor marinobufagenin (MBG), recently implicated in the genesis of essential hypertension. MBG achieves natriuresis via potent selective inhibition of the alpha-1-type sodium pump, expressed throughout the nephron. In as much as renal calcium retention is largely dependent on efficient activity of calcium-sodium exchangers situated in the basolateral membranes of tubular epithelium, it is evident that an increased intracellular sodium concentration consequent to sodium pump inhibition could blunt the activity of these exchangers. Thus, it is postulated that MBG mediates the impact of salt loading on renal calcium retention.

Different endothelin receptor subtypes are involved in phospholipid signalling in the proximal tubule of rat kidney

Phospholipid signalling mediated by endothelin (ET) receptor subtypes was studied in the rat proximal tubule. In freshly isolated proximal tubule cells, ET-1, ET-2 and sarafotoxin S6c (S6c) evoked an increase in 1,2-diacylglycerol (DAG), inositol 1,4,5-trisphosphate (InsP3) and phosphocholine (PCho), suggesting stimulation of both phosphatidyl-inositol 4,5-bisphosphate- and phosphatidyl-choline-specific phospholipase C (PLC), while ET-3 increased only DAG and PCho, presumably via phosphatidyl-choline-dependent PLC. Renal cortical slices were also stimulated by the above-mentioned agonists, followed by isolation of either brush border (BBM) or basolateral (BLM) membranes for which mass measurements of inositol lipids and DAG were performed. In BBM, DAG increased in response to ET-1, ET-2 and ET-3, and was followed by protein kinase C (PKC) translocation to the BBM, while in BLM, DAG formation and translocation of PKC were observed only in response to ET-3, suggesting spatial segregation of signalling systems between two membane domains of proximal tubule cells. Tyrphostine, pertussis toxin (PTX) or cholera toxin (CTX) did not influence ET-mediated signalling in either of the membranes, suggesting involvement of PTX- and CTX-insensitive G-protein-mediated stimulation of PLCbeta by ET receptors. ET-dependent stimulation of PLC in BBM and BLM was used as a tool to examine the presence of different ET receptor subtypes in these two cell membrane domains. BQ123, an inhibitor of ETA receptors, did not prevent ET-1-mediated signalling in BBM, but an ETA,B antagonist, bosentan, inhibited ET-3-mediated signalling in BBM. In addition, an ETB agonist, S6c, stimulated PLC in BBM. Neither BQ123 nor bosentan inhibited ET-3 signalling in BLM. Therefore, these data strongly suggest the presence of ETB receptors coupled to phosphatidyl-inositol 4,5-bisphosphate- and phosphatidyl-choline-dependent PLC in BBM and ETC receptors linked to phosphatidyl-choline-dependent PLC in BLM.

Diabetes mellitus: a disease of abnormal cellular calcium metabolism?

Although the pathogenesis of the diabetes mellitus syndrome remains poorly understood, both insulin-dependent diabetes mellitus and non-insulin-dependent diabetes mellitus predispose the individual to a similar spectrum of complications, including hypertension, macrovascular and microvascular disease, cataracts cardiomyopathy, neuropathy, and premature aging, suggesting that these complications develop along a pathway common to both diabetic conditions. Yet not all diabetic persons are affected by all of these complications or to the same degree. What causes this marked variability in the clinical manifestations of the diabetes syndrome remains an enigma. Accumulating data from animal models of diabetes and from studying patients with diabetes reveal that intracellular calcium levels are increased in most tissues. The activities of the membrane, adenosine triphosphatase (ATPase) associated cation pumps, which determine intracellular calcium level (i.e., calcium-ATPase and [sodium + potassium]-ATPase), are also altered. The nature of the alteration is often tissue specific and may depend on the level of blood glucose or insulin, or both. In this review we discuss the potential contribution of these changes in intracellular calcium regulation, whether acquired or genetically determined, to the pathogenesis of the diabetes syndrome, to the abnormalities in insulin secretion and action (mainly in non-insulin-dependent diabetes), and to the complications of both diabetes syndromes. Altered intracellular calcium metabolism may represent a common, underlying abnormality linking the metabolic, cardiovascular, ocular, and neural manifestations of the diabetic disease process.

Inositol lipid signalling occurs in brush-border membranes during initiation of compensatory renal growth in the rat

Using highly specific mass assays, concentrations of inositol lipids and 1,2-diacylglycerol (DAG) were determined in plasma membranes isolated from rat kidney cortex. Significantly higher concentrations of inositol lipids were determined in brush-border (BBM) than in basal-lateral (BLM) plasma membranes, although DAG concentrations were similar in both. After unilateral nephrectomy, a decrease in PtdIns(4,5)P2 and PtdIns4P, with a concomitant increase in DAG and translocation of protein kinase C (PKC), were observed in BBM but not in BLM isolated from the remaining kidney. On the other hand, stimulation of renal cortical slices with insulin-like growth factor II (IGF-II) or phenylephrine caused similar effects in BLM but not in BBM. Stimulation of phospholipase C activity with translocation of PKC only to BBM in one kidney was also induced by occlusion of blood flow through the contralateral kidney for 15 min. At 30 min after the occlusion was removed and reflow established, DAG concentration and the amount of PKC in BBM returned to control values. These results suggest that an early signal after unilateral nephrectomy is transmitted to cells through BBM and can be switched on and off by blood occlusion and reflow through the contralateral kidney, while hormonal signals caused by IGF-II and phenylephrine are transmitted to cells through BLM.

Characterization of calcium transport in the luminal and the basolateral membrane from kidney cortex of hypercalcemic rats bearing the Walker 256 carcinosarcoma

The Walker 256 (W256) carcinosarcoma did not significantly modify calcium uptake by brush border membrane (BBM) vesicles in the kidney of the host rat, compared with that in membranes isolated from control animals. However, it showed a tendency to increase at near equilibrium in W256 tumor-host rats, associated with a decreased BBM protein content. ATP-dependent calcium influx by basolateral membrane (BLM) vesicles from W256 tumor-bearing rats was also increased compared with that in control BLM. This stimulation was due to a decreased Km for calcium. Passive calcium permeability or the Na+/Ca2+ exchanger were unchanged in BLM from W256 tumor-host rats compared with control BLM. Pre-stimulation of control rat cortical tubules with either 10(-7) M parathyroid hormone-related protein (PTHrP) (1-34) or 10(-4) M N6,2'-O-dibutiryl cyclic AMP before BLM isolation, did not modify the ATP-dependent calcium uptake by BLM vesicles compared with control membranes. However, our results do not rule out that the stimulated ATP-dependent calcium influx in BLM from W256 tumor-host rats could be mediated by the interaction among PTHrP and other humoral factors. Our findings suggest a possible mechanism for the increased renal calcium reabsorption in this animal model for humoral hypercalcemia of malignancy.