Extracellular ATP stimulates polyphosphoinositide hydrolysis and prostaglandin synthesis in rat renal mesangial cells. Involvement of a pertussis toxin-sensitive guanine nucleotide binding protein and feedback inhibition by protein kinase C

Possible regulatory functions of protein kinase C-alpha and -epsilon isoenzymes in rat renal mesangial cells. Stimulation of prostaglandin synthesis and feedback inhibition of angiotensin II-stimulated phosphoinositide hydrolysis

Short-term treatment of mesangial cells with phorbol 12-myristate 13-acetate (PMA) decreases angiotensin II-induced InsP3 formation, but potentiates hormone-stimulated arachidonic acid release and prostaglandin (PG) E2 synthesis. Long-term treatment with PMA augments hormone-stimulated InsP3 generation (after 8 h treatment), but eliminates angiotensin II-induced arachidonic acid release and PGE2 formation (after 24 h treatment). By using specific antibodies it is observed that mesangial cells express two protein kinase C (PKC) isoenzymes, PKC-alpha and -epsilon. No PKC-beta and -gamma isoenzymes are detected. On exposure to PMA a complete depletion of PKC-alpha is observed within 8 h. In contrast, down-regulation of PKC-epsilon to 10-20% of that found in control cells requires a 24 h treatment with PMA. These results may imply that PKC-alpha mediates feedback inhibition of phosphoinositide hydrolysis, whereas PKC-epsilon is a candidate for regulating PG synthesis in mesangial cells.

Juxtamedullary afferent arteriolar responses to P1 and P2 purinergic stimulation

We assessed the responsiveness of rat juxtamedullary afferent arterioles to purinergic stimulation using the in vitro blood-perfused juxtamedullary nephron technique combined with videomicroscopy to allow direct measurement of arteriolar inside diameter. To minimize the contribution of endogenously formed angiotensin II, all rats were pretreated with enalaprilat (2 mg i.v.) for 30 minutes before the right kidney was isolated and prepared for study. Renal perfusion pressure was set at 110 mm Hg and held constant. Afferent arteriolar diameter averaged 20.9 +/- 0.8 microns (n = 41) under control conditions. Exposure to 1.0 microM 2-chloroadenosine induced a significant (11.1 +/- 3.2%) reduction in vessel diameter, whereas a 100 microM concentration induced an afferent vasodilation (7.6 +/- 1.5%; p less than 0.05). These data are consistent with the preferential stimulation of high affinity constrictor adenosine receptors (A1) at lower concentrations and activation of lower affinity vasodilator adenosine receptors (A2) at higher concentrations. In contrast, ATP elicited a significant afferent vasoconstriction of approximately 9.2%, 12.9%, and 10.0% at concentrations in the range of 1-100 microM (p less than 0.05). Treatment with ADP, at concentrations up to 100 microM, failed to alter vessel caliber significantly. Furthermore, the nonhydrolyzable ATP analogue alpha,beta-methylene ATP produced a rapid and potent vasoconstriction, which mimicked the response to ATP. These data reveal the presence of both adenosine-sensitive P1 and ATP-sensitive P2 purinergic receptors on rat juxtamedullary afferent arterioles and demonstrate that ATP can induce afferent arteriolar vasoconstriction directly without first requiring hydrolysis to adenosine.

Evidence for the presence of P2-purinoceptors at the surface of human articular chondrocytes in monolayer culture

Extracellular purines can act at purinoceptors to influence metabolic processes. Nucleotide-metabolizing ectoenzymes may modulate such purinergic effects, and their occurrence in a tissue may suggest the presence of purinoceptors. Thus, following the identification of ecto-nucleoside triphosphate pyrophosphatase in cultured human articular chondrocytes, we have studied whether these cells express P2-type purinoceptors. Release of prostaglandin E (PGE) was monitored, since articular chondrocytes synthesize and secrete PGE, and activation of P2-purinoceptors frequently results in enhanced prostaglandin production. Extracellular ATP and ADP stimulated PGE production, whereas AMP and adenosine had only limited effects. ATP concentrations as low as 5 microM were effective, and maximal responses were achieved at 50-100 microM ATP. GTP, UTP and ITP also elicited responses, but tended to be less effective than ATP at equivalent concentrations. Of the analogues of ATP that were tested, only adenosine 5'-(beta,gamma-methylene)triphosphate stimulated PGE production. The response to extracellular ATP was virtually abolished by indomethacin. Treatment of the cells with the P1-purinoceptor antagonist, 8-phenyltheophylline, or with pertussis toxin reduced both basal and ATP-stimulated PGE production, but did not substantially decrease the ratio of ATP-stimulated to basal PGE production. These results indicate the presence of P2-purinoceptors in cultured human articular chondrocytes, and suggest that extracellular ATP may have physiological and pathological effects in human articular cartilage.

Interactions between second messengers: cyclic AMP and phospholipase A2- and phospholipase C-metabolites

The article reviews several new findings on the interactions between phospholipase A2- and phospholipase C-derived metabolites and cyclic AMP, in view of the developments recently achieved in studies on intracellular signal transduction. A complex network of multi-directional regulative mechanisms in the airways and inflammatory blood cells is briefly outlined.

Comparison of extracellular ATP and UTP signalling in rat renal mesangial cells. No indications for the involvement of separate purino- and pyrimidino-ceptors

Extracellular ATP and UTP caused a rapid formation of InsP3, with similar kinetics and dose-dependences. ITP also displayed strong agonistic properties in terms of InsP3 production, whereas CTP was almost inactive. Pretreatment of the cells with pertussis toxin attenuated ATP- and UTP-stimulated InsP3 generation to a comparable extent, indicating that both nucleotides couple to phospholipase C by a pertussis-toxin-sensitive G-protein. Short-term (15 min) treatment of the cells with phorbol 12-myristate 13-acetate (PMA) produced a dose-dependent inhibition of ATP- and UTP-induced InsP3 formation. Furthermore, down-regulation of protein kinase C by long-term (24 h) exposure of the cells to PMA resulted in a comparable potentiation of phosphoinositide hydrolysis by both nucleotides. Preincubation of mesangial cells with ATP or UTP caused a pronounced cross-desensitization of subsequent nucleotide-stimulated InsP3 production. ATP and UTP displayed no additivity in terms of InsP3 formation, when used at maximally effective concentrations. In contrast, the peptide hormone angiotensin II interacted in an additive manner with either nucleotide in stimulating phosphoinositide hydrolysis. Reactive Blue 2, a putative P2y-purinoceptor antagonist, caused a rightward shift of both the ATP and UTP dose-response curves. However, since 2-methylthio-ATP was only a partial agonist in stimulating InsP3 formation, the mesangial-cell ATP receptor appears to be different from a classic P2y-receptor. In summary, these results provide no evidence for separate purino- and pyrimidino-ceptors on mesangial cells. In contrast, ATP and UTP may use a common nucleotide receptor for transducing their signals in mesangial cells.

Regulatory functions of protein kinase C in glomerular mesangial cells

Protein kinase C is a family of isozymes that are activated by hormone-stimulated phosphoinositide hydrolysis and participate in the signalling process by phosphorylating certain target proteins. In glomerular mesangial cells protein kinase C fulfills two major functions: it contributes to hormone-induced prostaglandin formation, and it acts as a negative feedback regulator of the inositol lipid signalling cascade. Furthermore, protein kinase C activates a phosphatidylcholine-degrading phospholipase D activity with as-yet-unknown cellular function.