Possible involvement of inositol phosphates and calmodulin in calcitonin-induced stimulation of phosphate transport in LLC-PK1 cells

Calcitonin potentiates oxygen-glucose deprivation-induced neuronal death

Calcitonin is a hormone that decreases plasma calcium through inhibition of osteolysis. It is used in the treatment of osteoporosis and other bone disorders. Salmon calcitonin is typically utilized in individuals for whom use of estrogen is contraindicated, for example, women at high risk for breast cancer. In addition to actions on bone, calcitonin may have effects on the central nervous system. Receptors for calcitonin are present on central neurons, and salmon calcitonin has been shown to alter neuronal activity. Since salmon calcitonin is used clinically, and it can have actions on neurons, the present studies were designed to determine whether salmon calcitonin could alter death of cortical neurons. The effects of salmon calcitonin on neuronal death induced by exposure of murine cortical cultures to serum deprivation, staurosporine, oxygen-glucose deprivation, kainate, and NMDA were tested. Salmon calcitonin had no effect on apoptotic cell death in cortical cultures. However, acute treatment with salmon calcitonin (1-1000 nM) caused significant potentiation of neuronal death induced by oxygen-glucose deprivation. Similarly, salmon calcitonin potentiated cell death induced by exposure to kainate. In contrast, it did not potentiate cell death induced by exposure to NMDA. In fact, addition of a high concentration (1000 nM) of salmon calcitonin attenuated NMDA toxicity. These results indicate that calcitonin is not a survival factor for cortical neurons; however, it can alter excitotoxic cell death. The most interesting, and disturbing, effect is the ability of low concentrations of salmon calcitonin to potentiate oxygen-glucose deprivation-induced cell death.

Phosphorylation of endogenous protein in primate kidney: effects of modulators of calcium/calmodulin-dependent kinases

1. Phosphorylation of endogenous proteins in response to calcium and calmodulin was assessed in membrane and cytosol from primate kidney. 2. Quantitative studies showed that calcium and calmodulin had little effect on phosphorylation in baboon kidney cytosol; in membranes, phosphorylation was significantly decreased by calcium and calmodulin. 3. Phosphorylation of specific proteins which had been electrophoretically separated indicated that calmodulin, in the absence or presence of calcium, enhanced bands of masses 37.1 +/- 1.1 kDa and 103.3 +/- 6.2 kDa (N = 6) in cytosol fractions of baboon kidney; in membrane fractions no effect was noted. 4. Similar results were found in normal human kidney.

Differential effects of activation of protein kinase C and cyclic-AMP-dependent protein kinase on sodium-dependent phosphate uptake in NIH 3T3 cells

Activation of protein kinase C (PKC) by phorbol ester (PMA), or by diacylglycerol analogue (OAG) treatment of NIH 3T3 cells resulted in the rapid (within 2-5 min) stimulation (approx. 2-fold) of sodium-dependent phosphate (Pi) transport. Conversely, preincubation of these cells with forskolin and cholera toxin, or incubation with 8-bromo-cAMP, to activate cAMP-dependent protein kinase (PKA), resulted in a decrease in Na+/Pi transport. Activation of either PKC or PKA did not change the Vmax of Pi uptake. However, activation of PKC did result in an increase, while activation of PKA caused a decrease, in the affinity for Pi. These results indicate that there is differential regulation of Na+/Pi uptake in NIH 3T3 cells by activators of PKC (stimulated) and PKA (inhibited) as a consequence of changes in the affinity of the transporter for Pi.

Macrophages express functional receptors for calcitonin-gene-related peptide

The present study was designed to investigate whether non-activated macrophages express calcitonin (CT) or calcitonin-gene-related peptide (CGRP) receptors. To this end, we first analyzed whether CT and CGRP induce a cAMP accumulation in macrophages. Macrophages were treated for 2 min with increasing concentrations of either CT or CGRP in the presence or absence of IBMX. A dose-dependent cAMP accumulation was measured in response to CGRP with a half-maximal effect attained with 1 nM CGRP. CT failed at all doses to induce an accumulation of cAMP. The effects of CT and CGRP on the activation of the Na-H exchanger were next assessed by spectrofluorometry by using the pH-sensitive dye 2,7 biscarboxyethyl-5(6)-carboxyfluorescein (BCECF). Steady-state pHi of macrophages in a 7.4, HCO3-free solution (HEPES-buffered) was 7.04 +/- 0.08 (n = 22). pHi recovery following an NH4+/NH3 acid load was inhibited by the removal of Na+ or by the addition of the amiloride analog EIPA; therefore recovery is dependent on Na-H exchange activity. CT had no effect on steady-state pHi but CGRP increased pHi in a dose-dependent fashion (10(-12) to 10(-6) M). The pHi change induced by CGRP was due to the stimulation of the Na-H exchanger as CGRP enhanced the rate of recovery (dpHi/dt) from an acid load from 45.3 to 77.2 microMs-1 (n = 8, P less than 0.002) and was completely blocked by EIPA. These data indicate that CGRP both enhances the activity of the Na-H exchanger and increases intracellular cAMP, thus demonstrating that macrophages express functional CGRP receptors.

Vasopressin stimulates phosphoinositide hydrolysis in LLC-PK1 cells

LLC-PK1 cells have been shown to possess vasopressin (VP) receptors (V2 type) that are coupled to adenyl cyclase to generate adenosine 3,5'-cyclic monophosphate (cAMP). To determine whether VP also stimulates phosphoinositide (PI) hydrolysis to generate inositol phosphate (IP) and diacylglycerol (DAG) messenger system in LLC-PK1 cells, we measured the release of IP in LLC-PK1 cells in the absence and presence of various concentrations of VP. In addition, we also determined the effect of an increase in osmolality of the incubation medium on VP-stimulated PI hydrolysis in LLC-PK1 cells. The methods involved the incubation of LLC-PK1 cells with [3H]inositol for its incorporation into membrane PI and the measurement of the release of [3H]IP in the presence of LiCl which prevents dephosphorylation. The osmolality of the incubation media was increased from 300 to 600, 900, and 1,200 mosmol/kgH2O by the addition of NaCl and urea. In an isosmotic incubation medium, VP (10(-8) M) produced a 100% increase in PI hydrolysis in LLC-PK1 cells. The effect was much greater at higher concentrations of the hormone. There was no effect of osmolality in VP-stimulated PI hydrolysis in LLC-PK1 cells up to 600 mosmol/kgH2O, but PI hydrolysis decreased significantly when the osmolality of the incubation medium was increased to 900 or 1,200 mosmol/kgH2O. Our results suggest that in LLC-PK1 cells, VP stimulates PI hydrolysis probably through VP receptors that are coupled to phospholipase C. Furthermore, VP-stimulated PI messenger system in LLC-PK1 cells is influenced by osmolality of the extracellular fluid.