beta-Adrenergic agonists stimulate Mg(2+) uptake in mouse distal convoluted tubule cells

Modeling Distal Convoluted Tubule (Patho)Physiology: An Overview of Past Developments and an Outlook Toward the Future

The kidneys are essential for maintaining electrolyte homeostasis. Blood electrolyte composition is controlled by active reabsorption and secretion processes in dedicated segments of the kidney tubule. Specifically, the distal convoluted tubule (DCT) and connecting tubule are important for regulating the final excretion of sodium, magnesium, and calcium. Studies unravelling the specific function of these segments have greatly improved our understanding of DCT (patho)physiology. Over the years, experimental models used to study the DCT have changed and the field has advanced from early dissection studies with rats and rabbits to the use of various transgenic mouse models. Developments in dissection techniques and cell culture methods have resulted in immortalized mouse DCT cell lines and made it possible to specifically obtain DCT fragments for ex vivo studies. However, we still do not fully understand the complex (patho)physiology of this segment and there is need for advanced human DCT models. Recently, kidney organoids and tubuloids have emerged as new complex cell models that provide excellent opportunities for physiological studies, disease modeling, drug discovery, and even personalized medicine in the future. This review presents an overview of cell models used to study the DCT and provides an outlook on kidney organoids and tubuloids as model for DCT (patho)physiology. Impact statement This study provides a detailed overview of past and future developments on cell models used to study kidney (patho)physiology and specifically the distal convoluted tubule (DCT) segment. Hereby, we highlight the need for an advanced human cell model of this segment and summarize recent advances in the field of kidney organoids and tubuloids with a focus on DCT properties. The findings reported in this review are significant for future developments toward an advanced human model of the DCT that will help to increase our understanding of DCT (patho)physiology.

Magnesium physiology and clinical therapy in veterinary critical care

OBJECTIVE

To review magnesium physiology including absorption, excretion, and function within the body, causes of magnesium abnormalities, and the current applications of magnesium monitoring and therapy in people and animals.

ETIOLOGY

Magnesium plays a pivotal role in energy production and specific functions in every cell in the body. Disorders of magnesium can be correlated with severity of disease, length of hospital stay, and recovery of the septic patient. Hypermagnesemia is seen infrequently in people and animals with significant consequences reported. Hypomagnesemia is more common in critically ill people and animals, and can be associated with platelet, immune system, neurological, and cardiovascular dysfunction as well as alterations in insulin responsiveness and electrolyte imbalance.

DIAGNOSIS

Measurement of serum ionized magnesium in critically or chronically ill veterinary patients is practical and provides information necessary for stabilization and treatment. Tissue magnesium concentrations may be assessed using nuclear magnetic resonance spectroscopy as well as through the application of fluorescent dye techniques.

THERAPY

Magnesium infusions may play a therapeutic role in reperfusion injury, myocardial ischemia, cerebral infarcts, systemic inflammatory response syndromes, tetanus, digitalis toxicity, bronchospasms, hypercoagulable states, and as an adjunct to specific anesthetic or analgesic protocols. Further veterinary studies are needed to establish the frequency and importance of magnesium disorders in animals and the potential benefit of magnesium infusions as a therapeutic adjunct to specific diseases.

PROGNOSIS

The prognosis for most patients with magnesium disorders is variable and largely dependent on the underlying cause of the disorder.

β1-Adrenergic receptor signaling activates the epithelial calcium channel, transient receptor potential vanilloid type 5 (TRPV5), via the protein kinase A pathway

Epinephrine and norepinephrine are present in the pro-urine. β-Adrenergic receptor (β-AR) blockers administered to counteract sympathetic overstimulation in patients with congestive heart failure have a negative inotropic effect, resulting in reduced cardiac contractility. Positive inotropes, β1-AR agonists, are used to improve cardiac functions. Active Ca(2+) reabsorption in the late distal convoluted and connecting tubules (DCT2/CNT) is initiated by Ca(2+) influx through the transient receptor potential vanilloid type 5 (TRPV5) Ca(2+) channel. Although it was reported that β-ARs are present in the DCT2/CNT region, their role in active Ca(2+) reabsorption remains elusive. Here we revealed that β1-AR, but not β2-AR, is localized with TRPV5 in DCT2/CNT. Subsequently, treatment of TRPV5-expressing mouse DCT2/CNT primary cell cultures with the β1-AR agonist dobutamine showed enhanced apical-to-basolateral transepithelial Ca(2+) transport. In human embryonic kidney (HEK293) cells, dobutamine was shown to stimulate cAMP production, signifying functional β1-AR expression. Fura-2 experiments demonstrated increased activity of TRPV5 in response to dobutamine, which could be prevented by the PKA inhibitor H89. Moreover, nonphosphorylable T709A-TRPV5 and phosphorylation-mimicking T709D-TRPV5 mutants were unresponsive to dobutamine. Surface biotinylation showed that dobutamine did not affect plasma membrane abundance of TRPV5. In conclusion, activation of β1-AR stimulates active Ca(2+) reabsorption in DCT2/CNT; an increase in TRPV5 activity via PKA phosphorylation of residue Thr-709 possibly plays an important role. These data explicate a calciotropic role in addition to the inotropic property of β1-AR.

Immunosuppressants inhibit hormone-stimulated Mg2+ uptake in mouse distal convoluted tubule cells

Immunosuppressants such as cyclosporinA and FK506 (tacrolimus) are widely prescribed to treat numerous disorders and to treat organ transplant recipients. However, cyclosporine A and FK506 are both known to produce hypomagnesaemia. The mechanism of this effect is still unclear. The present study determined the effects of immunosuppressant treatment on the parathyroid hormone (PTH) mediated Mg(2+) uptake and the mitogen-activated protein kinase (MAPK) activation in mouse distal convoluted tubule (MDCT) cells. The intracellular Ca(2+) and Mg(2+) concentrations in a single MDCT cell were measured by using the fluorescentdye Fura-2 AM and Mag-fura-2 AM, respectively. Cyclosporine A and FK506 illicited a transient increase of intracellular Ca(2+) from a basal level of 99 +/- 16 nM to 685 +/- 105 and 608 +/- 96 nM, respectively. In order to determine the Mg(2+) transport, the MDCT cells were Mg(2+)-depleted by culturing them in Mg(2+)-free media for 16 h, and the Mg(2+) uptake was measured by microfluorescence after placing the depleted cells in 1.5mM MgCl(2). The mean rate of Mg(2+) uptake, d([Mg(2+)](i))/dt, was 140 +/- 16 nM/s in the control MDCT cells. PTH increased the Mg(2+) uptake more than 2 times in this cell. Cyclosporine A (10 microM) and FK506 (0.1 microM) did not affect the basal Mg(2+)uptake (140 +/- 16 and 142 +/- 14 nM/s, respectively), but they inhibited the PTH-stimulated Mg(2+) entry, decreasing it from 248+/-12 to 147 +/- 7 and 148 +/- 14 nM/s, respectively. These effects were inhibited by L685818, which is a potent competitive antagonist of FK506. PTH stimulated the extracellular signal-regulated kinase1/2 (ERK1/2) protein synthesis. This PTH-stimulated ERK1/2 activation was inhibited by cyclosporine A and FK506. In the present study, the role of ERK1/2 activation on the PTH-dependent magnesium uptake was examined in MDCT cells, and we showed that immunosuppressants inhibit the hormone-stimulated Mg(2+) uptake into the MDCT cells by inhibiting the MAPK pathway.