Differential effects of phorbol esters on PGE2 and bradykinin-induced elevation of [Cai2+] in MDCK cells

Involvement of Intracellular Ca2+ in the Regulatory Volume Decrease After Hyposmotic Swelling in MDCK Cells

We examined the source of Ca(2+) involved in the volume regulation of Madin-Darby canine kidney (MDCK) cells with confocal microscopy and fluoroprobes. Hyposmosis induced a transient increase in cell volume, as well as cytoplasmic Ca(2+), which peaked at 3 to 5 min and gradually decreased to reach the initial value within about 30 min. This late decrease in cell volume, as well as the transient rise in cytoplasmic Ca(2+), was reduced in Ca(2+)-free solution and was abolished by pretreatment with thapsigargin. In conclusion, Ca(2+) released from the intracellular store contributes to the regulatory volume decrease following hyposmotic swelling in MDCK cells.

Involvement of EP1 and EP2 receptors in the regulation of the Na,K-ATPase by prostaglandins in MDCK cells

Prostaglandins are key regulators of ion transport in the kidney. In MDCK cells, which model distal tubule cells, the transcription of the Na,K-ATPase beta1 subunit is regulated by PGE1 and PGE2. To identify the EP receptors that mediate transcriptional regulation, transient transfection studies are conducted using the human beta1promoter/luciferase construct, pHbeta1-1141 Luc. The involvement of EP1 and EP2 receptors is indicated by studies with the EP1 selective agonist 17-phenyl trinor PGE2, and the EP2 selective agonist butaprost (which stimulate), as well as by studies with the antagonists SC-51089 (EP1 specific) and AH 6809 (EP1 and EP2 specific). Consistent with the involvement of Gs coupled EP2 receptors, is that the PGE1 stimulation is inhibited by the PKAI expression vector (encoding the protein kinase A (PKA) inhibitory protein), as well as by the myristolated PKA inhibitory peptide PKI. In addition to this evidence (for the involvement of EP2 receptors), evidence for the involvement of EP1 receptors in the PGE1 mediated stimulation of Na,K-ATPase beta subunit gene transcription includes the stimulatory effect of 17-phenyl trinor PGE2, as well as the inhibitory effects of SC-51089. Also consistent with the involvement of Gq coupled EP1 receptors, the PGE1 stimulation is inhibited by the PKCI vector (encoding the PKC inhibitory domain), the PKC inhibitor Go 6976, thapsigargin, as well as the calmodulin antagonists W7 and W13.

Osmolar regulation of endothelin signaling in rat renal medullary interstitial cells

We tested the hypothesis that endothelin (ET) responsiveness in the renal medulla is modulated by ambient osmolarity. Cultured renal medullary interstitial cells (RMICs) were incubated from 3 to 24 h in isosmolar culture medium (300 mOsm/kg H2O) or media rendered hyperosmolar (600 mOsm/kg H2O) by the addition of urea. Under hyperosmolar conditions, the peak of ET-evoked Ca2+ transient was blunted by 45-58% (P < 0.02) and PGE2 accumulation decreased from 16- to 2-fold above basal values (P < 0.001). To explore whether hyperosmolar conditions blunt intracellular signaling via modulation of receptor number or expression, kinetics of ET binding and Northern blot analysis of ETA receptor mRNA was performed. Under hyperosmolar conditions, ETA receptor density was reduced by 84% versus isosmolar conditions (238 +/- 12 vs. 1450 +/- 184 fmol/mg) (P < 0.01). In contrast to the ligand binding studies, ETA receptor mRNA was increased by 58% (P < 0.05) in cells grown under hyperosmolar versus isosmolar media. These observations indicate that in the hyperosmolar setting, ET-evoked intracellular signaling is blunted in RMICs due to ET receptor downregulation. Since ETA receptor mRNA is increased under hyperosmolar conditions, we conclude that ET receptor downregulation is the consequence of either decreased translation of message, increased degradation of receptor peptide, or increased internalization of specific receptor sites.

Acetylcholine and kinin augmentation of Cl- secretion stimulated by prostaglandin in a canine renal epithelial cell line

1. The actions of kinins and of acetylcholine upon transepithelial ion transport in a renal-derived cultured epithelium (Madin-Darby canine kidney cells, MDCK) have been investigated. 2. In voltage-clamped epithelial layers mounted in Ussing chambers and with prior stimulation of inward short-circuit current (SCC) by 5 or 10 microM-prostaglandin E1 (PGE1), both bradykinin (1 microM) and acetylcholine (0.1 mM) stimulate an additional, but transient, inward SCC. In the absence of PGE1 minimal effects of both bradykinin and acetylcholine upon SCC are observed. The SCC response to bradykinin and acetylcholine are attenuated with prior stimulation by 10 microM-adrenaline. 3. Measurements of bradykinin and acetylcholine-stimulated inward SCC with cation and anion replacement of the bathing media and the use of the Cl channel blocker 5-nitro-2(3-phenylpropylamino)-benzoic acid (NPPB) together with bumetanide to inhibit Na(+)-K(+)-Cl- 'co-transport', are consistent with the bradykinin- and acetylcholine-stimulated SCC being the result of basal to apical Cl- secretion. 4. Bradykinin (1 microM) is capable of stimulation of inward SCC from both epithelial surfaces, whilst acetylcholine is only effective from the basolateral surface. Kallidin (lys-bradykinin) was similar in effect to bradykinin from both epithelial surfaces whereas bradykinin (1-8) was ineffective, suggesting that B2 bradykinin receptors mediate the effect of bradykinin upon SCC. Dose-response relationships show that the response to kallidin and bradykinin was of higher sensitivity for additions to the apical cell aspects. 5. The data are discussed in relation to a model for epithelial Cl- secretion, and to the mechanism of natriuresis observed with kinins and acetylcholine in vivo.

The effect of hypo-osmolarity upon transepithelial ion transport in cultured renal epithelial layers (MDCK)

Regulatory volume decrease after exposure to hypo-osmotic media in MDCK epithelial cells results from activation of both K+ and Cl- conductances. Swelling-stimulated 86Rb(K) losses were observed only across the basal-lateral membrane and were relatively insensitive to 10 mM Ba2+. The effect of hypo-osmotic media upon MDCK epithelia mounted in Ussing chambers has been investigated. Exposure of the basal-lateral surfaces to hypo-osmotic media resulted in a transient stimulation of inward short-circuit current (Isc) followed by inhibition of inward Isc in both control layers and in layers where inward current (due to transepithelial Cl- secretion) was first stimulated by 5 microM prostaglandin E1 (PGE1). The transient stimulation of inward current by hypo-osmotic media was not markedly attenuated by 10 mM Ba2+ in PGE1-stimulated layers. After stimulation of inward (Cl(-)-secretory) current to high levels by 10 microM adrenaline, the predominant effect of basal-lateral exposure to hypo-osmotic media was an inhibition of the inward current. This inhibition was partially reversed by 40 microM 4,4'-diisothiocyanatostilbene-2,2'-disulphonate (DIDS). The stimulation, then inhibition, of inward Isc is likely to be the result of separate swelling-induced K+ and Cl- conductances (respectively) at the basal-lateral membrane. The swelling-stimulated Cl- conductance is distinct from the apical Cl- conductance regulated by PGE1 or adrenaline.

Calcium influx and intracellular calcium release in anti-CD3 antibody-stimulated and thapsigargin-treated human T lymphoblasts

Jurkat and MOLT-4 cultured T lymphoblasts were loaded with low concentrations (30-50 microM) of indo-1 and with high concentrations (3.5-4.5 mM) of quin-2, respectively, in order to follow the activation of calcium transport pathways after stimulation of the cells by a monoclonal antibody against the T cell antigen receptor (aCD3), or after the addition of thapsigargin, a presumed inhibitor of endoplasmic reticulum calcium pump. In the indo-1 loaded cells the dynamics of the intracellular calcium release and the calcium influx could be studied, while in the quin-2 overloaded cells the changes in cytoplasmic free calcium concentration ([Ca2+]i) were strongly buffered and the rate of calcium influx could be quantitatively determined. We found that in Jurkat lymphoblasts, in the absence of external calcium, both aCD3 and thapsigargin induced a rapid calcium release from internal stores, while upon the readdition of external calcium an increased rate of calcium influx could be observed in both cases. aCD3 and thapsigargin released calcium from the same intracellular pools. The calcium influx induced by either agent was of similar magnitude and had a nonadditive character if the two agents were applied simultaneously. As demonstrated in quin-2 overloaded cells, a significant initial rise in [Ca2+]i or a pronounced depletion of internal calcium pools was not required to obtain a rapid calcium influx. The activation of protein kinase C by phorbol ester abolished the internal calcium release and the calcium influx induced by aCD3, while having only a small effect on these phenomena when evoked by thapsigargin. Membrane depolarization by gramicidin inhibited the rapid calcium influx in both aCD3- and thapsigargin-treated cells, although it did not affect the internal calcium release produced by either agent. In MOLT-4 cells, which have no functioning antigen receptors, aCD3 was ineffective in inducing a calcium signal, while thapsigargin produced similar internal calcium release and external calcium influx to those observed in Jurkat cells.

Chloride secretion stimulated by prostaglandin E1 and by forskolin in a canine renal epithelial cell line

1. The actions of prostaglandins upon ion transport in a renal-derived cultured epithelium (MDCK) have been investigated. 2. Prostaglandin E1 (PGE1) stimulates an inwards short-circuit current (SCC) in voltage-clamped epithelial layers mounted in Ussing chambers. Measurements of transepithelial Cl- fluxes, cation and anion replacement of the bathing media and the use of the Cl- channel blocker 3-nitro-2(3-phenylpropylamino)-benzoic acid (NPPB) are consistent with the PGE1-stimulated inward SCC resulting predominantly from basal to apical Cl- secretion. 3. The response of MDCK epithelia is relatively specific for prostaglandins of the E series. PGE1 is most effective when applied to the basal bathing solutions, though near-maximal stimulation of SCC was possible with 1 microM-PGE1 added to the apical bathing solution. 4. Forskolin addition (10 microM) stimulates an inwards SCC in MDCK epithelia which displays similar characteristics and is of a similar magnitude to that observed with PGE1. In the presence of isobutylmethylzanthine, PGE1 and forskolin are capable of elevating intracellular cyclic AMP accumulation, suggesting that stimulation of inward SCC is mediated via a cyclic AMP-dependent mechanism.